Bio-Rational Pesticides

Bio-Rational Pesticides

Traditionally, landscape managers have viewed daily pest management situations through the eyes of "control" or "eradication." Ideally, the best strategy would be to identify and remove only the pest, causing minimal disruption to the system. Until recently, insect pest managers did not have the necessary tools to achieve this goal, but within the last decade, even within the last few years, many new and exciting products have emerged or have greatly improved in efficacy. We classify many of them as "bio-rational" pesticides. These compounds achieve several currently desired goals of pest managers and the greater public: they are very selective (targeting just the pest, usually do not persist in the environment, are much safer to handle and apply when compared to most chemical pesticides, and tend to preserve beneficial organisms. Some of the more commonly used and effective bio-rational pesticides are Insect (and mite) Growth Regulators (IGR's), Bacillus thuringiensis (Kurstaki) (B.t.), horticultural oils, insecticidal soaps, entomopathogenic nematodes, and NEEM.

Insect Growth Regulators (IGR's)

Insects, being arthropods, must go through a molting process ("shedding their skin") in order to become larger and to mature; their rigid exoskeleton does not stretch and grow along with them. There are complex, specialized chemicals within the insect's body that regulate this process. Some of these chemicals have been identified and synthesized (or imitated) and are available commercially for management of certain pests.  Applications disrupt the carefully orchestrated natural molting process and are lethal to target insects.

Bacillus thuringiensis (Kurstaki)

Commonly known as B.t., this bacterium has been available as an insecticide for many years. There are several strains of B.t. available and each acts on different groups of insects. Kurstaki is effective against Lepidopteran caterpillars and therefore is the most widely used type of B.t. in the Green Industry. The product is tank mixed with water and applied to foliage where Lepidopteran caterpillars are feeding. Caterpillar-type larvae are also found in other insect orders, such as the Hymenoptera, which we recognize as sawfly caterpillars. It is well known that B.t. is not effective against these caterpillars and other methods of management must be chosen for sawflies.

B.t. works best on the younger Lepidopteran caterpillars and is not recommended for older larvae. It must be ingested to be effective. Once inside the insect's gut, B.t. becomes active due to the "preferred" alkaline environment, and begins to multiply. Endotoxins are produced by the bacterium, which then kill the caterpillar. Once B.t. is ingested, the caterpillar will cease to feed within minutes. However, it may remain alive for several days, often making it difficult to convince clients that no further plant injury is occurring even though the insect is still present.

B.t should not be stored for long periods of time due to loss of efficacy. Also, once tank-mixed, it should be applied in a timely fashion. Care should be taken not to use high pH water (alkaline) in the spray tank because this may induce the bacterium to become active in the tank prior to being ingested by the caterpillar. This will reduce the efficacy of the product. Sticker-spreaders can be added to the spray tank to increase the persistence of the product on the foliage.

B.t has been the main choice for use against the Gypsy moth for decades in the Northeast and it has been a very effective tool in this regard. More recently, researchers have discovered an entomopathogenic fungus that occurs naturally (now) and which has been keeping gypsy moth populations very low in much of the Northeast in recent years. This fungus, like plant parasitic fungi, requires water to be active and successfully invade caterpillars. It has been suggested that the Northeast may not have to worry about massive and destructive gypsy moth outbreaks again due to the activity of this fungus (Entomophaga maimaiga). However, unusually dry weather in early summer can reduce the activity of the fungus and lead to increased gypsy moth populations.

Horticultural Oils

These products have been available for decades and were originally used by orchardists for insect pests that over-wintered exposed on the trees, such as scale pests. The majority of these products are highly refined petroleum based oils. Originally, they could only be applied when the tree was dormant; typically in late winter. Consequently these products were known as "dormant oil sprays". Now, horticultural oils have become so highly refined that, depending on tank-mix concentrations, they can be applied to trees and shrubs during the dormant and growing seasons, with some limitations.

Oils work primarily by covering and suffocating the pest organism. In some cases, they can disrupt certain membranes of the exoskeleton. The target pest must be present and exposed at the time of application and the oil must cover the pest in order to be effective. Once oil sprays are dry (as little as 15-20 minutes after application) they have no insecticidal qualities whatsoever. Therefore, oils work best on pests such as scale insects, spider mites, certain adelgids, and others with limited mobility. One attractive feature of oils is that they can kill all life stages, including eggs. Many other pesticide products only kill the immatures and adults. Oils should not be applied near open water sources in order to prevent contamination.

Extra care should be taken when applying horticultural oils (in this case, a summer-weight oil) when new foliage is just emerging from the buds. Phytotoxic reactions may occur on this tender new foliage. Also, oils should not be applied if freezing temperatures are predicted 24-48 hours after application; this also increases the chance of plant injury. In more southern states where high summer humidity is sometimes a harsh reality, care should be taken to avoid oil use at the times of peak heat and humidity. This includes both the time of day and the time of season. Some applicators in these areas do not apply oils during much of July and August. Note also that some plants are sensitive to oil application. Among oil sensitive plants are: beech, black walnut, maples, hickory, smoketree, azaleas, Japanese holly, redbud, spruces and Douglas fir. Always read the label for information on sensitive species.

A specific example of the usefulness of horticultural oils is in the case of the hemlock woolly adelgid. HWA or Adelges tsugae is an introduced species along much of the eastern seaboard where Tsuga canadensis (Canada hemlock) has its native range. This pest is quite destructive and can kill host plants, especially if additional stresses such as drought or soil compaction are present. Entomologists working in the range of HWA receive countless inquiries about this pest and its management. Horticultural oils are one of the best management tools for this pest, where application is practical. Total coverage is extremely important in controlling the HWA; any missed individuals will quickly re-establish the population to damaging levels. Larger trees, therefore, become a challenge and multiple applications of oil may be required. It is difficult to discern when an oil application has been effective against this pest. Normally, the HWA is not very visibly active or mobile. Pest managers must carefully inspect the insect under magnification and take note of what live hemlock woolly adelgids look like: nymphs and adults will be somewhat plump and should produce liquid when pierced with a fine probe. Viable eggs will be turgid (plump with moisture) and shiny. Two to three weeks after an oil application, affected nymphs, adults and eggs should appear more dull, produce little or no liquid when probed, and eggs in particular may appear shrunken. These distictions can be very subtle and making them is a challenge.

Insecticidal Soap

These commercial products are literally what their name states: soap. Or, more scientifically, potassium salts of fatty acids. They work best on soft bodied insects (and some spider mites) but usually are not effective against the egg stage. Like oils, the target pest needs to be present and exposed at the time of application. Once the material has dried, they have no insecticidal qualities. Soaps kill arthropods by disrupting the membranes in the inter-segmental folds of the exoskeleton and cause the organism to lose a lethal amount of body fluid (blood). Common targets for soap sprays are aphids, spider mites, mealybugs, some adelgids (including the hemlock woolly adelgid), and relatively young caterpillars (including sawflies). Older caterpillars may be annoyed by a soap application but they will rarely be killed by it. Stout bodied insects such as beetles should be managed in some other fashion.

One factor to consider when using insecticidal soap is that of water hardness. This relates to the amount of minerals in the water. Most municipal water is "soft water" (low in mineral content). Water with high mineral content such as most well water, is conversely known as "hard water." Insecticidal soap will not mix well in hard water and therefore will not be very effective as an insecticide. If water hardness is not known, the very simple "jar test" can be performed. Place a drop or two of an insecticidal soap product into a quart of water in a clear glass container. Cover and shake the mixture, then observe. If the water is of low mineral content (soft water) then the mixture should be sudsy on the top and somewhat clear throughout. However, if few suds are produced and the mixture is milky throughout, then the water has a high mineral content (hard water) and another water source must be found for tank mixing.

Entomopathogenic Nematodes

Most people think of nematodes (if they think of them at all) as being plant parasites and causing symptoms similar to that of plant pathogens. However, some nematodes attack and kill insects. "Entomo" refers to insects (as in the word entomology) and "pathogenic", of course, means "to create disease symptoms within." These nematodes do not directly kill the insect. They enter the insect via natural openings such as the mouth, anus, or spiracles (openings for respiration), carrying a bacterium. Once inside the host insect, the bacterium becomes active. The nematode feeds on this bacterium, and the waste by-products of the bacterium become lethal to the insect, killing it by bacterial septicemia. Nematodes require an aqueous environment or they become inactive. Entomopathogenic nematodes work well on wood boring larvae that keep their tunnels open, thus creating a perfect environment for the nematodes (dark and moist). Such wood borers include the clear-winged borers (family: Sesiidae) like the peach tree borer and the dogwood borer. Many nematode products are labeled for soil inhabiting pests such as beetle grubs and black vine weevil larvae. However, obtaining and maintaining the correct amount of soil moisture for several days is difficult and the desired level of management may not always be achieved. It is important to keep soil moist enough to allow the nematodes to "swim" towards their intended target. These nematodes require a moisture film around the soil particles in order to stay active and to be mobile. But if the soil is too wet, they cannot achieve traction on the soil particles and they will float helplessly in the saturated soil. If it is too dry they will become dormant.

NEEM

NEEM (azadirachtin) is an extract of the Neem tree (Azadirachta indica) that grows in India, Africa and elsewhere. It has had many uses for centuries, one of those as an insecticide. It can work in several ways: as a sterilant, a deterrent, an anti-feedant, and as an insect growth regulator. However, it is relatively recent as a commercially available product and its effectiveness is not yet at the level of the other bio-rationals discussed above. It works best when insect populations are low to moderate in size. The product needs to be applied as soon as the pest appears and then re-applied, as often as every week, as long as the pest is active. This becomes time-consuming (at the commercial level) and not necessarily cost effective. It is expected that more effective Neem products will eventually appear.

In the early 1990's, a new pest was introduced into the Boston area from Europe: the lily leaf beetle (Lilioceris lilii). This bright red adult beetle and it's larvae are voracious feeders of all true lilies, Solomon's-seal and fritillaria, among others. It is a devastating pest. Neem has been effective against this pest when utilized as described above.

Written by: Robert Childs
Revised: 10/2011