Good Insects in the Garden

(News article for May 29, 2020)

When extension agents get calls about insects, it’s often because one is, or appears to be, injuring a plant. However, the reality is that roughly 99% of insects are not “pests.” In fact, if you know what to look for, you may see insects that are helping to keep pest insects under control.

Many people have seen aphids on their vegetable or ornamental plants. These are sometimes called “plant lice.” They suck sap out of plants and excrete a sugary substance called honeydew. We often see sooty mold growing on that honeydew.

There are a number of beneficial insect and even some fungi that help manage aphids.

We sometimes see aphid “mummies.” The aphids have holes in them and look like they've been blown up like a balloon. This is caused by parasitoid wasps that lay eggs in the aphids. The eggs hatch, and the larvae feed on the insides of the aphids. The larvae pupate and become adults, which make holes in the mummies as they exit.

The larvae of some lady beetles (“lady bugs”) and of green lacewing insects also feed on aphids. Note that the larvae of these insects do not look like the adults, so you might not recognize them. Both look sort of like tiny alligators. Lady beetle larvae are often black and orange, while lacewing larvae are brownish. If lacewing insects are present, you may also see their eggs, which are each perched on a “stalk.”

Like aphids, thrips are common insects, though they’re often not noticed because they’re so small. Some are problematic because they feed on flowers and thus affect flower appearance or fruit development. Some also spread plant viruses, like tomato spotted wilt virus.

The minute pirate bug is one insect that preys on thrips. Like thrips themselves, minute pirate bugs are not easy to see with the naked eye, but with a hand lens, they can be observed to be black and white in color. Based on work done in Florida, it was found that having one minute pirate bug for every 40 western flower thrips was enough to provide adequate control.

If you’ve grown tomatoes, you very well may have seen tomato hornworms at one time or another. If you see white cocoons on the back of a hornworm, let that hornworm stay. The cocoons are those of a parasitoid wasp. These wasps lay their eggs in the hornworms. After eggs hatch, larvae emerge from the hornworms and spin cocoons on their backs. The wasps will pupate in the cocoons, emerge as adults and, hopefully, go lay eggs in other hornworms.

The final beneficial insect I’ll mention is the spined assassin bug. The adults of this insect are brown and readily seen with the naked eye. This is one of several different assassin bugs, which often hold other insects with their forelegs while feeding on them.

People sometimes inquire about buying beneficial insects for the garden. While this may be helpful in some cases, it’s a more practical strategy for an enclosed structure like a greenhouse.

In the garden, one of the main things to do to encourage beneficial insect activity is to avoid using broad-spectrum insecticides when possible.

The pyrethroid class of insecticides includes ones with active ingredients like pyrethrins, permethrin, bifenthrin, lambda cyhalothrin, and esfenvalerate. These insecticides are some of the most effective options we have for some insects, like leaffooted bugs and plant-damaging stink bugs. However, they’re broad-spectrum and may kill insects that you’d rather not kill, as well.

Some insecticides have a more narrow target range. We have a number of options for caterpillar pests. Insecticides with the active ingredients spinosad or Bacillus thuringiensis subspecies kurstaki are generally effective on caterpillars when they’re small.

Spinosad is also effective on some other insects like thrips and citrus leafminer. Spinosad can still negatively affect bees if they come in contact with it before it dries, so, as with any insecticide that can harm bees, wait and spray it in the late afternoon or evening when bees are no longer active.

Insecticidal soap (often called “potassium salts of fatty acids”) or horticultural oil products can help keep numbers of soft-bodied pests like aphids, thrips, and spider mites low. Good coverage of plant surfaces, including undersides of leaves, is essential when using these, since they work on direct contact with the insect or mite.

These insecticides are generally more effective when pest numbers are few and insects are small, so try to keep a close eye on plants so that you’ll spot problems before they get out of control.

Before using any insecticide, be sure to make sure it’s labeled for the plant on which you intend to use it, and follow label instructions.

Let me know if you have questions.

Contact Mary Helen Ferguson.

Aphid mummies on a pepper leaf. (Photo by M.H. Ferguson)