BIOLOGICAL CONTROL SOLUTIONS FOR CORN PESTS
Biological control with beneficial insects makes dollars and sense. Costs of sprays, scheduling sprays when workers are not present, managing residue and resistance problems, particularly the resistant corn earworm can be avoided. Difficulty of achieving satisfactory spray coverage by itself is incentive enough to seek more sustainable systems of pest management. A savings of 50 to 75 percent in pest control costs is often reported in the first two years of transition to predominantly biological control. Public and worker liability risks, even insurance costs, may be reduced. Even gaining a mere few weeks delay in the onset of spraying by making early releases of beneficials can yield dramatic monetary gains, with natural biological control substituting for weeks of pesticide inputs. As if this were not enough, there are also public relations benefits from using this "green", environmentally-friendly alternative to conventional chemical control. Certainly on organic farms, augmenting natural enemies makes sense.
CORN BIOLOGICAL CONTROL
Unsprayed corn fields host an army of beneficial arthropods, a valuable free source of natural pest control best nurtured, but all too easily and often destroyed by harsh pesticide regimens. Corn earworms, armyworms, aphids and other potential pests seldom pose major problems on corn in diversified organic farms where the indigenous pest-fighting natural enemies are preserved.
Among the hundreds of beneficial species commonly devouring corn pests are green and brown lacewings, pirate bugs, big-eyed bugs, assassin bugs, damsel bugs, spined soldier bugs, Staphylinid rove beetles, Carabid ground beetles, Collops beetles, lady beetles, six-spotted thrips, Tachinid flies, Phytoseiid mites, spiders and several dozen parasitic wasp species, including Trichogramma.
BIOLOGICAL CONTROL UTILIZES A COMPLEX OF NATURAL ENEMIES
Minimizing crop losses and optimizing crop production through biological control is dependent upon maximizing the diversity and distribution of the above plant species and resident arthropod wild life. To counter the forces acting against natural enemies of pests of corn in conventional farming systems it is necessary to manipulate these enemies as much as the crop is managed, both must be farmed together.
Slight changes in farming can overcome some of the cultural practices taken for granted under conventional chemical farming that form habitats for biological controls. Crop rotation, hedging, refuge management can make a difference in the behavior of both pests and the beneficials that attack them. Legume green manure crops add to the diversity of organisms provided by resident weeds. When managed by periodically, alternately cutting every other border, they keep the weeds from going to seed and further increase the diversity of composting organisms. This process adds humus to the soil and fosters the food chains that feeds the sets of beneficials in the biological control of insect pests and diseases.
In biological control programs, monitoring takes on a whole new meaning from that used in conventional total chemical eradication programs. Predator and parasite management augments biological suppressive forces so that pest population levels do not rise so explosively. The non economic pest population grows more and more natural enemies that later in the seasonal growth cycle will control major pests. The goal in IPM is to restore biological control and not eradicate the pests. For example, not all parts of the field get pests at the same time. Careful monitoring and sampling of the progress of biological controls can often identify the "hot spots" that can be treated with larger numbers of beneficials or spot treated with least toxic, low residual spray materials. Not all pesticides have adverse effects on the balance of pests and their natural enemies, and certain dosages of conventional pesticides are less disruptive to biological controls. It is important to point out that insects, mites, and weeds. etc. are pests only when they affect our way of life beyond tolerable limits. It is only the intolerable numbers that can be called pests!
Moths can fly several miles and lay many eggs distributing them widely throughout a planting of corn. They tend to congregate on bloom prior to oviposition. One can anticipate the pest pressure in corn at silk from monitoring population information gathered on whorls prior to development of tassels. When there are moths, there will soon be egg deposition. Moth flights can be anticipated from traps and moon cycles and from examining whorls while walking the field to count the numbers of moths seen resting there during daylight hours. Tassel worm damage reflects the level of control of both corn earworm and armyworms.
The beneficial insects (all natural, non genetically engineered) supplied by Rincon-Vitova Insectaries augment other natural occurring predators and parasites grown in the crop and covercrop refuges. The least pest damage to the market crops occurs where the covercrop refuges trap the pests away from the crop. Refuges of alfalfa and other legumes attract large numbers of soft bodied aphids, mites, worms that are prey to feed general predators of corn pests. Alternate strip harvesting of this covercrop refuge will keep the plants attractive to these herbivores throughout the season. On the other hand, cutting all of a mature alfalfa hay field, for example, can send large numbers of moths into a nearby corn planting. Including strips of covercrop plants provides the ecosystem for sets of natural enemies to increase in numbers by feeding on their prey. This "battle of the bugs" takes place without damage to the corn. The alternate strip cutting favors the biological controls.
Since corn earworm and armyworms can sometimes elude these hungry predators and parasites, augmentation with Trichogramma pretiosum and green lacewing can effect early suppression. Colonies of beneficials should first be released for the tassel worm (first generation moths) and again using larger releases when silk development begins (second generation moths). Since Trichogramma attack freshly laid moth eggs, they must be present as mated adults when moth flight begins. Ideally Trichogramma should be packaged in a swarming container, such as a paper cup, bag or jar, with a smear of honey so the female wasps are mated and fed prior to leaving the container.
Typical quantities used range from 100,000 to 300,000 per acre for the season, depending on pest pressure, amounts of natural biological control and tolerance for damage. Releases can be made over four to five weeks scheduled as closely as possible to moth flights and blanketing the tasseling and silking periods. Distribution should be heavier in "hotspots" of moth activity where most oviposition is occurring. When plantings are staggered, releases should start when the first planting is approximately a foot tall and followed by releases during the first flight of moths. Subsequent plantings should not require the same quantity, because subsequent generations of Trichogramma will be available.
Lacewing are predators on eggs and small worms, but best results occur when the natural enemy complex is in place attacking the pest eggs as they are laid in the developing whorl building biological control resources to suppress the next generation moth flight that attacks the corn ears. Rincon-Vitova cultures a vigorous strain of green lacewing, Chrysoperla carnea. The hatching larvae are distributed using a carrier, such as rice hulls or corn cob grit, to help distribute the larvae as far as is practical. Methods of application include hand broadcasting, hand-held blowers, ground rig and airplane mounted hopper systems. Five to ten thousand per acre is commonly used per release. Adults can also be attracted by artificial diets; however, lacewing are naturally attracted to corn for pollen and nectaries. Colonizing them early can help suppress aphids and mites, armyworm moth eggs and larvae of the species Spodoptera and Heliothis tassel worms. Lacewings are summer insects overwintering as adults. Early larval releases can place a generation of lacewings in the field before any natural overwintering adults begin laying eggs. Two releases a week apart helps to create over-lapping generations helping assure lacewing larvae feeding on a steady basis over subsequent plantings.
Minute pirate bug is a most important predator in corn. It is attracted by flower thrips, but eats almost any tiny insect or mite. Commercial sources are not yet economically practical, so covercrop or border plantings provide a field source for minute pirate bugs into the corn during the silking period. Where European corn borer is established, there are Trichogramma species that would be appropriate for augmentation, but, again, costs appear impractical at present.
MAINTAINING BIOLOGICAL CONTROL
Maintaining biological control in corn is an on-going process involving introduction and conservation of natural enemies and careful monitoring. Conservation of natural enemies is facilitated by phasing out hard pesticides interfering with biological control, as well as by periodic maintenance releases of beneficials. Season long biological control is predicated upon monitoring the progress of biological control throughout the various plant developmental stages. General predators that feed on a wide variety of prey enter the field by feeding on early season pests in the covercrops and resident weeds at the edges of the fields. Several generations later their off-spring form the basis of biological controls that enter the corn. Periodic releases of insectary grown beneficials timed to focus on the developing pest spots in the field is an alternative to spraying and killing beneficials as well as pests. Repeated spraying of resistant pests that are hard to kill devastates beneficials and creates ever more pests in the long term. Much conventionally treated corn has more worm damage than untreated corn, because of pesticide resistance. Worm-free corn is commonplace from organic farms where covercropping and augmentation of natural enemies is practiced.
The common sense approach would be to back away from conventional chemical controls by following a program of integrated pest management (IPM). Start releases early when the first pests enter the field. Treatments with Bt are time-consuming and temporary and not an ideal component of a sustainable and cost-effective program. If it becomes necessary, however, during the season to knock down runaway pest populations to levels that small populations of newly-introduced beneficials can easily mop up, use the least-toxic, low-residual spray materials available. The goal for all spraying (selective use of least toxic pesticides) is to lower the pest population to a tolerable level and still restore biological control.
CULTURAL CONTROLS Plant the legume covercrops for field insectary refuges for beneficial insects. As the season advances begin mowing alternate strips when plants start to bloom. Cut half and let this start to grow back before mowing the alternate strips. This management maximizes the populations of beneficials on the farm. Broad spectrum pesticides are avoided at all costs in early season. Release early season insectary grown beneficials that are the back bone of reestablishing biological control. It is like "restocking the fish pond" when one starts to rescue such natural enemy depleted farms from the pesticide treadmill.
SPRAYING Avoid pesticide (including herbicides and certain sprays for pathogens) to the extent possible that interfere with beneficials. Preventative chemical treatments, particularly systemic pesticides that interfere by almost eradicating early season pests must be avoided. It is essential that a few minor pest situations develop in order to obtain and maintain a buffering natural enemy complex within the farm ecosystem that will control major pest problems later on.
IPM in corn is based on the ecosystem concept that stresses the broad range of interactions of diverse sets of natural enemies affecting potential pests of this crop. Although conventional farming practices greatly simplify these predator/prey relationships, slight modifications in the way one farms can emulate more natural systems to take advantage of natural enemies. Getting started requires a desire to use these interactive beneficials throughout the year. Short term benefits from over reliance on pesticides will not lead to long term gains in biological control. The second and third year are always easier than the first. Biological control in corn is economical, safe and profitable.
Relevance of Ecological Concepts in Practical Biological Control, Dr. Peter W . Price, Beltsville Symposia in Agricultural Research #5 Biological Control in Crop Production 1981.
The Nature and Practice of Biological Control of Plant Pathogens, R. James Cook and Kenneth Baker 1983.