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Archive for Sunday, August 19, 2012

Garden Calendar: Not all bugs bad for plants

August 19, 2012

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In the plant world, horticulturists and gardeners are usually always battling some sort of insect pest, but this year is especially challenging because plants are already suffering from heat and drought stress.

Green lacewing larvae feed on bright yellow aphids on milkweed. Lacewing larvae can eat hundreds of aphids in their lifetime.

Green lacewing larvae feed on bright yellow aphids on milkweed. Lacewing larvae can eat hundreds of aphids in their lifetime.

 Immature eggplant lace bugs feeding on the underside of eggplant leaves. Although the damage is obvious, the insects themselves hide well.

Immature eggplant lace bugs feeding on the underside of eggplant leaves. Although the damage is obvious, the insects themselves hide well.

Whether we’re talking squash bugs or borers, the key to control is always proper identification and assessment of the population.

Why identify the insect? Because the insect you think is causing the problem may not be the real problem.

Lady beetle larvae and green lacewing larvae are two insect species that are commonly confused for “bad bugs” because of their menacing appearance. However, both insects feed on aphids and other insects that cause injury to plants rather than causing harm to the plants themselves.

Even if the insect in question is causing injury, it may turn into something nicer — like a beautiful butterfly. Swallowtail butterfly larvae are notorious for stripping the leaves from parsley, but I would rather watch the adults they become flit on the breeze than have the fresh herb at every meal. Another solution that works well in this case is to plant more of the plant than you need so you have some to share.

Another reason to identify the insect is that even if the insect causes injury, it may be just an incidental pest on the plant you find it on. I see squash bugs on sweet potato vines and other plants occasionally and boxelder bugs hanging out on trees other than maples or boxelders. Since insects are really good at seeking out their preferred food sources, it is unlikely they will stay on an unfavorable plant species long enough to do injury.

Finally, more than one insect might be causing the plant injury, and identification can help you determine if more than one insect is present. For example, I recently observed a few spotted cucumber beetles feeding on the leaves of the eggplant in the Master Gardener Demonstration Vegetable garden at the Douglas County 4-H Fairgrounds, 2110 Harper St. Although cucumber beetles are considered a pest of eggplant, I was more concerned with the mottling and discoloration of the leaves than the chewing those cucumber beetles were doing. Sure enough, eggplant lace bugs were the culprit doing the real damage but were barely visible on the undersides of the leaves.

Why assess the population? In the previous example, it would cost more time and money to control the cucumber beetles than the eggplant is worth. If there were more cucumber beetles and I was depending on the eggplant for supper, I would crunch numbers to see if control was worth the cost. The lace bugs, however, warrant treatment if I want to continue to harvest eggplant.

On an ornamental plant, I would consider the cost of replacing that plant in comparison to the time and money to control the insect. This is especially important with perennials because control over a number of years adds up.

On trees and other large plants, the insect population can be thought about in relation to the size of the plant. For example, an aphid population on a small, newly transplanted maple might warrant treatment while the same number of aphids on a full-grown maple is unlikely to affect the health of the tree.

Once you know what the insect is and whether it is worth trying to control, if it turns out to be a bad guy, control options are easier to find. Sucking insects like aphids and spider mites can be controlled with a hard stream of water created with a high pressure nozzle, and some insects can simply be scraped or picked from the plant. Horticultural oils, insecticidal soaps and spinosad products have low residuals and are less likely to harm beneficial insects than some conventional insecticides.

As always, make sure the insect you want to control and the plant you want to use it on are listed on the label. Different brands of pesticides, whether organic or conventional, contain different amounts of the active ingredient and different inert ingredients. Insecticides can cause injury to plants when used improperly. A good example is neem — it is an active ingredient in insecticides, fungicides and herbicides — so picking up the wrong formulation might kill what you are trying to protect.

The Kansas Department of Agriculture offers Insects in Kansas, a full-color guide to many of the approximate 20,000 insect species found in Kansas, for $25 plus shipping and handling. The book may be ordered by calling 785-532-5830 or emailing orderpub@k-state.edu.

There are also several websites with insect identification guides and keys, although special attention should be paid to the insects distribution since species vary across regions.

Douglas County Extension Master Gardeners can also help identify insects through their horticulture hotline, which operates Monday-Friday from 1 p.m. to 4 p.m. at the K-State Research and Extension—Douglas County office at the fairgrounds.

— Jennifer Smith is the Horticulture Extension Agent for K-State Research and Extension in Douglas County. Contact her or an Extension Master Gardener with your garden questions at 843-7058 or dgemg@sunflower.com.

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