Enemies in the Garden


    I’m a minimalist or, maybe, I’m just lazy. I prefer to think I am assisting the natural evolution of resistant varieties of our native flora by only using insecticides occasionally. I also don’t use a large variety of insecticides, maybe, because I want to live a couple extra years, but more likely it’s my minimalist attitude. Nine times out of ten, I will battle an insect pest with either horticultural oil or a systemic insecticide such as acephate or imidacloprid. I’ve read that because horticultural oil kills bugs mechanically by suffocating them rather than physiologically (e.g., disrupting important enzymes) it's very difficult for the pest to evolve any immunity or defense. I like systemics because they are normally effective for weeks or months, deterring re-infestation. 

    You can find horticultural oil (e.g., ORTHO Volck® Oil) and imidacloprid (e.g., Merit®) in nearly any garden shop. Unfortunately, acephate has become harder to find locally. Therefore, I now order it online. However, if you do buy acephate, be sure to buy an easily-useable liquid and not the super-concentrate powders made for large-acreage farming.

    Below is an alphabetical listing of pests (by most common name) I have encountered and battled over the years. By no means should you consider this a complete list of Hawaiian garden pests.

Ants (family Formicidae)

Ants are such a ubiquitous part of our fauna today that it’s hard to believe they were absent from Hawai‘i before human colonization. Yet, entomologists tell us, “Hawaii is one of the few places on Earth believed to harbor no native ant species.” Today, over 40 introduced ant species have become naturalized here (Krushelnycky, http://www.antweb.org/hawaii.jsp). The damage these invaders have caused to native arthropods, birds, and plants is immense (http://www.hear.org/ants/). While ants rarely harm native plants directly, indirectly, they disperse and protect harmful honeydew-producing insects such as aphids, mealybugs, and scale insects. 

    After many unsuccessful attempts, I no longer try to eradicate ants from the native plant nurseries, gardens, and restoration sites I manage. Instead, I focus my efforts on controlling ant populations and minimizing the damage they cause. My greatest diligence is in the nursery where tiny seedlings and young plants are most vulnerable. There, I use numerous boric acid bait traps which I refill with a mixture of: ⅓ cup white sugar; 1 tablespoon boric acid; 1 cup warm water. Additionally, when spraying insecticides such as horticultural oil or acephate on pest-infested plants, I often extend the spray to nearby plants, trays, and nursery benches. My control of ants is most cautious and restricted within restoration sites where I could potentially harm both native and beneficial alien arthropods. To discourage any pest, including ants, from attacking a new planting for the first few months, I sprinkle about three tablespoons of granular, broad-spectrum insecticide such as Spectracide Triazicide Insect Killer® or Ortho Bug B Gon Max Insect Killer® immediately around the base of the plant. After that, I normally don’t combat ants or other pests unless I see a life-threatening situation such as root mealybugs with companion ants. I will, then, thoroughly spray the plant with a systemic insecticide and reapply the broad-spectrum insecticide. In the garden, I am more preemptive at controlling ants. In addition to the above restoration site actions, I will use either the granular insecticides mentioned above or a granular ant bait such as AMDRO Fire Ant Bait® to kill large ant colonies I see near or on my favorite plants. 

Aphids (superfamily Aphidoidea)

These are tiny insects that suck fluids from the plant. They are usually seen on the leaves and young stems where they often distort the growth of these organs. They can also transmit viruses that cause disease. Some species of ants 'farm' aphids, protecting them from predators, transporting them to new areas of the plant or non-infested plants, and eating the honeydew the aphids excrete. Since aphids normally do not kill the plant, if you just ignore them, often ladybird beetles (i.e., ladybugs) and other predators eliminate them in a few weeks. If you wish to get rid of them immediately, spraying them with insecticidal soap is the safest way. (Often two or three sprayings a few days apart will prevent their reappearance.) Spraying with horticultural oil and almost any other type of insecticide (e.g., contact, systemic) will also kill these pests.

Photographs below show: Aphids on Hibiscus leaf and flower bud.

Black Twig Borer (Xylosandrus compactus)

The black twig borer has had devastating effects on many of Hawai‘i’s native hardwood trees such as māhoe (Alectryon macrococcus), mehamehame (Flueggea neowawraea) and uhiuhi (Caesalpinia kavaiensis). Fortunately, we are better able to combat this foe in the garden than in our native forests. The black twig borer attacks a young stem, hollows out the pith, inoculates the cavity with an ambrosia fungus – food for the borer’s young, and lays its eggs. The result is a dead stem, and a new generation of borers that need only crawl out of their nursery to find a new, healthy stem to repeat the process. 

    Several factors regulate the amount of damage caused by black twig borers. Controlling these is the best and most long-lasting defense against this pest. First, there is the plant’s location. Expect the most damage in damp, shady, windless sites, and the least damage in dry, sunny, windy locations. Therefore, plant borer-susceptible plants in the driest, sunniest, windiest part(s) of your garden. Second, is the health of the tree. Occasionally watering your native trees to avoid drought-stress or fertilizing them will also reduce or eliminate twig borer attacks. However, do not overwater your plant or you’re likely to face new problems from unfriendly soil microbes. (Over-fertilizing can also cause problems by making your tree irresistible to leaf-eating pests like Chinese rose beetles.) Third is the prompt removal of infested branches. Twig borer larvae within the dead branch take time to mature and then spread to new healthy branches. If you prune away and dispose of the infested branches quickly you can break the cycle of infestation. Disposing of the branches means taking them far offsite – don’t just toss them onto your compost pile. 

    I’ve heard conflicting reports on the effectiveness of insecticides to control the black twig borer. Some native plant enthusiasts believe spraying your tree with neem oil can ward off the initial attack. Personally, I used the systemic insecticide, acephate, and others have used imidacloprid to control the borer with some success. I’ve also heard of one study where a fungicide was used to kill the ambrosia fungus and, theoretically, starve the developing young. I’ve yet to find out how well this approach worked. Regardless of which insecticide you try, they seem to only be effective with repeated (and, probably, alternating) application. Therefore, I recommend you try the above-mentioned non-chemical methods before resorting to insecticides. 

Photographs below show: Black twig borer damage to a kauila stem. Dissected stem with adult black twig borer and (white) grub-like larvae. Twig borer hole with borer in the entrance.

Broad Mite (Polyphagotarsonemus latus)

Broad mites are insidious pests for three reasons. First, they are too tiny to see with the unaided eye – you need a microscope to see them. I don't have a camera on my microscope, therefore, please take a look at the electron microscope image below from the Agricultural Research Service of the USDA to see what a broad mite looks like. Second, the damage caused by broad mites is variable and easily misinterpreted as symptoms of other pests or problems. The symptoms/damage I have seen include an absence or stunting of growth (even following fertilizer treatments) and leaves that are bronze-colored, brittle or distorted. Third, my standard treatments of horticultural oil and/or systemic insecticide are ineffective at eliminating this pest.

    Frustrated after losing a bunch of aiea seedlings, I contacted the Agricultural Diagnostic Service Center (808.956.6706) at the University of Hawai‘i at Manoā (Tropical Agriculture & Human Resources) hoping they could help me. They did. After I described the symptoms over the phone, and without a photograph or sample, the agricultural agent quickly and correctly identified the problem – broad mites! He then prescribed a most effective treatment: one (1) Tbsp of wettable sulfur per gallon of water sprayed every 2-3 days on the afflicted plants. After about five treatments, my sole remaining aiea seedling regained its health and started growing again. Since then, I have seen broad mites attack keahi, māmane and maua seedlings. Each time, the sulfur treatment was effective at eliminating the broad mites and restoring these plants to good health.

Photograph below shows: The Broad mite (Polyphagotarsonemus latus), USDA BARC Public domain

Chinese Rose Beetle (Adoretus sinicus)

Chinese rose beetles are dull brown and about half an inch long. During the day, adults live in leaf litter, mulch, or slightly below the soil surface. At dusk, females emerge from the soil and begin feeding on the leaves of nonresistant plants such as koa, ko‘oloa ‘ula, ma‘o hau hele, and wiliwili. The less destructive males locate and mate with the females on afflicted plants. Eggs are laid in the soil and develop into white grubs that feed on humus, detritus, and decaying plant matter; reportedly, the grubs do not harm living plants. From egg to adult takes about seven weeks of development. 

    The damage caused by Chinese rose beetles is seldom fatal but does make the plant unsightly and, by reducing the photosynthetic capacity of the leaves, slows plant growth. The beetles are most active a few hours after dusk. Damage is often cyclical, perhaps, because of the seven-week lifecycle. And, is worst a few days after a heavy rain or on moonless nights. During long dry weather periods the beetles 'disappear' and leaf damage is minimal. You can accurately predict worst-damage nights by keeping a calendar and notes of these environmental factors.

    The easiest response to Chinese rose beetles in your garden is no response; if you can live with native plants with imperfect leaves, then, do nothing. As stated above, plant death due to this beetle is very rare. Planting your susceptible native plants under a bright light (e.g., yard floodlight, streetlight) will dramatically reduce rose beetle damage since the beetles shun bright light. If you control the light, you only need to leave it on for a few hours after dusk; an automatic light-timer makes this easy. Another solution is to handpick and kill the beetles. A good friend of mine and his wife use a record book to predict which nights they should spend their evening hunting beetles. Hey, it’s healthier than vegetating in front of the TV and is good, destructive fun! (If you have kids, make hunting Chinese rose beetles into a family-time game.) A physical barrier such as a cylinder of field fence covered with weedcloth will reduce rose beetle damage. If you make the barrier lightweight, you can easily deploy or remove it from your garden as needed. If you decide to use insecticides, systemic forms such as imidacloprid sprayed on the plants just before the predicted worst nights work best. These appear to deter the beetles from landing and feeding on your plants rather than actually killing them outright. (Some Hawai‘i gardeners have told me they use neem, in a variety of forms (e.g., neem wood chips, neem oil, fresh-cut neem leaves), to control Chinese rose beetle damage; I have yet to try this deterrent.) 

Photographs below show: Chinese rose beetles feeding, courtesy of Dr. Arnold H. Hara, CTAHR ©. Closeup of Chinese rose beetle courtesy of Emmy Engasser, Hawaiian Scarab ID, USDA APHIS PPQ, Bugwood.org CC BY-NC 3.0 US.

Flea Beetles (tribe Alticini)

Flea beetles are perfectly named. Adult flea beetles are tiny (1-3 mm). When disturbed, they use their very strong hindlegs to jump far away. When not threatened, they crawl on or fly around their host plant. Flea beetle larvae live beneath the soil where they generally cause insignificant damage. While there are many species, most flea beetles attack plants in the cabbage (Brassicaceae) or nightshade (Solanaceae) families. 

    For me, flea beetle attacks have been limited to my ‘ōhelo kai, ‘aiea, and native pōpolo. I’m not an entomologist, but I suspect the culprit is the tobacco flea beetle (Epitrix hirtipennis). Flea beetle problems are most common during the hot summer months. And, more difficult to control on plants in the garden than those in the nursery. The damage flea beetles cause, numerous tiny holes in the leaves, is normally not fatal (except, perhaps, to seedlings), but it certainly makes the plant unsightly and may reduce growth or stress the plant (which can lead to other problems). 

    All beetles, flea beetles included, have a hard exoskeleton that protects them from many contact insecticides. Fortunately, killing them isn’t a necessity; stopping them from chewing holes in your native plants is the goal. There are various non-chemical ways of reducing flea beetle damage including: 'trap crops' that lure the beetles away from your 'harvest crop', sticky traps placed among your plants, and cover-screening to physically keep the beetles away from the plants. However, being a lazy horticulturist with no intention of eating my native Solanaceae, I’ve always used the easiest chemical solution for dealing with any flea beetle infestation: horticultural oil mixed with a systemic insecticide. It works. Normally, one spraying keeps the beetles away for two or more weeks. I say “away” because I’m not certain if the spray actually kills them or just makes them disappear. If you’re dead set against using a systemic insecticide, I encourage you to experiment with neem oil, peppermint oil, diatomaceous earth, or any other organic that makes your native plants taste or smell bad (to a flea beetle).

Photographs below show: Flea beetles on pōpolo ku mai (Solanum incompletum). Sweetpotato flea beetles on pōhuehue (Ipomoea pes-caprae subsp. brasiliensis); Photograph courtesy of UHM Emeritus Plant Pathologist, Dr. Scot C. Nelson - CC0 1.0 Public domain

Fungal Diseases

Fungal diseases/infections vary tremendously in their lethality when it comes to native Hawaiian plants. Some, like powdery mildew, will kill a plant's leaves but not the entire plant, while others such as Ceratocystis lukuohia and C. huliohia (collectively called Rapid ʻŌhiʻa Death or ROD) have a greater than a 90% mortality rate within 2-3 years. Following, I describe my experiences with fungal infections of native Hawaiian plants from least deadly to most deadly.

Sooty Mold

    Sooty mold is a collective term for many different non-parasitic Ascomycete fungi that grow on plants as well as non-biotic objects like fences, garden furniture, stone walls, and even cars. The mold feeds on either a sugary secretion produced by the plant, or on honeydew secreted by sap-sucking insects such as aphids or scale insects. While unsightly, the mold does little if any harm to the plant other than possibly reducing photosynthesis and gas exchange. Rather, its presence is nearly always a sign of a more serious problem — an infestation by sap-sucking pests that should be treated. The mold itself can be removed by gently washing the leaves, etc., with either water or a dilute soap solution. 

Powdery Mildew 

    This is a common disease caused by many different species of fungi that are usually host-specific. The infection is visible as white (or yellow) powdery spots most commonly seen on the plant's leaves, but may also be visible on flowers, stems, and fruits. Often, infected leaves, flowers, and fruits will yellow, dry out, and fall off. Despite this damage, it is rare for the entire plant to die from an infection. Infections are most common during periods of high humidity. However, interestingly, wet leaves can inhibit infection. For most native Hawaiian plants susceptible to powdery mildew like wiliwili, infections are seasonal and go away without treatment. However, if the infection is persistent or severe, you can (1) move the plant to a sunnier and less humid location with better air circulation, or (2) spray the infected plant with a solution of horticultural oil and baking soda, vinegar and water, neem oil, or wettable sulfur. Do not use wettable sulfur in combination with any type of oil, and do not spray plants with any type of oil if the air temperature is over 90°F or the plant will be exposed to direct sunlight for more than an hour after spraying; a late afternoon or early evening treatment is safest. These treatments will usually not save the infected leaves, etc., but will prevent the infection from spreading to healthy regions of the plant.

Myrtle or ʻŌhiʻa Rust (Austropuccinia [formerly Puccinia] psidii)

    This fungus, native to tropical America, has spread rapidly (since about 2000) throughout the Pacific and is now present in Hawai‘i (2005), China (2009), Australia (2010), South Africa (2013), and New Zealand (2017); efforts to contain myrtle rust have been largely unsuccessful. The rust attacks a broad range of host plants in the Myrtle family (Myrtaceae) with different strains infecting specific species or genera. In Hawai‘i, myrtle rust has infected six native plant species and at least 24 non-native species according to Anderson (2012). The endangered endemic nioi (Eugenia koolauensis) has been most affected. Also attacked are the non-endangered indigenous nioi (Eugenia reinwardtiana) and ‘ōhi‘a lehua (Metrosideros polymorpha) which has largely escaped major damage. The most affected species has been the invasive non-native rose apple (Syzygium jambos), which likely fueled the rapid spread of the fungus throughout the Islands.

    Mryrtle rust spores have a distinctive yellow to orange color, visible on the circular lesions the fungus creates on leaves, buds, and fruits. The leaves and buds then deform and become necrotic. Severe infections result in repeated defoliations and eventual death. Myrtle rust is spread by the transport of infected plant material, contaminated equipment, wind and water, animals including humans, and vehicles.

    Currently, there are no approved fungicides to treat myrtle rust in Hawai‘i. (However, I will not discourage anyone from trying any fungicide available here; it's unlikely to make things any worse!) Therefore, the best treatments, for now, involve good sanitation practices, such as removing and bagging and or destroying infected plant parts as soon as symptoms appear, as well as keeping the plant's foliage dry when watering to reduce the spread of fungal spores.

Fusarium Wilt

    This is a deadly disease caused by the fungus Fusarium oxysporum which infects and clogs the vascular system of a plant, causing it to wilt, turn yellow, and eventually die. Over a hundred different host-specific types, referred to as forma specialis, of F. oxysporum have been identified, each attacking a different group of plants from many different plant families. Among native Hawaiian plants, many species are likely susceptible to F. oxysporum, however, only a few forma specialis have been definitively identified. One of these is Fusarium oxysporum f. sp. koae which infects and kills our native koa (Acacia koa) and koai‘a (A. koaia).

    Fusarium oxysporum lives in the soil, where it can persist for many years without a host plant. Infection is more likely at high temperatures in warm moist acidic soils. Plants stressed by other factors (e.g., insect pests, water-logged soil) are more susceptible to infection.

    Currently, there is no cure or effective longterm treatment for fusarium wilt. While a few fungicides such as azoxystrobin, prothioconazole, and thiophanate-methyl have been found to offer temporary control of F. oxysporum, they are effective for only a short period (i.e., days). Rather, the only practical control methods today are: (1) planting fusarium-resistant plant varieties, or (2) improving soil conditions such as better drainage and raising the soil pH. Unfortunately, for native Hawaiian plants, the development of resistant strains has only been attempted for koa (Acacia koa); this research is currently ongoing and shows promise.

    If your plant is in a container and you think it has fusarium wilt:

    If your plant is in the ground and you think it has fusarium wilt:

Photographs below show: Sooty mold on gardenia with scale insects and ants courtesy of UHM Emeritus Plant Pathologist, Dr. Scot C. Nelson - CC0 1.0 Public domain. Powdery mildew on native mint courtesy of J.K. Lindsey ©. ʻŌhiʻa rust on ʻōhiʻa courtesy of UHM Emeritus Plant Pathologist, Dr. Scot C. Nelson - CC0 1.0 Public domain. ʻŌhiʻa rust on nīoi courtesy of David Eickhoff CC BY 2.0. Infected (left) and dead (right) koa trees from koa (Fusarium) wilt. Vascular staining from koa wilt. (Both koa photographs courtesy of Dr. Scot C. Nelson - CC0 1.0 Public domain.)

Lace Bugs (family Tingidae)

Adult lace bugs are about ⅛ inch long and often have elaborate dorsal surfaces. Clear cells in these surfaces and the wings give the bugs a lace-like appearance. Wingless nymphs are normally darkly colored and have spines. Both gather on the undersurface of leaves where they pierce leaf cells and suck out the contents. The result is spotted or yellowing leaves, visible from both above and below the leaf. This damage is permanent; the leaf does not repair itself even after the lace bugs have been eliminated. Females insert tiny eggs in leaf tissue and cover them with brown or black excrement. Adults and nymphs also produce a dark excrement that speckles the leaves. While the leaf damage they cause can be significant and unsightly, lace bug infestations rarely kill plants. Often natural predators such as parasitic wasps, lacewings, ladybird beetles, spiders and mites will eliminate or control the lace bugs on the plants without intervention. If you do decide to battle the bugs, you can use a forceful spray of water directed at the undersurface of the leaves to wash away most of them. Additionally, almost any contact insecticide will kill lace bugs. Try using the safer types such as horticultural oil, neem oil, insecticidal soap and pyrethrin. 

Photographs below show: Lace bug on ko‘oko‘olau. Lantana lace bug on naio.

Mealybugs (family Pseudococcidae)

Mealybugs are small (less than ¼ inch long), sexually-dimorphic, soft-bodied insects closely related to scale insects. Most visible are females which lack wings but often retain legs in order to move very slowly. Males, smaller than females, are wasp-like flying insects. Immature nymphs, called crawlers, are very tiny but more mobile than females. Females secrete white, powdery, wax filaments that cover their body. Females and nymphs feed on plant sap and can be found on all plant parts but are most common on stem-tips and roots where they cluster tightly together; males do not eat. A root infestation can be difficult to detect. Use your finger to gently dig around the base of the plant to discover mealybugs attacking surface roots. Ants near the plant’s base are also a good clue because they farm the mealybugs for their sugary secretions. For native plants in containers, carefully remove the plant from the pot to inspect the roots for mealybugs.

    Mealybugs are a serious pest capable of killing even large plants. Small infestations can be removed by hand, a strong jet of water, a cotton swab dipped in alcohol, or by pruning and discarding the infested plant parts. For large infestations, I use a mix of horticultural oil and systemic insecticide. Spray the mix directly on mealybugs attacking above-ground plant parts. For mealybugs attacking the roots, drench the soil (or media) thoroughly near the base of the plant or, for potted plants, immerse the entire plant in the mix. Repeated treatments, several days apart, are required for complete control. (Others have used insecticidal soap, neem oil or diatomaceous earth to control mealybugs.) Ladybird beetles and their larvae are voracious mealybug predators and should be encouraged in the garden and nursery. In contrast, ants often reintroduce and transport mealybugs and, therefore, should be controlled as best as possible.

Photograph below shows: Mealybugs with protective ants on kolomona (Senna gaudichaudii).

Rats and Mice (order Rodentia)

Early Polynesians likely introduced the Pacific or Polynesian rat (Rattus exulans) to Hawai‘i, either accidentally or intentionally, approximately 1,000 years ago. This introduction had devastating effects on the native flora and fauna, effects that continue to be felt today. Particularly at risk were and still are Hawai‘i's native birds, both flying and flightless, as rats have been known to consume their eggs, nestlings, and adult birds. The Pacific rat is also accountable for the extinction or severe decline of numerous native Hawaiian plants, either by directly consuming them or their seeds and seedlings. One notable example is the native Hawaiian palms, or loulu (Pritchardia spp.), whose seeds have been decimated by rats over time, leading to the disappearance of large loulu forests, especially in the lowlands.

    Shortly after James Cook's arrival in Hawai‘i, two additional rat species were introduced: the black rat (Rattus rattus) and the Norway rat (Rattus norvegicus), along with the house mouse (Mus musculus). The black rat is the most arboreal of the three species, while the Norway rat is the largest.

    Rats and mice pose significant threats to native Hawaiian plants in cultivated settings. While there are numerous methods devised by humans to eradicate these rodents, no method is foolproof, as it only takes one rat or mouse to cause serious damage or death to a native plant. Therefore, I recommend utilizing two different types of barriers: cages for small plants and trunk shields/guards for larger plants, to prevent these animals from coming into contact with the plant and causing harm.

    Plant cages and trunk shields can be purchased online or at local garden and landscape stores. However, I prefer to create my own. For cages, I utilize hardware cloth due to its small mesh size that can even exclude tiny mice, and it is available in various sizes. To create a protective barrier for your plants, you can construct a cylinder with a top by cutting cloth to size and securing it with zip-ties or wire. This cage can be placed over a potted plant or anchored to the ground using fabric staples typically used for weed-cloth. For a visual guide on how to make a hardware cloth cage, you can refer to this instructional YouTube video: https://www.youtube.com/watch?v=jkg-GnnlnI0.

    For larger plants, consider making a trunk-shield using either empty 2L plastic soda bottles or corrugated plastic drainage pipe. These materials are slick enough to deter rats and mice from climbing up past them. Avoid using metal or plastic trunk-shields with ventilation holes, as rodents can easily exploit these openings to climb. To create a trunk-shield with drainage pipe, cut it to a suitable height (2-3 feet) and slice it lengthwise to wrap around the plant's trunk. For soda bottles, remove the top and bottom, then slice lengthwise. To increase height, stack multiple bottles around the trunk. Like the plant cages, secure the trunk-shield with fabric staples for added stability.

Root-knot Nematodes (Meloidogyne spp.)

Root-knot nematodes (RKN) are microscopic roundworms that attack the roots of plants, causing the development of knots or galls. Infected plants often have stunted growth, wilt easily, are chlorotic (yellowing on leaves), or do not flower. However, sometimes RKN cause only slight swellings on the roots, while other symptoms of RKN can be easily confused with nutrient problems. There are hundreds of different species of nematodes that infect plants, but less than a dozen are economically serious in Hawai‘i. The most common species of root-knot nematode in Hawai‘i are Meloidogyne incognita (southern root-knot) and M. javanica (Javanese root-knot). A professional soil assay is needed for an accurate RKN diagnosis. Here in Hawai‘i, such testing can be done by the UHM College of Tropical Agriculture and Human Resources (CTAHR) Agricultural Diagnostic Service Center.

    There is no silver bullet when it comes to eliminating or controlling root-knot nematodes. In fact, our ability to control this serious pest is really quite limited, and an infected native Hawaiian plant is most often a soon-to-be-dead native Hawaiian plant. One helpful piece of advice I can share is to adopt the habit of routinely pulling up and inspecting any native plant that dies under your care. With these autopsies, you will be better able to plan future actions and plantings in that section of your landscape.

    Here are some control methods for RKN:

Photographs below show: Damage to tomato and noni roots by root-knot nematodes. Microscope view of nematode. All photographs courtesy of UHM Emeritus Plant Pathologist, Dr. Scot C. Nelson - CC0 1.0 Public domain https://www.flickr.com/photos/scotnelson/albums/72157635415455383/

Scale Insects (superfamily Coccoidea)

The many species of scale insects (about 8,000 according to Wikipedia) vary widely in size, form, and color (see below for some examples). However, they all fall into two main types: armored and soft. Armored scale have a cover or shield that can be removed without dislodging the insect from the plant. In contrast, the shield of soft scale is actually part of its body; pulling away the shield dislodges the insect from the plant. Additionally, soft scale often produce honeydew while armored scale do not. Most often when you recognize scale insects on your plants, you’re looking at the (nearly) immobile females. Juvenile scale are very tiny and called 'crawlers' because they do (slowly) move, dispersing away from their mother. Male scale are also small, have wings (to find new females), do not feed, and only live a few days. Scale insects live on stems, leaves, and fruits, where they feed on the plant’s sap. (I’ve never seen scale insects on roots.) Some species are extremely harmful, even deadly, while others are almost benign and just make a plant unsightly. 

    Natural predators such as ladybird beetles, parasitic wasps, lacewings, and mites are best for long-term control of scale insects. Unfortunately, ants often protect the scale from these predators. If the infestation is small, try removing or killing all the scale with your fingers, or prune and discard the infested leaves and stems. Avoid using contact insecticides such as sevin or malathion to kill scale since these chemicals kill everything, often making a second infestation of scale or another pest such as mites even worse. When I do resort to chemical control, I nearly always spray my plants with horticultural oil which smothers the scale and is relatively harmless to beneficial insects. Neem oil and canola oil are also good sprays. You can also try a systemic insecticide but be aware that not all systemics are equally effective. For example, imidacloprid will not control cottony cushion scale or most armored scales. After treatment, the dead scale often remains attached to the plant. Therefore, scrape away the scale with your fingernail to determine if it is dead or alive before you repeat any treatment. 

Photographs below show: Cottony cushion scale. Barnacle Scale. Snow scale on a stem. Lobate lac scale. Unidentified soft scale. The last two photographs courtesy of Jeffrey W. Lotz and Gilles San Martin from Namur, Belgium (uploaded by Jacopo Werther) via Wikimedia Commons (https://commons.wikimedia.org/wiki/File:Scale_insects_(7244837120).jpg)

Snails & Slugs (class Gastropoda)

There are a lot of online sources of information about how to combat snails and slugs that might attack your native Hawaiian plants. Therefore, it would be difficult and redundant for me to try to replicate or combine all the available online information in one space here. Rather, I prefer to direct you to one of the better websites I have come across, produced by a local expert here in Hawai‘i. That site is:

Managing Slugs, Snails, and Flatworms in the Home Garden to Reduce the Risk of Rat Lungworm Infection by Cynthia Nazario-Leary, PhD (Extension Agent in Urban Horticulture, Department of Tropical Plant & Soil Sciences)

https://manoa.hawaii.edu/ctahr/farmfoodsafety/wp-content/uploads/2017/05/CTAHR-Snail-Slug-Mgmt-HG-RLWD.pdf

    After taking a look at Dr. Nazario-Leary's website, you may ask, "Which of the management methods she describes do I prefer or most commonly use?" Well, I tend to like and use the more permanent methods that require the least amount of repeated labor. These are: 

Phototographs below show: Giant African African Land Snail courtesy of J.M.Garg, CC BY-SA 4.0). Leatherleaf Slug courtesy of Michal Maňas, CC BY 2.5. Gray Garden Slug courtesy of Bruce Martin, CC BY-SA 3.0.

Spider Mites (family Tetranychidae)

Spider mites are very tiny creatures (that look like tiny moving dots to the naked eye) closely related to spiders. Like spiders, they produce webs on the underside of the leaves and branches of plants they attack. The webbing is a good way to distinguish spider mites from other small insect pests that attack the underside of leaves such as aphids and thrips which do not produce webs. Unlike spiders, spider mites live in large colonies with a single leaf sometimes hosting more than a hundred mites. Spider mites have oval bodies and eight legs except for newly-hatched larvae which only have six legs. They do best in hot dry weather, and, under these conditions, reproduce very rapidly, producing a new generation in less than a week. Spider mites damage their host plant by sucking out the contents of leaf cells. This results in the leaf having a stippled coloration. If left untreated, the leaf eventually yellows, dies, and drops off the plant.

    In small numbers, spider mites do not threaten the life of a plant, and, frequently, natural predators like other mites, lacewing larvae, and some types of thrips keep them under control. However, large numbers should be dealt with – but not with broad-spectrum insecticides such as carbaryl (Sevin) or organophosphates that can actually worsen the infestation by killing mite predators. Rather, I typically control spider mites with sprays of horticultural oil or insecticidal soap. Others, prefer using a miticide, a chemical that only kills mites. However, if you opt for a miticide be sure to follow the label directions exactly. It’s also sometimes possible to reduce spider mite populations on your plants by spraying the undersides of the leaves frequently with water or temporarily increasing the humidity of your garden (e.g., sprinklers go on three times a week for 10 minutes rather than once a week for 30 minutes). Macrophotograph on the right courtesy of Gilles San Martin – originally posted to Flickr as Tetranychus urticae with silk threads.

Photographs below show: Red spider mites on leaf. Macro-photograph of red spider mite courtesy of Gilles San Martin - originally posted to Flickr as Tetranychus urticae with silk threads, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=11208762

Spittlebugs (superfamily Cercopoidea)

Spittlebugs are known for the frothy mass they produce while feeding on plants. There are no native Hawaiian species of spittlebugs. However, there are three invasive species (from three distinct families) firmly established here. More commonly seen are spittlebug nymphs (see photo below) that live in the spittle, while adults, called froghoppers because they can jump like leafhoppers, are less visible. Spittlebugs feed on a variety of plants such as grasses, roses, clover, strawberries, herbs, and many other garden plants, by piercing the plant's stem in order to suck out the plant's sap. In many cases, but with important exceptions like the Two-Lined Spittlebug (Prosapia bicincta), spittlebugs cause little damage and are not life-threatening to the plant. However, spittlebug infestations can sometimes cause leaves to lose their shape. In my experience, spittlebugs are a relatively rare pest on native Hawaiian plants, and are most commonly seen on naupaka or native sedges and grasses like mauʻu ʻakiʻaki.

    There are a number of ways to control or eliminate spittlebugs. These include:

    Recently, a very damaging spittlebug has invaded Hawai‘i, the Two-Lined Spittlebug (TLSB), Prosapia bicincta. It was first detected in 2016 and by 2021 had infested 180,000 acres on Hawai`i Island. For more information on TLSB or to report a sighting, visit this UHM College of Tropical Agriculture and Human Resources website: https://www.ctahr.hawaii.edu/oc/freepubs/pdf/IP-52.pdf

Photographs below show: Spittlebug nymph on naupaka kuahiwi. Nymph in spittle courtesy of monicaborden13 CC BY-NC-ND 2.0. Two-lined Spittlebug adult courtesy of John Flannery CC BY-SA-2.0.

Thrips (Order Thysanoptera)

First, the singular (just one insect) and plural (two or more insects) for this pest is 'thrips'; yea, it’s counterintuitive! Second, prior to myoporum thrips arriving in Hawai‘i, I had very few encounters or problems with thrips on my native Hawaiian plants. Therefore, the information and advice given below is largely a summary of the online writings of others. 

    Entomologists have described approximately 6,000 species of thrips. Most are very small (1 mm long or less) and slender, with fringed wings (as adults) and distinctive asymmetrical mouthparts. The majority feed on plants by puncturing and sucking out the contents, although a few species are predators. Many species are pests of commercially important food and ornamental plants, and a few are vectors for plant disease viruses; some such as the myoporum (naio) thrips can cause extinction-level damage and death while others are a minor nuisance. Pest species generally attack the softer parts or new growth of plants resulting in scarring, dead tissue patches or trails, and galling of leaves. Thrips feeding is usually accompanied by visible black flecks of excrement. Because most thrips are tiny, they can be difficult to see and identify. One way to determine if thrips are infesting your plants is to put a blank sheet of white paper beneath the damaged leaves or flowers and shake the basal stem. If there are thrips, some will fall off, and you will be able to get a better look at them on the paper with a magnifying glass.

    Thrips rapidly evolve resistance to insecticides. This evolution, along with their rapid reproduction, high mobility (by flight or wind), and ability to shield themselves from predators and insecticides within galls, deformed leaves and tiny crevices can make eradication or control difficult. Methods of eradication or control include: pruning infested leaves and stems, insecticidal soap, horticultural oil, neem oil, pyrethrin, spinosad, diatomaceous earth, kaolin clay, and sticky traps. Biocontrol agents for pest thrips include: predatory thrips, lacewings, minute pirate bugs, mites, and certain parasitic wasps. As previously mentioned, my experience with thrips is quite limited. Of the treatments above, I have only used pruning, insecticidal soap and horticultural oil with varied success. For more detailed information on thrips, I recommend the UCIPM webpage on thrips.

Photographs below show: Assorted life stages of Tubuliferan thrips courtesy of Scot Nelson & Dick Tsuda CC0 1.0, Myoporum thrips, and damage by thrips courtesy of Center for Invasive Species Research CC BY-NC-ND 2.0.

Whiteflies (family Aleyrodidae)

Whiteflies normally attack the undersides of leaves. There, they suck sap from the plant causing the leaf to yellow and die. A whitefly egg hatches as a tiny crawling nymph. The nymph then grows through three more nymphal stages called instars that are immobile and look similar to scale insects. The familiar winged adult emerges from the last instar. Like aphids, whiteflies can transmit plant diseases and viruses and produce honeydew. The honeydew, in turn, is consumed by sooty mold or harvested by ants that protect the whiteflies from natural predators such as lacewings and ladybird beetles. Complete elimination of whiteflies using pesticides is difficult since the insects quickly evolve resistance. However, repeated sprayings of insecticidal soap, horticultural oil, neem oil, or a systemic insecticide can greatly reduce whitefly infestations. Frequently spraying the undersurface of the plant’s leaves with water or vacuuming up the adults can also help. Whiteflies are attracted to yellow and repelled by reflective objects, therefore, yellow sticky traps and aluminum foil can be an effective treatment. 

Photographs below show: Whiteflies on Gouania vitifolia leaf. Closeup of whiteflies courtesy of Stephen Ausmus - US Department of Agriculture Public domain.

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