Squash Vine Borer

2012 was a really bad year for squash vine borer in Michigan.  The outbreak was likely in part due to the mild winter of 2011 and perhaps the drought.  It was the worst season for this pest I’ve experienced.

As a formally trained entomologist and proponent of Integrated Pest Management, I’ve always found it very helpful to understand the life cycle of a pest, and integrate multiple types of control strategies in fighting it.

Squash vine borer is clearwing moth (Melittia cucurbitae, a moth in the family Sesiidae) common in the US east of the Rockies.  The adults are somewhat wasp-like and are active at night.  They lay eggs singly on many squash family plants.  They generally prefer winter squash, but can attack summer squash, pumpkin, gourd, and occasionally cucumber and muskmelon.

Eggs are small (about 1 mm / .04 in long) and reddish, and usually laid at the bases of plants and on leaf petioles starting around June.  Eggs hatch into caterpillar larvae about two weeks after being deposited.

Larvae develop inside the squash vines, going through four “instars” (growth stages) over about 4 weeks.  After completing their feeding, caterpillars burrow into the soil.  In warm climates, the caterpillars may pupate and emerge as adults for a second generation in summer, while in colder climates, caterpillars overwinter in the soil, pupating and emerging in spring to early summer.

There are a range of options for preventing damage from this pest, and gardeners should begin scouting, looking for signs of the borer, by early June in most northern climates.  There are pheromone traps that can be used to trap adult moths available commercially, as another scouting option.

In scouting, check the bases of plants and lower leaf petioles for signs of the pest.  Look for eggs and small holes with frass (insect droppings) or ooze coming out.

Exclusion can be effective to prevent egg laying.  Low-tech methods include wiping the bases of the stems to squish and remove eggs and small larvae on a weekly basis, or wrapping the base of plants with nylon pantyhose pieces or aluminum foil.

Floating row covers can be placed over plants to exclude adults, but need to be removed before plants bloom to ensure fruit production.  This is challenging in areas where there is a second generation.  Note that crop rotation should be used with this strategy, to ensure that adult borers do not emerge from the soil under the covers.

Physical control can be quite effective.  Borer larvae can be killed and removed from stems, if they are caught early, without harming the plants.  By the time vines have started to wilt, it’s generally too late, so regular scouting is needed to catch the larvae when they are still small.

At an entrance hole where a larva burrowed into the stem, carefully slice the stem carefully lengthwise a short distance, and kill any larvae inside.  Some gardeners use a small piece of wire to stab and remove larvae.  After surgery is complete, close up the stem and cover it with soil.  Plants will usually recover from their treatment quite well, if larvae are destroyed early enough.

Biological control of this pest is another option.  There are commercially available insect-eating nematodes (microscopic roundworms) products available commercially.  Syringes are be used to inject nematodes into damaged stems.  Note that beneficial nematode products have a shelf life, and anyone using them should examine a sample of the solution to ensure that the nematodes are active and alive.  This can be done with a dissecting microscope or hand lens.

Genetic resistance is also potentially useful in controlling this pest.  There is some variation in susceptibility to being damaged by squash vine borer among different squash varieties and species.  Hubbard types are very susceptible.  Solid stemmed types (including butternut squash, Cucurbita moschata) are less damaged by the pest, as are types that root easily at nodes.  Crookneck squash and green striped cushaw (Cucurbita argyrosperma, aka Cucurbita mixta) are also reported to be resistant to damage by this pest.

Chemical control is of course possible.  Note that chemical controls (pesticides) are not effective (other than systemic action chemicals) on larvae after they enter the plant stems.  As eggs are usually laid close to where the larvae will burrow into the plants, there is a relatively small window where the larvae are active and exposed to pesticides.  To minimize the amount of pesticide used, just the bases of plants can be sprayed.  Try to ensure that the leaf petioles are treated, not the leaf surfaces.  Weekly applications during egg-laying season may be required for good control.

In addition to conventional pesticides (including organic options like pyrethrin), there are some “softer” chemical options for control, like BT (Bacillus thuringiensis),  insecticidal soap, and kaolin clay.  Regular (every 5 to 7 days) applications are typically used throughout the egg-laying season.

I suspect that it might be possible to use trap cropping in combination with chemical control, as well.  Planting attractive, night-blooming species to attract adult moths, and treating those plants with a contact pesticide late in the day, might help reduce egg laying.

Keep in mind when using commercial chemical pesticides (even organic compounds), a homeowner is legally required to read and follow the label instructions.  Even organic pesticides can potentially be dangerous to the applicator and non-target species, if improperly handled or used.

Comments are closed.