Insect Pest Management

In this appendix, we will expand upon the general principles introduced in the appendix “Principles of Pest Management” as they pertain to managing insect pests of crops. Note that this appendix focuses Other Insect Control Methods
on nonchemical methods of insect management. We discussed chemical control generally throughout the manual and specifically for insects in the chapter “Insects and Their Relatives.”

Other Insect Control Methods

Insecticides are only one tool in your insect management plan. Nonchemical methods can have a significant impact on pest insects and can greatly reduce your need to use pesticides.

Insect-Resistant Crop Varieties

Insect-resistant varieties of crops affect the degree of damage caused by plant-feeding insects. No method of control is perfect and there are problems associated with developing and utilizing resistant varieties:

• Time and cost are great. It can take years to develop acceptable resistant varieties.
• It is difficult and time consuming to discover and incorporate resistance factors into acceptable varieties. For example, some highly insect-resistant varieties yield so poorly that they are unacceptable from a production standpoint.
• Biotypes of several insect pests have adapted to plant resistance factors so continual research is needed for new resistance to stay ahead.
• A variety that is resistant to one insect may not be resistant to others. Being that most of our crops are attacked by more than one insect, if you apply insecticides for other pests you get little immediate benefit from using the resistant variety.

Despite these disadvantages, resistant varieties can be powerful tools in managing insects:

• They have selectivity of action for the pest and so don’t harm other organisms in the environment.
• Once a resistant variety is developed, the cost of growing that variety is no greater than growing a susceptible one (unless genetically engineered). This results in a cost saving over the use of chemicals.
• With low-value crops, the cost of insecticide applications may not make growing the crop worthwhile. However, resistant varieties could be economical.

There are four mechanisms of plant resistance to insects: non-preference, antibiosis, tolerance, and transgenic. An interaction of all mechanisms is possible.

Non-preference

Non-preference plants have either physical or chemical traits that make them less suitable to insects for egg laying, feeding, or shelter. Some of the more common physical characteristics include color, the wavelength of light reflected from the plant canopy, the presence or absence of plant hairs, leaf shape, and the form and structure of fruiting bodies.

Antibiosis

Antibiosis involves the presence or absence of plant chemicals that directly affect insect development and survival by adversely affecting the insect’s physiology. For example, many corn hybrids contain chemicals that lead to poor survival of first-generation corn borers when the corn plants are young. Unfortunately, the concentration of these chemicals decreases as the corn plants grow. This is why we suggest you start checking for borer damage after corn is 18 inches tall.

Tolerance

Tolerance refers to a plant’s ability to outgrow, repair, or withstand insect damage. For example, some small grain varieties may replace the destroyed original stalks by tillering, and some dent corn hybrids may have a greater capacity to regenerate roots destroyed by corn rootworms. Insect pests can overwhelm tolerant varieties if populations are great enough or if they are present for long periods of time.

Transgenic Crops

Genetically engineered crops have resistance to key insect pests. For example, transgenic corn hybrids are resistant to European corn borer and/or corn rootworm. In each case, a gene from the bacterium Bacillus thuringiensis (Bt) was incorporated into the plant.

The gene allows the plants to produce a toxin that causes the insect to stop feeding. Although the insect does not die right away, the toxin eventually causes the gut to rupture, killing the insect. The Bt gene is quite specific to each target pest. That is, the Bt gene for European corn borer will not control the corn rootworm and vice versa.

Bt has been commercially available as a microbial foliar insecticide for moth and butterfly larvae, such as European corn borer, for decades. Bt spray formulations are applied to leaves and other areas where the insect larvae feed. Bt sprays have a relatively short residual in the field, thus a well-timed single application or, more typically, multiple applications based on pest insect scouting are necessary to maintain control.

Unlike Bt microbial spray formulations, which may only last for days in the field, the Bt toxin in transgenic Bt corn is active for the life of the plant. This can lead to more consistent and economic insect control when target insect populations reach economic threshold levels. However, there can be a downside to the use of transgenic crops. Because the transgenic plant is, in effect, exposing the target insect to pesticide active ingredient whenever that insect feeds on the plant, you can expect those pests to eventually develop resistance to the toxin in the plants (in the same way that insects develop resistance to foliar applied pesticides as we discussed in the chapter, “Pesticide Resistance”). Several examples already exist where insects have become resistant to Bt foliar insecticides and transgenic hybrids due to repeated use. To preserve the long-term effectiveness of transgenic Bt, growers must use these crops in a way that will delay the onset of resistance.

A key component of resistance management for transgenic crops is to preserve a portion of the pest population that has never been exposed to Bt and, thus, remains susceptible to it. To achieve this, the EPA requires growers to plant “refuge” areas within or close to the transgenic plants to a hybrid that does not contain the gene from Bt (Figure 1). Also, do not use Bt sprays in the refuge areas. Though resistance to the Bt toxin may develop within the insect population that feeds on the transgenic plants, such resistance will not be selected in those areas where insects do not encounter the toxin. Bt-resistant insects that mate with Bt-susceptible insects will produce offspring having a lower level of resistance than the resistant parent. If you do not plant refuge areas, if/when resistant insects develop, they will mate only with other resistant insects, and the transgenic crop will eventually become ineffective.

As with resistance management strategies for conventional insecticides, a refuge strategy will be most effective when used in conjunction with other control tools such as cultural or biological control. A well-designed IPM program incorporating a variety of management strategies, which includes a monitoring component to detect resistance, is therefore, an important prerequisite to avoiding resistance and maintaining the long-term effectiveness of transgenic crops.

Biological Control

Biological agents can have a profound effect on insect populations, but cannot always maintain the desired degree of suppression of many pests. Annual crops are poor areas for survival of natural control agents because the continuous interruption of the crop cycle by farming practices does not provide a stable habitat. Even perennial field crops are frequently disrupted by harvest or grazing. Nevertheless, many insects could regularly cause economic damage if natural organisms were not functioning.

It is important to understand the differences between the terms “natural” and “biological” control. Natural control involves the combined action of biotic agents (e.g., predators, parasitoids, pathogens) and abiotic factors (e.g., rainfall, wind, humidity, heat, cold). The term biological control as it applies to insects refers to the use of biotic agents.

Predators and Parasites
Predators capture and eat other creatures (called prey). Lady beetles and aphid lions (lacewing larvae) are examples of insect predators. Parasites live in or on the bodies of another living organism (the host); they are usually much smaller than their host and a single individual usually doesn’t kill the host. Parasitoids are often placed in the overall parasite category but are actually a special kind of parasite. They are usually about the same size as the host, which they kill, and require only a single host for development into an adult. The tiny wasps that attack alfalfa weevils in Wisconsin are excellent examples of parasitoids. During some years, over 90% of the alfalfa weevils collected have been parasitized by these wasps.

Introducing Natural Enemies
Introducing natural enemies works better against pests that have been introduced into this country than against native pests. Native pests have already evolved stable relationships with their natural enemies so releasing more natural enemies does not dramatically affect the pest population. Introduced pests, on the other hand, were brought to this country accidentally (e.g., in grain brought to colonial America by settlers) and without their natural enemies. Thus, they have been able to reproduce unhindered. By introducing natural enemies from the pest’s native area, we have been able to bring several pests under control.

Conserving Beneficial Insects
Applying insecticides only when absolutely necessary is one of the most important ways to conserve and enhance pest insects’ natural enemies present in your field. Most modern insecticides are broad spectrum and kill pest and beneficial insects. You can also try to maintain suitable habitat around field edges to attract beneficial insects.

Pathogenic Organisms
Many viruses, bacteria, fungi, protozoa, and nematodes kill pest insects. These pathogens are selective against specific insects or groups of closely related insects. We have been able to mass produce some of these agents (e.g., some fungi that infect insects) and use them as we would chemical insecticides. Their high degree of selectivity is an advantage over broad-spectrum chemical insecticides that kill both good and bad insects. Unfortunately, you only get effective control when conditions favor pathogen reproduction; since you cannot manipulate these conditions, control is sporadic.

Fertility Control
Many insects, particularly butterflies and moths, use pheromones to locate mates. These have been used to disrupt mating (e.g., coddling moth on apples) but are used most frequently to monitor populations as an aid in timing other control measures.

Cultural Control

Some insect pests can be controlled by cultural practices. These cultural practices are directed at “weak points” in the insect’s life cycle and are generally something the farmer does anyway.

Crop Rotation
For crop rotation to work, the alternate crop must be an unacceptable food source for the insect pest. The insect also must have limited dispersal activity and a long life cycle. There is no advantage to crop rotation if a pest can easily move from crop to crop or can complete an entire life cycle before you can rotate crops.

Date of Planting
In Wisconsin, we worry about corn earworm damage only in very early- or very late-planted sweet corn during most years. Sweet corn planted between these two extremes usually escapes significant damage. Cole crops planted either before or after peak emergence of overwintering cabbage maggots, which occurs in early May, will avoid severe damage from this pest.

Weed Control
Some insect problems are directly related to poor weed control. Armyworm moths deposit their eggs on grasses. If weeds such as quackgrass, foxtail, or nutsedge are plentiful in your corn field, you could get damage from those armyworms hatching from the weedy grasses. Because armyworms do not typically lay eggs on corn leaves, we would not expect to see this situation in a field relatively free of attractive weeds. Stalk borers, hop vine borers, and potato stem borers also lay eggs on weeds in cornfields or in field borders. These pests, however, lay their eggs during late summer and the larvae do not hatch until the following spring. Control grass weeds in the field and watch for damage in rows along the field edge that may result from larvae moving into the field.

Time of Harvest
You can time a crop harvest to disrupt a developing pest population and eliminate the need for an insecticide. The best example of this in Wisconsin applies to the alfalfa weevil. Economic populations usually are not reached before alfalfa is mature enough to harvest. If the weevils are reaching damaging levels, immediate harvest can be as effective as insecticides because the exposed larvae often desiccate or starve.

Tillage
On its own, tillage cannot adequately control insects, but it can help reduce some pest populations by exposing them to desiccation (drying out), predation, or freezing and thawing. Tillage also turns under plant litter that serves as a habitat for many insects.

The probability of insect problems increases as you go from conventional to conservation tillage. Therefore, consider the following aspects when you plan a conservation tillage program:

• Simultaneous attack by more than one species: In some years, farmers that no-till plant corn in grass sod have armyworm, black cutworm, and stalk borer infestations all at the same time. You can control these insects by timely insecticide application.
• More stable environment for insects: Reduced disturbance o f t he topsoil and increased surface debris may contribute to insect problems. Surface debris can serve as a food source, be attractive to egg-laying cutworm moths, maintain a moist habitat and reduce the chance for desiccation of soft-bodied insects, provide hiding places, and lessen harmful temperature fluctuations. Although slugs are not insects, they are a potential pest of no-till crop production. The thick mulch present when corn is planted in sod has led to problems with slugs. The decaying mulch also can be attractive to egg-laying seed maggot flies.
• Some problems are more severe: Compared to other forms of conservation tillage, no-till operations have more frequent insect problems, and can experience severe insect damage and simultaneous attack by several species.
• Increased scouting needs: Because the potential for pest damage is greater, you need to carefully watch for developing problems. You may not need to take action, but scouting is still good insurance that pays regardless of your tillage program.
• Increased insecticide use? The increased potential for insect problems calls for increased insecticide use in some cases.
• Weed relationship: Weed scientists are concerned that some weeds will be more difficult to control in reduced-tillage programs. Escape weeds can result in increased problems with armyworm, stalk borer, and hop vine borer.

Insecticides and IPM

A full awareness of potential drawbacks before selecting and using insecticides allows us to use them effectively in integrated pest management programs.

• Use field scouting and a treat-when-necessary approach as opposed to routine preventative treatments.
• Remember that eradication or 100% control is not necessary to prevent economic damage. Also, a residual population of pest insects is needed to maintain a population of beneficial insects. Without this residual population, beneficial insects would either starve or move out of the field, setting the stage for future insect problems.
• Carefully time insecticide applications to attack insects at the weakest point of their life cycle. Use field scouting to time these applications.
• Use preventative treatments sparingly and use the most selective compounds available.
• Be selective in the use of insecticides to control insect populations while minimizing effects on nontarget organisms.