Adult squash vine borers mimic wasps to dupe predators wary of dealing with stinging insects. Photo: Sue Witte
Just drought, or is something nefarious behind my wilted vines?
Over the past several weeks, curious fans of Bug of the Week have sent images of eyed click beetles and dobsonflies that inspired yarns describing the marvels of these six-legged creatures. This week the trend continues with a close look at the squash vine borer, a true nemesis of vegetables in the garden. One of my favorite gardening pastimes is growing ornamental gourds, those amazingly diverse members of the Curcurbitacea, kin of cucumber, squash, and zucchini. In late October the largest of these will be transformed into Jack O’ Lanterns and the small ones will decorate my home. Imagine my dismay upon returning to my garden after a short journey to find my vines withered and one step away from death’s door. The miserable condition of these vines could not be pinned on a summer drought, not with the rainfall we have enjoyed this year. Clearly this was the work of some diabolical insect.
While pondering my wilted leaves, I saw several red and black waspish insects zipping among the vines, but these were not wasps. Rather, they were the clearwing moths, the adult stage of the squash vine borer. The squash vine borer is a moth that mimics a wasp both in coloration and behavior and in this way gains a modicum of protection from predators like birds that have learned not to attack flying critters resembling stinging insects.
A female squash vine borer tidies up between bouts of laying eggs. It only takes a moment for the squash vine borer to deposit an egg on the underside of a leaf.
Frass marks the spot of the squash vine borer larva.
Upon inspection of leaves and stems visited by the female moth, I discovered small reddish eggs which spelled doom for my vines. In a week or so after the bombs are dropped, the eggs hatch and small caterpillars crawl earthward. They bore a hole into the hollow stem, often near the junction of leaf and vine. As the caterpillars feed, they push out grainy, yellow or green pellets of excrement known as frass. Frass is a sure sign of squash vine borers. Frass spewed from a bevy of holes lining each meter of my poor vines. Upon splitting open the vine, I was amazed to see several large and meaty caterpillars tunneling through the tender tissue within the stems. Larvae dining on vascular tissues disrupt the movement of water and nutrients to the leaves. When enough larvae are present, the vine and its attendant leaves wither and die. My vines clearly had enough larvae.
After completing their development, caterpillars spin a cocoon, turn into pupae, and then emerge as adult moths that wage another round of war on the vines. After delivering the coup de grȃce, the second generation of fully developed larvae spins cocoons. Inside these refuges the larvae or pupae escape the rigors of winter. Next spring, when foolish humans have replanted pumpkins or their relatives, adult moths will emerge from the earth to search for and ravage the next crop.
Tiny caterpillars will hatch from reddish eggs deposited by the female squash vine borer.
To defeat this pest, some people place nets over their vines in spring to prevent adults from reaching the stems to lay eggs. This practice of using floating row covers may help, but be sure to remove the covers when the vines begin to produce flowers, otherwise pollinators can’t do their job and you will have no vegetables. One study found that by injecting entomopathogenic nematodes into infested stems, borer larvae were killed on par with applications of conventional insecticides. Several insecticides are labelled for use against this pest, but if you go the pesticide route follow label directions and precautions precisely. Blue Hubbard squash is highly attractive to these fiends and by planting a sacrificial row of Hubbards early in the season you can lure the borers in and then destroy the plants and caterpillars, thereby reducing the crop of pests for curcurbits planted later in the season. This technique is called a trap crop.
My favorite strategy is to get down and dirty with the pest, hunting and crushing eggs and carefully inspecting vines looking for holes bearing frass. My experience has been that infestations often originate on the primary stem near its exit from the soil. When I find a likely hole, I carefully make a longitudinal incision along the stem to reveal the borer within. Once located, the caterpillar is easily dispatched with a sharp implement. As mentioned above, curcurbits vary in their susceptibility to this terror, with Hubbards and pumpkins favored and cushaws and butternuts less frequently attacked.
Several large, meaty caterpillars ravaged the internal plumbing of my wilted pumpkin vines.
I often battle the squash vine borer; sometimes I win and sometimes I lose. The good news is that my gourds are filling out early this year and despite my losses I should have enough to fulfill my dreams of Jack O’Lanterns and decorative baskets full of gnarly and weird gourds.
References
Bug of the Week thanks Sue Witte for the nice image of the adult squash vine borer that served as the inspiration for this episode. The wonderful references, “Insects of Farm, Garden, and Orchard” by Davidson and Lyon, and “Biology and Management of Squash Vine Borer in Organic Farming Systems” at the website listed below, were used as sources of information for this Bug of the Week. To learn more about the squash vine borer, please visit the following website: