Managing Insects in Organic Crops
Managing insects in the farm ecosystem is challenging. At a glance, it might seem that one insect is not significantly different from another, and that all are pests. But, because insect populations are interrelated, intervening to control them may prove to be more disruptive than beneficial.
Specific insect types must be considered when control strategies are devised. No single control method will be adequate for all. Successful management depends on incorporating a number of control strategies.
The ideal is to establish equilibrium between beneficial insects and pests and to use cropping practices that reduce insect populations. To reach that goal, consider the following management factors.
Planting fields with different crops effectively controls plant diseases and weed problems. It can also be used to reduce insect populations for: 1) species that feed on a single crop or related crop; 2) species where the overwintering stage is associated with that crop; and 3) species that have limited mobility.
Crop rotations reduce insect problems by denying easy access to the crop and increasing the abundance and effectiveness of insect predators and parasites. The diversified habitat provides these parasites and predators with alternative food sources, shelter and breeding sites.
Crop diversity is essential to limiting insect populations. A balanced cropping system, using a combination of cereal, oilseed, pulse, and biennial or perennial legume crops, helps limit insect populations. A diverse rotation will contain a larger number of insect species adapted to the various crops within the system. This in turn creates a proportionally higher number of natural enemies to any insect pest. Each crop type should be separated as far as possible within the rotation sequence.
Repeated cropping of large areas to single or closely related crops year after year encourages insects that feed on those crops to multiply rapidly and become pests. Single crop farming should be avoided.
Cabbage aphids, flea beetles, diamondback moth, cabbage butterfly, wheat midge, red turnip beetle, wheat stem maggot and wheat stem sawfly are a few of the many insect pests that can be regulated with diverse crop rotations.
Removing or destroying crop residue and alternate host sites can help control some insects. It is most effective against insects that overwinter in crop residue or lay eggs in it, and where crop residue protects insects from exposure to extremely cold temperatures.
Winter survival of wheat stem sawfly, Hessian fly, bertha armyworm, alfalfa plant bug and some cutworms increases when crop residues are left undisturbed.
Where wheat stem sawfly is a problem, shallow fall tillage is recommended. Tillage will also reduce bertha armyworm survival but may not be practical as a control strategy given the adult moth's flight range. Where tillage is required, attention should focus on maintaining a balance between crop sanitation and soil erosion protection. This will require leaving some crop residue on the soil surface.
In alfalfa seed fields, where cultivation is not practical, the alfalfa plant bug can be controlled by burning crop residues in late fall or early spring. Burning destroys the eggs laid in the alfalfa stems the previous summer and fall. In alfalfa produced for hay, burning is not required. Normal cultural practices, such as haying for forage, will remove the eggs and prevent a population build up.
Also attend to field margins and fence lines where weeds or volunteer crop growth occurs. The close proximity of volunteer plants could provide alternate habitats for insects to complete their life cycle until an appropriate crop occurs in the nearby field.
The size of a field can be important in determining the intensity of an insect problem. In general, it is a good idea to keep field sizes small.
Weeds should always be controlled to reduce their impact on crop yield and quality. Cultural control of insects may require a slight deviation from this concept. Sometimes a low, in-crop weed population may even be beneficial. In the case of aphids and leaf hoppers, their numbers on certain crops decrease as the diversity of host plants (weeds) increases. There are specific situations where this would not be true, such as for insects that are host specific.
The increase in plant diversity that results from allowing a certain amount of weed growth has been shown to have a positive effect on the diversity of insect species. The diversity may increase the number of predators attacking a pest, as has been observed in alfalfa stands. For example, an increase in plant diversity resulting from allowing a low population of weeds to grow increases the activity of parasitic wasps, which would help to control an outbreak of alfalfa caterpillar (Colias eurytheme).
Planting should be timed to provide the crop with a competitive advantage over insects. Early seeding reduces crop damage caused by grasshoppers. It may also markedly reduce aphid damage on cereal crops. The crop grows and becomes vigorous and passes the susceptible stage before aphid populations reach damaging levels. Similarly, seeding wheat early may reduce wheat midge damage.
In the case of wireworm in cereal crops, avoid very early or late seeding. Problems with sweetclover weevil can be reduced by sowing clover early, before grain crops. Hessian fly problems in winter wheat can be avoided by seeding after mid-September, thus preventing egg laying in newly seeded crops.
Delayed seeding can also be an effective way of avoiding problems. Waiting until the insect has finished its life cycle before seeding or until the spring insect migration is complete can reduce crop loss.
Crop damage from army cutworm can usually be controlled by delayed planting. This allows the overwintering cutworms to finish their development before the crop emerges. However, a cool spring may slow larval development, extending the delayed seeding time interval beyond a practical limit.
Similarly, barley thrips can be controlled with delayed planting. In the absence of barley, the migrating thrips colonize the non-crop host plants instead of the barley.
Increasing plant densities using higher seeding rates can have a positive effect on insect pest control. By having more plants in the field, a given aphid population will have less of an impact upon individual plants.
In the case of aphids, leafhoppers and flea beetles in canola, the amount of bare ground present in a plant community appears to influence abundance. These insects are attracted to the contrast between a green host and a dark soil background. Increasing seeding rates obscures this contrast. Depending on environmental conditions, increasing the plant density could change plant geometry (less tillers and leaf area) and/or the "lushness" of the plant. Either of these factors could alter the attractiveness of the crop to insect pests.
Higher plant densities may result in certain insects being more abundant. Increased problems with the true armyworm have been reported in densely seeded stands of fall rye.
Summerfallow and Stubble Management
Depending on the geographic area, summerfallow can be an important aspect of a cropping rotation. It may be used for effective insect, weed or disease control. The management of summerfallow can have a large influence on certain insect populations.
It is important to keep volunteer crops and weed growth to a minimum, since these plants serve as a temporary host, defeating the purpose of summerfallow, which is intended to break the life cycle of many insects.
Destruction of green growth in summerfallow fields should begin early in the year before insects like grasshoppers begin to hatch. This will destroy their food and starve newly hatched grasshoppers. Early June tillage of stubble infested with wheat stem sawfly helps bury their larvae, increasing mortality rate. Shallow tillage in the fall will also increase mortality rates of this pest.
Severe infestations of the red turnip beetle in canola fields can be prevented by destroying their eggs with late fall or early spring tillage. Spring tillage to remove cruciferous weeds and volunteer canola will control the larvae, by eliminating food sources. Another sporadic pest of canola, the bertha armyworm, may be reduced in number by fall and spring cultivation. Pupae of this insect are thus exposed to freezing, diseases and predators.
Controlling winter annual and volunteer crops in the fall is critical. Several kinds of cutworms hatch in fall and overwinter as partly grown larvae. If available, these larvae will feed on weeds and volunteer crops until freeze-up and again in the spring. One species of the army cutworm sometimes causes damage to winter wheat and fall rye.
In years when it is abundant, this cutworm may also seriously damage spring-seeded crops in fields where sufficient green growth in the fall provided food for the young larvae. Redbacked and pale western cutworm moths also lay their eggs in the fall until mid-September, but eggs do not hatch until the following spring. Moths of the redbacked cutworm, which is primarily a pest in the parkland areas, usually concentrate their eggs in patches of weeds in summerfallow.
For the pale western cutworm, sequencing agronomic practices is important. It prefers to lay eggs in loose soil. Where this insect is a problem, summerfallow should be cultivated before the middle of August and left to crust over or cultivated after the middle of September. In the spring (May), a delay of five or more days between cultivation and seeding can prevent infestations. The larvae die if they feed after they hatch and then are deprived of food for several days or cannot feed at all for 10 to 14 days.
Summerfallow can be useful in cereal-only cropping systems to reduce the likelihood of insect damage, particularly to the first subsequent crop. This interrupts the build up of resident insect populations. Similarly, allowing a 10-day break between the harvest of a spring wheat crop and the emergence of a winter wheat crop is critical for the control of wheat curl mite. A time period less than this will allow the insect to transfer from one crop to the other, continue its life cycle, and infect the winter wheat with wheat streak mosaic.
The function of seeding barrier strips of crop is to lure insect pests into a specific area. The pest problem can then be concentrated, making control easier. This approach can work for grasshoppers where damage is usually initiated from the edge of a field inwards.
Wheat stem sawfly may be lured into a temporary trap crop of a susceptible variety placed around the crop. This area is then swathed for hay or silage in July to remove some of the larvae population. A permanent trap crop of smooth brome grass around a field will reduce the number of larvae surviving in ditches and headlands.
Adequate, balanced soil nutrition is essential for crop quality, yield and moisture use efficiency. An imbalance in fertility can affect plant quality in several ways. Excess nitrogen, in relation to other nutrients, can result in plants becoming more succulent and thus more attractive to insects. Nitrogen is also responsible for stimulating vegetative growth, producing more leaf area. This would allow an overall higher pest infestation per field. At the same time, the enhanced growth rate resulting from this nutrient may offset potential insect damage. This depends on the particular growing conditions at the time of the pest problem.
A nutrient imbalance can readily occur where large applications of manure have been applied. Soil tests should be conducted regularly to monitor nutrient levels and allow corrections to be made.
Beyond a pre-established level necessary to maximize crop efficiency and yields, other factors become important.
Selection and use of resistant cultivars is a cost-effective form of insect control. For example wheat stem sawfly can be effectively controlled in the southern prairies by planting one of the "solid stem" wheat varieties. These cultivars are resistant to damage and should be used where crop yield is reduced by more than 10 percent. Other insects, such as the pea aphid, can be a pest on field pea, alfalfa and clover. Several pea varieties are not severely damaged by this aphid. In addition, certain varieties of alfalfa developed in the U.S. are resistant to the pea aphid.
Not all plants are equally attractive to insects. Factors such as leaf and stem toughness, pubescence (leaf hairs), nutrient content, water content and secondary compounds or toxins all influence an insect's growth or reproduction. Grasshopper development and reproduction is significantly better on wheat, followed by barley, which in turn are both better than oat. Similarly, grasshoppers usually won’t feed on certain varieties of field pea (CV. Sirius). This insect will selectively feed on lentil pods, while causing minimal damage to stems. Many of these properties are used by plant breeders to develop resistant varieties.
Insect resistance can also occur simply as a function of plant architecture or geometry. Certain cereal crop varieties with pubescent leaves are less desirable to cereal leaf beetles. Within a species, differences in tillering, leaf shape and size and plant height all alter the environment available to the pest. Similarly, differences in maturity between varieties can be used to advantage, depending on the specific pest problem. Some factors may simply reduce the insect's developmental rate, reducing the problem to an acceptable level.
In any balanced ecosystem, biological control by natural predators is constantly occurring. The more diverse a cropping system becomes, the greater the spectrum of insect species within it. This in turn leads to the development of more natural predators.
Biological control of wheat midge occurs from a parasitic wasp. This wasp can destroy about 40 percent of the overwintering population of wheat midge.
Ladybird beetles, ambush bugs, assassin bugs, lacewings and a host of other insects are predators of aphids, bertha armyworm larvae, sunflower beetles, etc. The pea aphid, which is a main pest of field pea, alfalfa and clovers, is reduced or controlled by damselflies, minute pirate bugs, ladybird beetles, lacewings, and hover fly larvae. In cereal crops, ladybird beetles can have a noticeable impact on aphid populations, usually keeping them in check.
Insects such as beet webworm are found in a wide range of weeds and crops, including canola, mustard, flax, sweetclover, alfalfa and sunflowers. They are attacked by a number of parasitic insects. Some of these parasites destroy the overwintering pupae. This is particularly true in lighter textured sandy soils.
The above examples of biological control occur naturally in a healthy, balanced system. We often think of biological control in terms of adding or "inoculating" an existing system with a foreign predator. The hope would be to gain pest control similar to the above examples. Microbial insecticides such as Bacillus thuringiensis (BT) would fall into this category. This bacterial pathogen is effective against a wide range of caterpillars (Lepidoptera). One great advantage of microbial pesticides is the specificity of their toxic action and their safety to non-target organisms. These products cause minimal environmental disruption.
Conservation of natural enemies refers to the practices designed to enhance the colonization and/or survival of natural enemies within the crop. Possibilities include providing pollen sources, nectary plants, shelter, or nesting sites. This approach is particularly well suited to perennial crops, where a long-term equilibrium between host and natural enemy can be established. This approach to pest control is relatively new and requires further research to be fully evaluated.
Synthetic insect hormones have been developed to duplicate naturally occurring insect (pest) chemicals. In nature, these products regulate insect body functions. Their mode of action is other than direct toxicity. When applied these products disrupt insect development, such as growth, rather than causing immediate death.
Depending on the type and amount, certain types of hormones (pheromones) may be used as attractants to monitor population levels or simply attract insects into a trap. These products can be categorized as having low mammalian toxicity, low use volume, target species specificity and natural occurrence. These products have potential, but require further development.