Sep 17, 2018Array of methods needed to curb the pesky squash vine borer
The squash vine borer (Melittia curcubitae) is a menace to squash, zucchini, and pumpkins and presents major challenges for gardeners and smaller-scale farmers – by the time these crops have wilted, it’s usually too late to stop the pest.
Squash vine borers are native to North and South America, and their range extends from Canada and the United States east of the Rocky Mountains all the way south to Argentina. The damage to squash and related plants starts when adult females lay eggs on the plants’ stems and leaves, which hatch into larvae about 1-2 weeks later. The larvae burrow into plant stems and crowns, eating surrounding tissue and causing the all-too-familiar wilting damage to the plants. Through a series of instars, larvae mature and leave the plants to burrow into soil, spinning a cocoon to pupate over the winter. Pupae then emerge as adult moths, which begin mating and starting the egg-laying process anew.The borer is a bit of a chimera. As an adult, it looks like a wasp but is actually a moth (part of the order Lepidoptera). But it’s the larvae that cause wilting trouble, not by infesting squash directly but instead by eating away at the vines that supply water and nutrients.
The insects cause a great deal of damage to the squash crop, valued in the United States alone between $120 million and $191 million. They cause a great deal of damage to small farms and gardens, because larvae tend to concentrate on a small number of individual plants, but commercial producers have reported losses up to 25 percent of their crop. Even a single larva can cause yield reductions of up to four percent in pumpkins.
To review the status of vine borers and their impact on cucurbit crops, Eric Middleton, an entomologist at the University of Minnesota, examined the life cycle and natural history of M. curcubitae and the methodologies used or being considered to control the pest. His review was published in August in the open-access Journal of Integrated Pest Management.
Most gardeners and farmers resort to monitoring using a pheromone trap to determine vine borer infestation. Injury levels vary according to type of crop. Five adults per trap every week is a standard level to determine economic injury, though some researchers have suggested levels as low as two adults and as high as 12 adults for vining squash and pumpkins.
Managing vine borers has always been very challenging, so it probably comes as no surprise that growers find themselves using a variety of methods to stave it off:
- Crop rotation. Rotating crops can prevent planting squash on infested land (because larvae overwinter in soil) and can be effective especially if fields are far apart.
- Resistance. Planting cucurbit cultivars that resist vine borers can help prevent infestations, but many varieties that resist the vine borer are not as frequently grown and are not as popular with consumers.
- Sanitation and tillage. Disking or tilling old crop into the soil can destroy larvae by killing them directly or burying them too deep in the soil to survive. However, the techniques are not as effective against heavy infestations and reduce beneficial pollinator populations.
- Trap cropping. Planting more attractive varieties around the squash crop can divert borers away from the favored crop. However, the trap crops (like blue Hubbard squash) must be significantly more attractive to draw borers away.
- Planting timing. In northerly latitudes, early plantings can see greater damage, while later planted squash mature after borers have laid their eggs. Early plantings in lower latitudes, however, can see less damage.
- Row covers. Lightweight cloth can exclude insects, preventing egg laying, if the covers are fastened firmly to the ground. Covers must be removed to allow pollination, which can be problematic because pollinators are active at the same time as squash vine borers.
- Chemicals. Insecticidal chemicals are common and effective, but timing is important to hit larvae before they are inside the squash stems. Pyrethroids are the most common chemical used against vine borers. Spinosad can be used in organic as well as conventional agriculture, but formulations not allowed in organic farming have been seen to work against borers. Carbaryl also reduces borer levels comparable to pyrethroids but isn’t as consistent against the pests. Other alternative chemicals—some of which can be used in organic applications– include Kaolin clay, neem, or thyme oil. Not much scientific research has been conducted on how well these work on squash vine borers, though, and the studies that exist suggest weak effectiveness.
- Biologicals. Little is known about biological control methods against the squash vine borer. Some parasitic wasps and species of ground beetles can feed on borer eggs but don’t destroy significant populations. Nematodes show more promise, in one study providing as much protection as a now-banned insecticide, endosulfan, but larval mortality never exceeded 40 percent. Bacillus thuringiensis (Bt) sprays and injections are showing more promise as a control over borers and may be a viable option for organic and conventional growers.
This range of control, and the persistent manner in which squash vine borer can infest plants, show the need for a comprehensive integrated pest management (IPM) strategy, based on knowing the natural history of the pest, the geography and climate (and size) of the growing area, and grower preferences. So more needs to be studied, however, on the most effective control methods for this persistent pest, especially on smaller plots.
– Andrew Porterfield