|Cocoons of a wasp that parasitized a tomato hornworm. Eric Sideman photo
Beneficial insects are part of complicated relationships in ecosystems, and we are just beginning to understand those relationships, said Kathy Murray, Ph.D., an entomologist and Integrated Pest Management (IPM) program coordinator with the Maine Department of Agriculture, Conservation and Forestry. Murray is also a member, along with MOFGA’s organic crop specialist Eric Sideman and others, of the Maine Integrated Pest Management Council, which promotes and enhances IPM.
Speaking at a MOFGA-sponsored session at the Maine Agricultural Trades Show in Augusta in January, Murray said we can manipulate the environment a little so that beneficial insects work for us or so that we don’t interfere with their work.
“Mother nature’s little helpers,” she said, include parasites, such as tiny wasps that lay eggs in caterpillars. An example is one of the beneficial wasps that attacks tomato hornworm. After the wasp’s eggs develop into larvae that eat the inside of that caterpillar, the wasps emerge and attach themselves as silk-covered cocoons, which can be seen on the outside of that caterpillar just before it dies. Beneficials can also be predators; and they can be non-insects, such as spiders, which eat insects; predatory mites; and pathogens. “Most of the time we don’t have to do anything about these organisms, as long as we don’t spray pesticides that will kill them,” said Murray.
Nature can be complex and endlessly fascinating, Murray continued. Our ecosystem is full of generalist predators (i.e., they eat many different organisms), including ladybug adults and larvae, ants, lacewings, and ground beetles (which eat weed seeds and other insects). Other organisms can, in turn, kill beneficials. A parasitic fly, for example, changes honeybee behavior – “zombifies” them – so that they can’t find their way back to the hive and they don’t pollinate as they’re supposed to. Sometimes a natural enemy can spur a pest to cause more damage, such as a parasite that attacks caterpillars, causing them to eat more plant material to provide more nutrients to the parasite.
“It’s part of nature, and nature has a way of balancing things out usually,” said Murray. Pest populations build, then predator populations build and knock down pest populations, then predator populations decline as their food source is consumed, then pest populations rebound … and so on.
We disturb this balance on farms by, for example, cultivating monocultures and tilling soil. Bringing organisms back into balance is part of biological control, which involves three basic approaches – conservation, importation (classical biocontrol) and augmentation.
|Our ecosystem is full of generalist predators (i.e., they eat many different organisms), including ladybug adults and larvae. This adult is feeding on an extra-floral nectary (a nectar gland that is not part of a flower) on an elderberry stem. English photo
The first line of defense – conservation biocontrol – conserves natural enemies by not killing them (by minimizing pesticide use, for example) and by providing habitat to support them. Murray quoted Carl Huffaker, one of the fathers of IPM: “When we kill off the natural enemies of a pest, we inherit their work.”
Providing habitat can include farmscaping – landscaping a farm with strips, borders or banks of flowering plants to attract and support natural enemies. For example, create diverse plantings that include small, open-faced flowers that provide natural enemies with pollen, nectar and shelter from the elements and from their enemies. Murray recommended the publication “Farmscaping: Making Use of Nature’s Pest Management Services” (https://articles.extension.org/pages/18573/farmscaping:-making-use-of-natures-pest-management-services). She also noted that the Natural Resources Conservation Service offers financial and technical assistance for such plantings through its Environmental Quality Incentives Program (EQIP), which includes the organic initiative, pollinator protection, filter strips (which could also include insectary plantings), water quality protection and high tunnels.
If you must use a pesticide, said Murray, select one that least impacts natural enemies; spot treat where needed rather than treating an entire field; and apply the pesticide when natural enemies are not in a susceptible stage. She recommended “Resource Guide for Organic Insect and Disease Management” (free download at https://web.pppmb.cals.cornell.edu/resourceguide/), which describes the impacts of many organic pesticides on natural enemies.
Many common pests, such as Japanese beetles and European corn borers, are not native, so their natural enemies – left behind in China or Europe – do not control them adequately here. In these cases, importing and augmenting the ecosystem with a natural enemy may help. For example, some predatory mites can be released to prey on two-spotted mites.
Inoculative release, said Murray, involves releasing a small number of natural enemies early in the season to establish a reproducing population. Trichogramma ostriniae, a parasitic wasp, can be released to help control European corn borers.
Inundative release means releasing a large number of natural enemies to overwhelm the pest population – e.g., releasing nematodes to control grubs in turf and around ornamentals. We don’t expect the nematodes to build up reproducing populations, said Murray, because our cold winter soils kill them.
When augmenting natural enemies, consider whether the biocontrol organism is a generalist, such as ladybugs, a specialist or both; whether the lifestyles and habitat requirements of the biocontrol organism are compatible with the climate, habitat and timing of your system; the availability and cost of the biocontrol; and its demonstrated effectiveness.
Augmentation has worked well in closed spaces, such as greenhouses and barns. Aphids, said Murray, can be controlled with parasitic wasps, predatory midges or lady beetles; whiteflies with parasitic wasps or lady beetles; spider mites with predatory mites; thrips with predatory bugs, beetles and nematodes; and fungus gnats and shore flies with nematodes.
In livestock operations, the housefly and stable fly can be controlled with parasitic wasps, predatory mites and predatory beetles.
Greenhouse aphids can be controlled with predatory midges (Aphidoletes aphidimyza), which are introduced as fly pupae, which soon emerge as adults and lay eggs, which hatch into small, orange, slug-like larvae. Parasitic wasps also work, including Aphidius colemani, which is good at mobile searches; Aphidius ervi and Aphelinus abdominalis. These tiny wasps lay their eggs inside aphids, mummifying the aphids into hard pear shapes. The parasites emerge as fully formed adults, ready to parasitize other aphids.
To maintain and increase beneficial insect populations, banker plants provide habitat and/or food. That food may be pollen or alternative hosts for the beneficial insects – such as an aphid that feeds only on that banker plant and won’t move onto crop plants. For example, a particular grass aphid will feed only on grass plants, so it can live on barley. A barley banker plant, then, can support beneficials by harboring that grass aphid for their feed. The beneficials migrate from banker plants to crop plants when pests appear on crops. They can return to the banker plants once they control the crop pests.
Banker plants may grow inside cages, screened from natural enemies, while aphid populations build up on them. Then banker plants are removed from the cages and dispersed throughout the greenhouse, and parasitic wasps are released to feed on the pests and on aphids on banker plants. Scout for mummies to see how well the system worked and whether it’s cost effective, said Murray, adding, “Don’t spray.”
A relatively simple banker plant system for thrips biocontrol in greenhouses involves the minute pirate bug, Orius insidiosus. Pollen from ornamental peppers, such as Black Pearl and Purple Flash, is produced when plants are about four months old and enhances Orius insidiosus populations. You don’t have to bring in another pest to infest the pepper, said Murray.
More Control Organisms
Beneficial nematodes, which are fairly hardy, relatively inexpensive and easy to apply, can control western flower thrips, shore flies and fungus gnats in greenhouses. When you get them, make sure they’re alive, said Murray. If they’re perfectly straight, they have died; if they’re worming around, they’re alive. Nematodes need water. They swim into natural openings, such as breathing holes, of insects, where they reproduce inside the pest and then release a bacterium that is toxic to that pest. Then they are released from the host and swim through the film of water. So water them into the soil with irrigation water or apply them on a rainy evening. If you put them into a drip irrigation system, make sure screens in the system are not less than 50 mesh.
Nematodes are easy to ship. They come packaged in a dry mix or in a sponge and can be stored for a little while. Some growers dip plug trays or cuttings in a bucket of water containing the sponge on which the beneficial nematodes were shipped.
To control stable and house flies, first keep manure very dry, get it outside and spread it on the field, said Murray. Then, if flies still get out of control, tiny parasitic wasps called Muscidifurax raptor and M. raptorellus, which attack the pupal stage of the fly, are effective in the Northeast. One farmer, said Murray, sprinkled these pupae right in the front of the calf hutch, where the calf couldn’t stomp on them, and they provided up to 80 percent control. Release them every week for a month so that they build up sufficient populations, Murray advised. Her website, www.maine.gov/ipm, has instructions for releasing these wasps, available from IPM Labs.
Trichogramma ostriniae, an egg parasitoid, is specific for controlling the European corn borer (ECB) in sweet corn. “It’s tiny,” said Murray. “Just put out a single packet. Timing is critical. Put ECB pheromone traps out; when you start catching male moths, it’s time for control.” Order the wasps in April, she said, and plan to release them in early June for early corn. (A table of degree days can help, as can pest alerts from Cooperative Extension.) They complete their life cycle inside the borer eggs, killing them and turning them black. Then new wasps emerge and attack other corn borer eggs. A later release may be needed for later corn. (See the Cornell fact sheet “Using Trichogramma ostriniae to help manage European corn borer in sweet corn, peppers, and potatoes” at https://nysipm.cornell.edu/factsheets/vegetables/swcorn/trich_ost.pdf)
Research has shown that the ECB threat is down throughout the Northeast, said Murray, due to the planting of genetically engineered Bt corn. This has created a “halo effect,” giving biological control options such as Trichogramma wasps a better chance of success.
Mark Guzzi, owner of Peacemeal Farm, said he has used Trichogramma for corn borer control. Adam Tomash, a homestead-scale gardener who attended Murray’s talk, said he sprinkles a little spinosad on corn silks within a week of when corn pollen has started to fall and silks have formed. “The timing is a little tricky, but I usually do it before the silks start to dry down,” said Tomash. “Sooner if I see evidence of worms on the ears. I have had excellent control with this, sometimes approaching 100 percent.” Murray noted that growers using pesticides and beneficials together will want to select products carefully to ensure that beneficials are protected. Several biological control suppliers publish compatibility charts.
A mobile app called “Greenhouse Scout,” developed by Dr. Betsy Lamb of Cornell University, costs $9.99 and provides information on beneficials, insects controlled, identification tools (including photos), pesticide biocontrol compatibilities (which pesticides will or will not harm the biocontrols), life cycles of natural enemies, how to apply each biocontrol and how to see if it’s alive. The app allows you to record and track pests and beneficials and has predictive graphing capabilities. Look for “Greenhouse Scout” in the iTunes app store or in android apps on Google Play.
The Association of Natural Biocontrol Producers (anbp.org) lists companies that sell biocontrol agents and posts “Guidelines for Purchasing and Using Commercial Natural Enemies and Biopesticides in North America.” Some biocontrol companies provide excellent technical support, said Murray. A BioBest representative comes through Maine regularly and will come onsite to help. IPM Labs (ipmlabs.com) in upstate New York has excellent service by phone.
Biological Control, Cornell University, https://www.biocontrol.entomology.cornell.edu/index.php
Gotpests.org (https://www.maine.gov/dacf/php/gotpests/index.html) is a good resource, designed mostly to help customers identify pests. Cooperative Extension also provides good information.
Miles, C., C. Blethen, R. Gaugler, D. Shapiro-Ilan and T. Murray. 2012. Using entomopathogenic nematodes for crop insect pest control. PNW Extension Publication 544, https://cru.cahe.wsu.edu/CEPublications/PNW544/PNW544.pdf
Maine Department of Agriculture, Conservation and Forestry, https://www.maine.gov/ipm
Murray, Kathy, Maine Department of Agriculture, Conservation and Forestry, [email protected]
SARE (USDA Sustainable Agriculture Research and Education), https://www.sare.org/
University of Maine Cooperative Extension, https://extension.umaine.edu/