Gardening in Almonte: Squashed!!

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David

As always there are winners and losers in the garden. This year has been great for veggies that like moisture and cooler conditions, such as salad greens. I have also had tremendous results with potatoes and carrots – a major caveat is that results depended on how heavy the soil is and how well drained.

A huge disappointment for me this year has been the squash crop, or rather the lack of a crop. I had made a fairly significant investment in time, seed and garden area in a couple of community gardens. Part of my reasoning was that it would be a good crop to follow on land that was used for potatoes last year. It also seemed to me that squash would be relatively easy to store and distribute as required, as most of the crop would be destined for the Food Bank.

About midway through the season, I noticed that some plants were wilting and others were simply not flourishing. I had attributed it to the continued wet conditions and poor drainage in some areas. Rather too late, I got down on my hands and knees and took a much closer look. What I saw shocked me – most squash plants were being eaten and destroyed by a pest that was attacking them from inside the stem, as the following photo shows:

The squash vine borer is a pest on pumpkins, squash, marrow and gourds and not one that I am aware that I had encountered before – this year it seems to be everywhere, in community and private gardens in Mississippi Mills and in Carleton Place. The adult is an attractive clear-winged moth with black and orange body and orange legs fringed with long, black hairs. The larva is the destructive stage of this pest as it feeds within the vine and causes the plant to wilt and/or collapse often causing death. Although squash is the preferred host, butternut squash is apparently immune to this pest.

The adult borer resembles a wasp. It is about 1/2 inch long with an orange abdomen with black dots. The first pair of wings is metallic green while the back pair of wings is clear, although that may be hard to see as the wings are folded behind them when they at rest. Eggs are flat, brown, and about 1/25 inch long. The larvae are white or cream-colored with brown heads, growing to almost an inch in length.

Beginning in late June or early July, squash vine borer adults emerge from cocoons in the ground. Adults are good fliers for moths and resemble wasps in flight. These moths are unusual because they fly during the day while nearly all other moths fly at night.

Soon after emerging, they lay eggs singly at the base of susceptible plants. Approximately one week after they are laid, the eggs hatch and the resulting larvae bore into stems to feed. The larvae feed through the center of the stems, blocking the flow of water to the rest of the plant. The larvae feed for four to six weeks, then exit the stems and burrow about one to two inches into the soil to pupate. They remain there until the following summer. There is one generation per year.

Often the first symptom of a borer attack is wilting of affected plants.  Wilting may occur only in strong sun at first but if the problem is left unchecked, the plants eventually collapse and die. Closer observation of a wilting plant often reveals holes near the base of the plant filled with moist greenish or orange sawdust-like material called frass (bug poop).

Squash vine borers are challenging to prevent or manage. Once the larvae invade the stem, it is difficult to treat. I have tried making a lengthwise slit in the stem and digging out the larvae and them mounding soil over the damaged stem – this seemed to have limited success. Other strategies that I am planning include:

  • Planting vine crops that are usually not attacked by squash vine borers, such as butternut squash, cucumbers, melons, and watermelons.
  • Planting a second bed of summer squash in early July after adult borers have finished laying eggs.
  • Promptly pulling and destroying any plants killed by squash vine borers.
  • Wrapping the bottom part of the stem with tin foil in mid-June as a barrier to deter the moth.

Another possible approach is the use of floating row covers over your vine crops when they start to vine (or for non-vining varieties, starting late June or early July) or when you first detect squash vine borer adults. The barriers need to be kept in place for about two weeks after the first adult borer has been seen and they need to be securely anchored to prevent adults from moving underneath it. You also have to ensure that squash were not planted in the same area the previous year as squash vine borers overwinter in the soil near their host plants. When the adults emerge the following summer, they may end being trapped under the row cover instead of being kept out. And most importantly do not use floating row covers anytime crops are flowering as they rely on access by pollinators.

[Material for this column was obtained from the Ontario Ministry of Agriculture, Food and Rural Affairs website and from the University of Minnesota website.]