Q: My pumpkin and squash vines started to wilt. I investigated the plants and found a white grub inside the vine. Do you know what it is and how to prevent it next year? I think some of the vines are going to survive, so that is good news.
A: Good work on finding the larvae of the squash vine borer. There are four life cycle stages to the squash vine borer: egg, larva, pupa and adult. The adults are clearwing moths that fly in the daytime resembling a wasp.
The pupa spends the winter in the garden soil. It is hard to see, even if the soil is tilled up, and it is pretty much impervious to insecticides. Rotating crops and tilling the soil are good ideas for many reasons, but they are not effective at preventing the borer. After hatching into an adult, the female moth lays eggs on the main stem of the vines and the leaves. The eggs hatch, and the caterpillars on the main stem make a hole and bore into the hollow stem. The caterpillars on the leaf walk down to the main vine and bore in when they get there.
The main efforts should be preventing the eggs from hatching and preventing the caterpillars from getting into the vine. Once they are in the vine, they eat for about a month without doing noticeable damage, and they are virtually impossible to reach with insecticides at that time.
In northern states, there is only one generation per year. After killing the vine, the caterpillar goes into the soil and pupates until the following spring. There is only one adult generation, and they are usually gone between mid-June and mid-July depending on where you live. Waiting until after this time to plant susceptible vine crops will be OK unless you are really far north and you plant a slow to mature crop.
A pumpkin variety that matures in 90 days after planting will be ready by mid-October, even if planted in mid-July. A variety that matures in 120 days will miss Halloween.
Plants set out into the garden earlier than July can be easily protected with row covers. They are wide strips of cloth that let sunlight, air and water through, but the mesh is so tight that insects can’t get in. The vines can grow well under the cloth, but the flowers can’t be pollinated by bees, since they also can’t get in. You can plant a self-pollinating variety, hand pollinate the flowers or wait until after the beginning of July to take the covers off for the rest of the summer to let the bees work. Don’t plant the vines in the same place as a previous year, because the borers could hatch into adults under the row cover.
In southern states where there are more than one generation of borers each summer, the row cover can be left on most of the season.
Without row covers, you can spray the vines twice a week with insecticidal soap or with a product containing the bacteria Bacillus thuringiensis, which only affects caterpillars of moths and butterflies. These sprays will kill the tiny eggs and caterpillars before they get into the vine.
As the vines grow out from the main plant, they can be occasionally covered with soil to cause them to send out new roots in several locations. If a borer damages a section of the vine near the original plant where the roots are, the remaining sections can remain alive with their own roots.
If you miss a borer, you can inspect for the hole and for the droppings that it pushes out that are known as frass. Slit the vine with a knife where you see the frass, and you should find the caterpillar. This of course damages the vine too, but it can prevent the whole vine from being killed, especially if there are new roots in other places.
