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Over the Fence

Where gardeners come to find out what's happening out in the yard.

Natural Enemies of Destructive Insects

Posted by Richard Hentschel -

There are a great many beneficial insects in the home landscape that can help gardeners manage destructive insect populations without ever opening the pesticide cabinet. Common to the yard are insect predators, parasitic wasps and natural pathogens that all work to our advantage. Some insects provide immediate control like lady bugs that feed on aphids. There are others like the parasitic wasp that lays their eggs on or inside an insect pest that later emerge to find additional pest to become hosts to their eggs. Predator insects can have an appetite for all forms of the pest, often eating eggs, young adults and adult forms of the insect.

Natural pathogens could be fungi, bacteria and nematodes. These can attack the larvae or caterpillar stages above or below ground. Bacillus thuringiensis (Bt) is a common example. Cabbage worms and the tomato hornworm are common targets in the vegetable garden. The natural rise and fall of lawn insects like sod webworm that live in the soil can be attributed to natural pathogens too. With all the wet weather we have been experiencing, bacteria and fungi are doing very well.

The goal is to invite and keep those beneficial insects in your yard so they are there when there an outbreak of destructive insects. Gardeners can initially introduce lady bugs for example, but without something to eat to keep them in the landscape, they move away to find something to eat elsewhere. One approach is to plant what is called trap crops or plants. These plants are very attractive to destructive insects which then are available to the predators. These plants are never treated with an insecticide. Later when there is an outbreak in the garden or on a shrub, predators are there to take care of the problem. Trap crops are typically placed at the edge of the garden or nearby. Gardeners can also augment existing populations by introducing additional predators to help out those already in the garden and landscape.

A more passive approach would be to be sure your garden contains flowers that those natural enemies are drawn to. Plants in the carrot, aster, pea and mustard families are good examples. The carrot family contains several of our herbs we use (dill, fennel), the aster family has several of our flowers we like (coneflower, coreopsis, cosmos) and the mustard family has the mustards, sweet alyssum and others.

Biological management of destructive insects then involves a wide range of naturally occurring enemies which together can manage insect populations. If there is still a need to use a pesticide, use one that is safe for the predators. An example of one that is not friendly is a miticide, making no distinction between a predator mite and a destructive one. Bt is a natural pesticide that can control specific worms without harming desirable moths and butterflies. In all cases when there seems to be a problem in the landscape or garden, be sure to identify the insect first to be sure that is the one actually doing the damage. Predator insects also have different stages that may look like they are the ones doing the damage. Lady bug adults are easy to spot, yet the larvae stage looks much different and is often mistaken for the insect doing the actual damage.


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