Insects play an incredibly important role in our gardens, landscapes and forests. They play a critical role in the production of our food as pollinators of fruits and vegetables. Some, in their flight from plant to plant, promote botanical diversity as they spread pollen and thus become one of the natural distributors of plant diversity through genetic exchanges. And we often forget that there is a wide range of insects that are beneficial to us as they prey on other insects that might be destructive in horticulture and agriculture.But on the other end of the spectrum, there are insects that we as gardeners and growers loathe. These insects seem to fall into three categories based on how they feed or on how they reproduce and how they fit into this puzzle has a great deal to do with how we control them or figure out what other insects will control them. In fact, one of the first things we look for when a new invasive insect becomes a problem is: What is its nemesis? What insect in its native habitat feeds on it and keeps its destructive nature in check?
For years, greenhouse growers had horrible problems with whitefly on poinsettias and all kinds of poisons were being used to kill these tiny white flies. Then it was discovered that a tiny wasp, an almost imperceptible insect called Encarsia formosa, parasitized the whiteflies. When introduced into a greenhouse, they’d clean out the whitefly without the use of chemicals. And yes, you can buy these tiny wasps online if you’d like to introduce them into your garden.
You may have had the opportunity to find a tomato hornworm in your garden. These caterpillars of the five-spotted hawk moth will munch on tomato foliage and can chew it down to nothing in a matter of days. This rarely happens though because they can easily be picked off the plant and, more often than not, a small wasp called Cotesia congregata will lay eggs on the hornworm and the wasp pupae feed on the hornworm and kill it.
We aren’t always so lucky though, and as gardeners we need to know how to identify insects, their damage and how to control them. In doing this we also need to keep in mind that the sprayer filled with chemicals is not always the answer. Even when we do have to use a sprayer there is a wide range of materials that can be used that begin with soaps and oils, then microbials and botanicals and—only as a last resort—man-made chemicals that are usually neurotoxins or stomach poisons.
One of the things you’ll notice in this guide is that the insects that we deal with in the garden can fit into one of three categories depending on how they feed or how they reproduce. First, there are the suckers. These insects have mouthparts that are designed to pierce plant foliage with needle-like mouth parts that are inserted into the leaf or stem. Once inserted, the insect removes the plant liquids. In doing so, the insect can also be transmitting a virus, as I noted in last week’s column. But as you may have noticed, what goes in also has to come out, and many insects excrete a sugary excrement called honeydew. In some cases we know there’s a sucking insect problem by finding the honeydew on our car windshields, lawn furniture or on the foliage below, where a black mold will grow on the honeydew aptly called sooty black mold.
The most ubiquitous sucking garden insect is probably the aphid. There are hundreds of species of aphids and they can be green, orange, black and white. Aphids are fairly easy to control with soaps, oils and botanicals, but you need to knock them down early. As with most insects, as the seasonal temperature rises so do their reproduction and damage. Aphids are also social insects, so there is never just one. Most are wingless, but when a colony gets too large a winged female is born who flies away and starts a new colony.
Another sucking insect is the tiny, often missed two-spotted spider mite (TSSM). The problem is that it’s really not an insect. Using the wrong “insecticide” on it not only won’t kill it but may cause it to reproduce more rapidly. Mites have eight legs while true insects have six. These mites show up once it gets hot and dry, but they first appear in small numbers early in the spring and that’s when they can be controlled with, of all things, sprays of water. Later on, appropriate miticides have to be used, some of which are pretty benign when it comes to environmental concerns. You’ll find the TSSM on privet but also on a wide range of garden, landscape and house plants like hibiscus and gardenia.
Then there are the chewers. These insects generally chew on foliage, but some will chew on fruits and vegetables. The most common chewers that we deal with are probably the Japanese beetle, the rose chafer, the Oriental beetle and a wide range of caterpillars. In most cases, the chewers feed from the leaf margin inward or directly on the flowers. When you see holes directly in the center of a leaf, that’s usually a clue that a slug or a snail has been at work.
When chewers like beetles are present some gardeners simply pick them off. Others will use contact insecticides that get absorbed through the insect, or they use insecticides that the beetle ingests while feeding.
There are also the caterpillars that are chewers. These include the gypsy moth caterpillar and the fall webworm caterpillar that is all over the place this summer on fruit trees and berry bushes. These chewers can be controlled with a biological material called Bt (Bacillus thuringiensis) that is sprayed on the foliage then ingested by the caterpillar as it feeds. It will work only on the caterpillars, but beware: It doesn’t know the difference between a webworm or a butterfly caterpillar like a monarch. So spray only on the target plants and not randomly.
Lastly, there are the ovipositors. These are insects that insert their eggs (oviposit) into the stems, branches and foliage of plants where the developing larvae feed on the interior of the plant. Many of these are wood-boring insects (borers) that you might find entering a peach tree, or the emerald ash borer of ash trees or an apple or other fruit maggot. Borers can be very difficult and expensive to control in larger trees and the chemicals used for these controls are anything but benign as they tend to be systemic, making most parts of the plant potentially poisonous.
In garden plants there is a leaf miner that oviposits on the foliage of columbines that leaves behind galleries of squiggles on the foliage. The columbine leaf miner rarely kills the plant but the damage can be unsightly. There’s also a leaf miner of the garden perennials in the Ligularia family that do the same but never seem to kill the plant. Neither of these miners has the status that calls for any kind of control, just beware that they exist and aren’t fatal.
In a nutshell, that’s it. Suckers, chewers and miners/borers. And for those with lawn insects, these too fall into either the chewers or sucker category. And where there’s a problem with any garden insect that you need to control, remember the mantra “do the least harm.” Use chemicals only as a last resort. Mother Nature isn’t perfect. Your garden doesn’t need to be either. Just beautiful and fruitful. Keep growing.
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