INSECT MANAGEMENT
Commercially Available Parasites and Predators
Biological control is not easy and is never 100 percent effective. Neither natural nor introduced beneficial insects will reduce a pest population to zero. When pest populations fall below a certain level, beneficials move on in search of new food sources. Thus, when little or no insect damage to the crop can be tolerated, reliance on natural enemies is not appropriate. An example of zero tolerance is in commercial pepper production for processing where loads can be rejected if even a few fruit are infected with pepper worms.
See also the example of beneficial nematode control on pickleworms in cucumbers.
Proper timing of release relative to pest populations is critical even when limited insect damage is acceptable. Beneficial insects need time to increase their populations sufficiently to reduce pest populations. They must be released before severe damage occurs, but releases when no pests are present or populations are very low are rarely effective either because the parasites need pest hosts to survive. If pest populations are low, predators may simply fly away from the release site in search of better hunting grounds.
Release of beneficials can take two forms. The first is inoculative releases of low numbers of natural enemies in the hope that they will become established and provide future control. The second, mass releases designed for immediate control, may or may not result in permanent establishment of the beneficials, even if the insects released actually control the pest as planned. Mass releases are costly, and permanent establishment is far from certain so they are most likely to be a cost-effective control for pests with only one or two generations per year.
Releasing under favorable conditions and providing a favorable habitat for released insects will aid in their success. Releasing predators when temperatures are moderate (not mid-afternoon on a hot sunny day or dawn on a frosty morning), spraying a few plants around the release site with sugar water, and placing beneficials in a shaded area, away from the wind, can keep them from immediately dispersing. Night releases may also slow dispersal.
It is important to restrict insecticide use both before and after introducing beneficials. If it is necessary to reduce pest populations before introducing beneficials, only "soft" chemicals which are target specific and have low residual activity should be used. Most conventional pesticides have some residual toxicity and biologicals are often less resistant than pests.
Lady beetles. Lady beetles are one of the most widely ranging and well known predatory insects. Although both adults and larvae of the lady beetle feed on soft-bodied insects, insect eggs and mites, beetles must reproduce in a field to be effective. If prey is not abundant, introduced adults will move to another area to lay eggs, often before significantly reducing aphid populations. They may fly away even if prey is available. Lady beetles collected while hibernating must fly to burn off stored fat before they will feed, while those collected during the feeding phase in the spring remain on plants at the release site and eat. Suppliers should provide information on the phase when lady beetles were collected. Releasing lady beetles in the evening will encourage them to stay in the field at least temporarily since they don't fly at night. Spraying plants in the release site with water or a sugary solution will help slow their immediate dispersal.
Green lacewings. Green lacewings are found naturally over wide areas of the United States. They are usually purchased as eggs (which should not be stored in the refrigerator). Recommended release rates are 50,000 to 200,000 eggs per acre. Larvae should be placed on plants as soon as they begin to hatch since they will eat each other if no other food is available. Larvae feed mainly but not exclusively on aphids. Lacewings have controlled caterpillar and aphid pests in cotton, corn, and apples.
Lacewings have also been used for leafhopper control. One dispersal method in vineyards combined lacewing eggs with corn grit in containers mounted above the vines, and spread by a tractor driving through the vineyard. The eggs tended to sift to the bottom of the container, giving a greater egg density at the beginning of the rows. Approximately one third of the eggs were crushed in the process or died from other causes. About 20 percent of the eggs hatched within 4 days. Therefore, the application rate of lacewing eggs needs to take such losses into account to achieve a 25 to 50 percent egg hatch at the release date. Lacewing releases should be timed to coincide with the hatch of each leafhopper brood.
Wasps. Trichogramma wasps lay eggs (oviposit) in the eggs of moths and butterflies. After hatching, the developing wasp larvae destroy the host egg. Different species of Trichogramma prefer different hosts. Trichogramma pretiosum is the species sold for control of caterpillar pests of vegetables.
Recommended rates per release are 50,000-300,000 wasps per acre. They are usually released as larvae inside host eggs. Releases should be timed to correspond with the start of egg laying by the pest. Survival of wasps will be increased if they are allowed to emerge and feed for 24 hours on a water-honey solution before field release.
Although not yet available commercially, other parasitic wasps, Cotesia plutellae, C. marginiventris, and Diadegma insulare attack fall armyworm and corn earworms, armyworms, and the diamondback caterpillar, respectively. Beneficial nematodes. Steinernema feltiae and other species of parasitic nematodes kill insects by injecting them with bacteria. They are promising control agents for insects living in moist, confined locations, such as the soil. Nematodes are typically applied to the soil as drenches or sprays but they can also be added through the drip irrigation system. Their use against above-ground insects is limited because they do not survive in dry environments, such as on plant leaves.
In the lab, these nematodes have killed beetles, flies, bugs, aphids, whiteflies, caterpillars, lacewings, and grasshoppers. In the field, however, for reasons not completely understood at present, their range of controlled insects is currently much more limited. They have been shown to kill only a few insects, including carpenterworms (a borer), black vine borer (a root weevil) and Japanese beetle larvae. In sweetpotato under field conditions in South Carolina, significant control was achieved in the first year of a test but not the second. Poor results the second year of the test were attributed to saturated soils providing an adverse environment for both nematodes and soil insects. Under laboratory conditions, S. carpocapsae killed 99.3 of the soil insects present. S. carpocapsae was also compared to the insecticide esfenvalerate (Asana®) in controlling pickleworm of cucumber in South Carolina. Nematodes were sprayed weekly at rates of 1 billion per acre in a water volume of 80 gallons. No fruit damage occurred in insecticide treated fields. The nematode treated field had 2.2 percent infested cucumber fruits and untreated control fields had 11.6 percent infested fruit.
Time of addition of nematodes appears to be critical. Recent reports suggest that only certain stages of larvae may be susceptible to parasitic nematodes, possibly accounting for variable results in field tests. They found that larvae of the western corn rootworm were most susceptible to S. carpocapsae when the second and third instar rootworm larvae predominate in the field. Addition at this point allowed sufficient nematode reproduction to control corn rootworm populations. Egg and pupal stages were not susceptible, so if nematodes are added too early, they will not become established.
Another study illustrating the importance of application time and method tested S. riobravis to control corn earworm in field corn in Texas. Best results were seen when nematodes were applied subsurface as well as to the surface (81 percent versus 45 percent parasitism). Almost complete parasitism (95 to 100 percent) was found when nematodes were added when 10 percent of the corn earworms had left the ear to pupate or when 50 percent of the earworms had developed to large larvae. Only 11 percent parasitism was seen in control plots. Adding the nematodes earlier reduced the amount of parasitism, unfortunately. Thus in sweet corn damage would not be prevented in the current crop, but population buildup in the soil would be avoided which would help protect later developed corn in adjacent fields as well as reduce movement from corn to other susceptible crops.
Commercial suppliers of beneficial nematodes suggest their use on white grubs, cutworms, armyworms, fungus gnats, carrot weevils, borers, cabbage maggots, and wireworms. To obtain best results with commercially supplied beneficial nematodes, the supplier's instructions must be followed carefully.
Typically, nematodes can be stored in the refrigerator for a short time after arrival because they are in a dormant state. Before applying the nematodes, this dormancy must be broken by stirring them in room temperature (over 65 degrees F) water to provide oxygen. After dormancy is broken, they must be used immediately. They prefer a moist soil and are damaged by light and so should be applied in the evening. Beneficial nematodes move faster in sandy soil than clay.
Other important general considerations in purchasing beneficials are finding a good supplier and dispensing the insects in the field. Currently there are no certification or quality assurance standards for insect-rearing and few protections for the customer if insects fail to establish. Thus when a release for biological control fails, it is hard to know where to place the blame. Insects supplied may have been the wrong type, they may have died in transit, or while the grower was holding them before release or they may have been poorly placed in the field so that there are too many in some areas and none in others. Even distribution in the field is often difficult. The number of live insects supplied is not easy to determine and may be more or less than the number ordered.
In dispersing, insects are often mixed with a solid media, such as corn grit, and may end up in higher concentrations the bottom of the container, as in the lacewing egg example, or can even crawl to the sides or top of the dispensing container in reaction to light, heat or other stimulus. Thus shaking the container gently will help distribute the insect more evenly. The best general guideline for a grower utilizing biological controls is to learn as much as possible about the lifestyles of the beneficial and pest and the requirements of the biological. Locating a good local insectary is ideal because the insects spend little time in transit and the insect supplier may be willing to visit the fields to provide follow-up advice. Where no local supplier is available, the best option is to choose a supplier with a good reputation who is willing to supply educational materials and to provide follow-up advice and possibly replace insects where growers are unable to establish purchased populations.
Potential future predators and parasites. Many other insects have the potential to suppress pests. On potatoes, predaceous stinkbugs (Perillus bioculatus) have been shown to reduce populations of Colorado potato beetle. Predatory mites that attack spider mites are routinely used in European greenhouses. They are potentially also useful in the field, possibly in conjunction with a pheromone product 'Stirrup-M' which stimulates spider mite movement so that they can be caught more easily by predatory mites.
bridgesj@unity.ncsu.edu