Pest Management in Organic Systems: Insects
Organic Crop Production
Pest Management in Organic Systems
The term “organic” describes production systems that optimize natural processes. Certified organic growers use a wide range of IPM practices that comply with the standards of the USDA’s National Organic Program (NOP) (www.ams.usda.gov/nop, phone: 202-720-3252).
In organic systems, the goal is to alter the production system so that pests do not find plants, are controlled by natural enemies (biological control), or their damage is kept to a minimum (Table 1.11-1). Vigorous, healthy plants are more able to withstand damage caused by arthropods and disease. Therefore, healthy soil and plants are the foundation of organic production. Described here are general principles of insect pest management in organic production. More information on biologically based (biointensive) pest management, including information on suppliers of biological control agents, can be found in the ATTRA publication Biointensive Integrated Pest Management available at attra.ncat.org or by calling 1-800-346-9140.
Biological control is the use of living organisms to maintain pest populations below damaging levels. Natural enemies of arthropods fall into three major categories: predators, parasitoids, and pathogens. Predators catch and eat their prey. Some common predatory arthropods include ladybird beetles, lacewings, syrphid flies, carabid (ground) beetles, minute pirate bugs, nabid bugs, big-eyed bugs, and spiders.
Parasitoids (sometimes called parasites) do not usually eat their hosts directly. Adult parasitoids lay their eggs in, on, or near their host insect. When the eggs hatch, the immature parasitoids use the host as food. Many parasitoids are very small wasps and are not easily noticed. Tachinid flies are another group of parasitoids. They look like large houseflies and deposit their white, oval eggs on the backs of caterpillars and other pests. The eggs hatch, enter the host, and kill it.
Pathogens are disease-causing organisms. Just as many other organisms get sick, so do insects. The main groups of insect disease-causing organisms are insect-parasitic bacteria, fungi, protozoa, viruses, and nematodes. Biological control using pathogens is often called microbial control. One very well-known microbial control agent that is available commercially is the bacterium Bacillus thuringiensis (Bt). Because not all formulations of Bt are approved for use in organic systems, it is important to check with your certifier. Several insect-pathogenic fungi are used as microbial control agents, including Beauveria, Metarhizium, and Paecilomyces. These are most often used against foliar insect pests in greenhouses or other locations where humidity is relatively high. Nuclear polyhedrosis (NPV) and granulosis (GV) viruses are available to control some lepidopteran pests (moths and butterflies). Insect-parasitic (entomopathogenic or insecticidal) nematodes in the genera Steinernema and Heterorhabditis infect soil-dwelling insects and occur naturally or can be purchased. As with all biological control agents, it is especially important to match the correct microbial control agent with the correct pest in order for them to be effective.
Biological control can be applied (inundation or augmentation of natural populations) or natural (conservation of natural enemies). Many biological and microbial control agents are commercially available for purchase. Information about rates and timing of release are available from suppliers of beneficial organisms. The quality of commercially available biocontrol agents is an important consideration. Biological and microbial control agents are living organisms, and, as such, must not be mishandled during shipping, storage, or application.
In many cases, purchasing biocontrol agents should not be necessary. Natural enemies are common and a grower just needs to know how to attract and keep the natural enemies in their system by providing environmental conditions conducive to their survival. “Farmscaping” describes the creation of habitat to enhance the chances for survival and reproduction of beneficial organisms. For example, many adult predators and parasitoids feed on nectar and pollen, so it is essential to have these resources nearby. Having several species of pollen- and nectar-producing plants in an area will provide resources more continuously than only having one species. Organic mulches and crop residue moderate fluctuations of temperature and moisture and can provide hiding places for soil predators such as carabid and staphylinid beetles, spiders, and centipedes. Other habitats provided by farmscaping include water, alternative prey, perching sites, overwintering sites, and wind protection. The success of farmscaping efforts depends on knowledge of pests and beneficial organisms.
In field crop systems, there are several well functioning biological control systems. A major successful biological control program was the release of parasitoid wasps to control alfalfa weevil populations. There are six primary wasp species helping to regulate weevil populations, with three being most common. These parasitoid wasps, along with a fungal pathogen, have successfully reduced the percentage of fields sprayed in Pennsylvania from nearly 100 percent in the 1960s and early 1970s to less than 5 percent in most years. Organic growers have the option of managing their forages to encourage maximum effectiveness of the wasps, thus eliminating the need for pesticides. Another example of successful introduction of a beneficial organism is in the population regulation of cereal leaf beetles in cereal crops (wheat, barley, oats, rye, triticale, etc.). These wasps have again significantly reduced the number of fields sprayed to control the pest. Again, organic producers can manage to optimize the effectiveness of these parasitoids. A third example of effective biological control is that of the European corn borer. Although, this insect is still a major concern in vegetable and field corn production and significant acreage is planted to new transgenic Bt corn hybrids (not allowed in organic production), its populations receive considerable regulation from beneficial organisms. During the 1930s, 1940s, and 1950s, the USDA imported and release at least 25 beneficial parasitoids in an attempt to regulate the species populations. Smaller re-releases occurred during the 1960s through 1990s. Several of these parasitoid species have become established and contribute to the regulation of corn borer populations. In the early years after introduction of the European corn borer into the United States, open-pollinated varieties were commonly infested with ten or more larvae per plant, and yields were completely eliminated. Today, with beneficial organisms and improved crop resistance (traditional, not transgenic methods), it is uncommon to see more than an average of three or four larvae per plant. In fact, the large majority of fields have less than one larva per plant. Research has shown that infestations below one larva per plant seldom cause measurable yield reductions. All crop producers should recognize, whether organic or other, that only 1 to 5 percent of the population of the majority of insects survive from the egg to the damaging stage. This tells us that nature, either through effects of weather conditions or natural enemies, is doing a lot to keep pest numbers down. In field crops, many times, we can tolerate infestations of a pest with out significant economic loss.
Given the great help we receive from nature, we are fortunate in field crops to have very few major insect pests that consistently cause economic losses. Exceptions are potato leafhopper in alfalfa and corn rootworm in corn. All other pests tend to be cyclic and only hit a few fields each year. Knowing about the life requirements of these pests can be a great help in designing effective organic production systems.
Many organic cultural practices are also carried out as best management practices that are applicable to all types of crop management systems and are discussed elsewhere in this guide. Some examples of cultural controls are crop rotation, sanitation, cover crops, resistant varieties, maintenance of biological diversity, appropriate planting dates, and plant spacing. As a general rule in field corn, early plantings have fewer losses to insect pests. Timing of cutting in alfalfa can be an effective management tool against potato leafhopper. Because the life cycle of the pest is about 28 to 30 days, cutting schedules of less than 30 days can reduce damage from leafhoppers. Although leafhoppers may reduce the quantity of forage harvest, increased protein content of the hay may partly offset this. Crop rotation is an effective method of eliminating corn rootworm injury in corn. Corn rotated annually to another crop will not have corn rootworm infestations. Continuous corn production in a field allows eggs to be laid in the field that will hatch the following year and result in larval feeding on corn roots.
When designing cultural controls for a production system, crop rotation and cover crops can lead to specific insect and other invertebrate pest problems. Slug populations can build up in sods and hay fields and then cause problems in field corn and soybean fields in the following years. The cover crop provides an ideal environment for slugs since they need shelter from the sun, which will cause them to desiccate. Tillage between crops, when slugs are present, can help reduce population before planting into the field. Other pests of field corn that can increase in frequency with cover crops and abundant residue include sod webworm, black cutworm, true armyworm, stalk borer, white grub, seed corn maggot, and wireworm. Plowing under residues at least 10 to 14 days before planting can reduce the likelihood that high populations will survive to damage the crop; however, this may not be effective if wireworm and white grub population are present. Methods to monitor for these pests can be found on the Penn State Entomology Web site (http://ento.psu.edu/extension/on-line-guides) under the Field Crop Training Manual. This manual also provides information on the association of key pest with specific types of environmental conditions.
Although listed as a cultural mechanism, host plant resistance is a major method in field crops to manage key pests. Within the last 5 years, new potato leafhopper–resistant alfalfa cultivars have reached the market place. These cultivars are the result of traditional selection methods. The newest varieties now have about 70 to 80 percent resistance to potato leafhopper and have agronomic characteristic similar to nonresistant cultivars. Selection of these cultivars can greatly reduce the effects of this major pest of alfalfa. By eliminating major issues with this pest, the only other pest of concern is alfalfa weevil; and with efforts to conserve its natural enemies, the frequency of losses can be greatly minimized.
Selecting a good corn variety can also minimize the impact of both European corn borer and corn rootworm. Over the years, plant breeders have increased both tolerance and resistance of corn hybrids to these pests. In their corn variety guides, however, they do not directly indicate the type or level of resistance to either pest. Insight into a hybrid’s ability to deal with both pests is its standability index and ear holding capabilities. Hybrids that consistently rate well in these characteristics tend to have less yield loss from these pests (particularly harvest losses). Although not listed in most variety guides, root system size can be important in tolerance of corn rootworm infestations. In general, a plant with a larger root system will tolerate more feeding by the pest. This, coupled with early planting, will reduce yield impacts by corn rootworm. The downside to these cultivars, however, is that if the plant is putting more energy into the root system than above ground parts, its yield capability may be less than cultivars with smaller root systems.
Soybeans and small grain crops also have been bred to tolerate certain pests. However, this information is often not provided in crop seed guides. Wheat varieties are available that have resistance to Hessian fly and greenbugs. Conducting your own evaluations can provide significant information on which cultivars to grow in an organic system.
Physical and Mechanical Control
Methods in this category use some physical component of the environment, for example, temperature, humidity, or light, to suppress pest populations or damage. Some examples are tillage, flaming, and flooding. Floating row covers over vegetable crops exclude pests. Heat or steam sterilization of soil is commonly used in greenhouses for control of soil-borne pests. For field crops, flaming can be used for alfalfa weevil control when timed properly, but you must be careful not to catch materials on fire that might threaten humans or wildlife. Tillage of corn can help minimize slug damage by helping the plant grow more rapidly (this is not a guaranteed outcome). The principle behind this approach is that the sun will warm up the soil around the plants faster and allow them to outgrow the pest’s feeding. In years when we have significant overcast and cool conditions, slugs can cause damage despite this practice.
Some types of pesticides are allowed in organic production. Lists of allowed substances are available from accredited certifiers; products are reviewed by the Organic Materials Review Institute (OMRI) (omri.org, phone: 541-343-7600). Because there are differences among certifying agencies, you should check on the status of specific compounds to determine if they are allowed, restricted, or prohibited in your organic system. Some products with allowable active ingredients may contain unacceptable adjuvants, so it is important to check the label and the certifying agency before using a material. Some examples of allowable active ingredients are Bacillus thuringiensis (Bt), pyrethrum, insecticidal soaps, diatomaceous earth, azadirachtin, horticultural oils, and insect-parasitic (entomopathogenic) nematodes and fungi.