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.
MONITORING
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.
BENEFICIALS
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.
GETTING
STARTED
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.
References:
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.