Beat the garden bug blues this growing season
Story and Photos by Dr. Blake Layton
Looking forward to this year’s vegetable garden? So are many insect pests. Much like people, insects have strong preferences about which vegetables they like best. Tobacco hornworms love tomatoes but can’t stand cabbage or broccoli, while cross-striped cabbageworms’ preferences are the exact opposite. Stink bugs have a more eclectic diet – they especially like peas and beans – but there are few vegetables they won’t eat. Every vegetable in the garden has several different insect pests that attack it, and controlling these pests is one of the many challenges to having a productive vegetable garden.
Successful insect control begins before the garden is planted and continues until after the final harvest. There are many non-insecticide control methods home gardeners can use to help manage insects. Crop selection, variety selection, planting date, tillage and mulching practices, handpicking and foot-stomping, and prompt post-harvest crop destruction are a few examples. But there are times when insecticides are needed to control insect populations that have gotten out of hand, or will get out of hand if you don’t spray, and this article focuses more on this aspect. There are too many insect pests to cover them all, but let’s consider a few of the most common. Organic controls are listed when available and are indicated by an asterisk (*). Few insecticides are labeled for use on every vegetable in the garden. Before applying any pesticide in your garden read the label carefully to be sure the product you are about to use is labeled for the crop on which you plan to use it!
Cutworms
Cutworms can really get the gardening season off to a disappointing start. What happened to those plants we set out yesterday? Cutworms chew through the stem of newly set transplants or emerging seedlings and eat the fallen plant. Depending on species, cutworms may overwinter in the soil as partly grown caterpillars, or moths may lay eggs on weeds in early spring. Once the garden is tilled, there is nothing for these partly grown caterpillars to eat – except your emerging seedlings or newly set tomato and pepper plants.
Control: Organic gardeners can use “cutworm collars” made from aluminum foil, wax paper, or plastic bottles to physically protect new transplants. A quicker and easier method is to spray a 6-8 inch band of soil around the base of newly set transplants with permethrin or bifenthrin.
Tobacco hornworms
These are the large green caterpillars that are so destructive to backyard tomatoes. Fully-grown caterpillars can reach 4 inches in length – and they have to eat a lot of tomato leaves to get that big. These caterpillars will occasionally damage fruit, but this usually happens only after they have eaten all the leaves they can find.
Control: See control for tomato fruitworm below. Hand-picking can also be somewhat effective, or at least vengefully gratifying.
Tomato fruitworm
They’re called fruitworms because they feed directly on the fruit and a single caterpillar can damage several tomatoes. Newly hatched caterpillars begin by boring into marble-sized fruit, moving on to larger fruit as they grow. Fully mature caterpillars are about 1¼ inch long and usually bore into large green or ripening tomatoes in the area around the stem. Heavy infestations can destroy more than half a crop.
Control: To protect your tomatoes from fruitworms, hornworms and stink bugs, apply weekly sprays as soon as plants begin blooming. Products containing spinosad* are effective against fruitworms and hornworms, but will not control stink bugs. Products containing permethrin or bifenthrin will control all three of these major tomato pests. If you are going to spray for insects, it makes sense to spray for tomato diseases at the same time. Uncontrolled tomato diseases can be more damaging than insect pests. Fungicides such as chlorothalonil, mancozeb, and copper-based products can usually be tank-mixed with insecticides and applied together, but check labels to be sure.
Squash vine borer
This stem-boring caterpillar causes large squash and pumpkin vines to suddenly wilt and die. The eggs are deposited on the leaves and stems by day-flying moths and the hatching larvae bore into the stem and feed toward the base of the plant, growing larger as they go. Heavy feeding at the base of the stem destroys the vascular system and causes sudden wilting.
Control: Early planting is one of the best non-insecticide defenses against vine borers. Insecticide control depends on killing newly hatched caterpillars before they bore into the plant. If you want to extend the life of your plants, apply weekly preventive sprays once plants begin to bloom. Protect pollinators by spraying late in the day. Spinosad* will control newly hatched vine borers, but it will not control squash bugs. Products containing permethrin or bifenthrin will control both pests. Spray the leaves to control vine borers and spray the base of the plant to control squash bugs. Permethrin has a one-day pre-harvest interval on squash. Bifenthrin is arguably a bit more effective, but it has a three-day PHI, which is a bit long for yellow squash and zucchini.
Colorado Potato Beetles
Both the VW-shaped adults and the pink, soft-bodied larvae feed on potato leaves and heavy infestations can cause severe defoliation, resulting in fewer and smaller potatoes. Eggplant and tomatoes are attacked less commonly.
Control: Hand-picking can be effective for light to moderate infestations. Spray as needed with spinosad* or permethrin to control heavy infestations.
Cole Crop Caterpillars
“Cole crops” is a somewhat archaic term for brassica crops – such as cabbage, broccoli, bok choy, and turnips. Several species of caterpillars specialize in feeding on cole crops, but cabbage loopers, cross-striped cabbageworms, and diamondback moths are the three most common. All of these caterpillars cause damage by eating leaves.
Control: Watch for early signs of leaf-feeding, especially those tiny little windowpane-like areas newly hatched caterpillars make when they feed from the underside of the leaf and leave the clear upper epidermis intact. Spray as needed with products containing spinosad*. Bt* products are another, somewhat less effective option.
Stink bugs
These shield-shaped sucking insect pests attack many vegetables: peas, beans, tomatoes, okra, corn, and more, and there are several species of stink bugs. Damage varies depending on the plant being attacked. Pea and bean seeds are shriveled in the pod; okra pods are curled; tomatoes have white or yellow splotches under the skin. There are several generations per year and populations increase significantly as the growing season progresses.
Control: There are no good organic controls for stink bugs. Hand picking can reduce numbers on small crops, and frequent spraying with a natural pyrethrin* product will provide some control. Fortunately, pyrethroid insecticides such as permethrin or bifenthrin are quite effective on most species of stink bugs. Spray as needed once you start seeing, or smelling, stink bugs.
Leaf-footed bugs
These sucking insects have long, stilt-like legs and the rear legs have a flattened, leaf-shaped segment. Leaf-footed bugs do not usually become problematic until late summer or fall. They are especially damaging to tomatoes, peppers, and tomatillos. Damage is similar to that caused by stink bugs.
Control: Leaf-footed bug control is similar to stink-bug control except that adult leaf-footed bugs are more likely to fly away when they hear you coming with the sprayer, only to return later. Be stealthy and spray early in the morning when cooler temperatures make them move more slowly.
Cowpea curculio
This small grey weevil is one of the most damaging insect pests of southern peas. The females chew through the pod and deposit eggs inside the seed, which hatch into small legless grubs. The egg-laying scars are fairly easy to spot once peas are shelled. Picking them out is a bit time-consuming, but the alternative is to have those damaged peas with the little grubs inside with your cornbread. Do a good job with your insecticide sprays and there will not be many damaged peas to pick out.
Control: Timing is the key to successfully controlling cowpea curculios. As soon as the first blooms open, spray with bifenthrin or carbaryl. Spray again in five days and spray a third time five days later. The goal is to kill the female curculios before they can lay eggs. Blooming peas attract large numbers of pollinators, as well as predators such as paper wasps. Protect these beneficial insects by waiting until after sunset to spray.
Know the Pre-harvest Interval
Before applying any pesticide to any edible crop you need to know the pre-harvest interval, or PHI. This is the amount of time one must wait to harvest a crop after it has been treated. Pre-harvest intervals vary greatly depending on the product being used and the crop being treated, ranging from 0 days to 14 days or more. For example, the pre-harvest interval for carbaryl (Sevin) is one day on asparagus, three days on tomatoes, and fourteen days on turnips. Where can you find this important information? It’s right there on the product label. Read the label carefully, before you spray!