Extension@YourService: Late-Season Pests

— Written By Leeann Crump

As summer begins to wind down, there are several pests that can make their appearance and cause some consternation for homeowners. Seemingly appearing out of nowhere, these critters can damage plants in the landscape and garden. Being able to recognize the signs of a pest and what steps one can take to mitigate their damage – or even if mitigation is needed – can take some of the angst out of the situation.

I’ve already gotten a call from a homeowner about a regularly occurring late-summer pest, the Azalea caterpillar. These critters rear their heads (literally!) on azalea, and sometimes on blueberry bushes, at the end of August or first week of September. The moth lays eggs on the underside of the leaves and when the caterpillars hatch they feed all together in a clump, called feeding “gregariously”, on the leaves on which they were laid. Put this pest’s appearance on your calendar and every year scout your azaleas for a couple of weeks during the activity window. The caterpillar can do a surprising amount of damage, though it’s typically not life-threatening to the azalea. If you catch them early, when the caterpillars are still grouped together, it’s easy enough to simply snip off the end of the branch and dispatch them with the sole of your foot. If you wait too long, not only have the caterpillars gotten larger, they are more dispersed around the plant. Typically, they are too few to really warrant an insecticidal spray: simply knock the caterpillars into a jar of warm soapy water and in short order you will have cleared them out of the bushes.

Another caterpillar pest is the orange striped oakworm, and sometimes the yellow striped oakworm. The adult moth emerges midsummer and lays eggs in oak trees. Caterpillars hatch and begin feeding. Some years the numbers of oakworms are so great it can be dangerous – or, at least, gross – to stand under an oak tree and have frass (another word for caterpillar poop) rain down on you. If you miss the joy of having a car or patio covered in greenish caterpillar frass, you may not even know the orange striped oakworm is in the trees above, munching away. When the caterpillars have reached their mature size, they descend from the tree and begin looking for a place to pupate. Often this is when homeowners first notice them, as they crawl around on the ground, up the screen porch, or through the carport. While it can be unnerving to see all the caterpillars roaming around, there’s no need to spray them. They’ve done all the damage they can do in the tree, and any defoliation that’s occurred will not cause any long-term damage to the tree, which will dropping leaves within a couple of months anyway. Additionally, because the caterpillars are mature, pesticides are not very effective against them.

I once had a call from a homeowner very concerned about a small brown worm that was decimating her parsley. She was calling to find out what she could spray to kill the pest and save her parsley patch. I explained this was the well-known parsley worm aka black swallowtail caterpillar. Parsley worm – blech! Black swallowtail caterpillar – aww! Adorable! It all depends on one’s perspective! The black swallowtail caterpillar, when very small, is rather brown and unattractive looking. As it eats and grows it takes on the lovely pale green, with black and yellow dots and stripes, that alerts us that this is a caterpillar with a greater destination in store. After losing all my parsley for several years to “parsley worm”, I now plant extra for the sole purpose of hosting and supporting as many black swallowtail caterpillars as I can, for the joy of knowing I helped these beautiful creatures get a start. If you’d like to host swallowtail caterpillars, plant parsley, dill, or fennel. In the wild Queen Anne’s lace is a host. All are members of the carrot family.

With caterpillar pests, it’s great to be able to identify the caterpillar and the moth or butterfly it will become before planning a wholesale assault. Often, by the time we see the caterpillars, they are a mature size that is hard to kill with pesticides. If you feel you need to use a pesticide, a product with Bacillus thuringiensis (known as Bt, for short) is a good option. This biological pesticide contains a bacterium which will only kill caterpillars that eat it, without harming honeybees or other pollinators. It is readily available in several formulations. Even better than using a pesticide is to have natural helpers like birds who feed on caterpillars and even wasps, which are predators or parasitizing on many caterpillars. This creates a healthy balance in your garden that keeps pests from getting out of hand while supporting the animals that need to eat insects to live and raise their young.

For more information on horticulture, you may reach Paige Burns at:  paige_burns@ncsu.edu; 910.997.8255; 123 Caroline Street in Rockingham, NC.