Now is the time when a small pest that has the potential to do a large amount of damage will be hatching. I am speaking of the Squash Vine Borer, the larval stage of the clearwing moth Melittia cucurbitae, an insect so synonymous with the squash family that it has cucurbit in its name.
The adult clearwing moth, unlike many other moth species, is diurnal and is therefore active during the day. With its orange abdomen and clear wings it is often is mistaken for a wasp. The adults are now emerging from the soil where they have over-wintered as pupae. Anecdotally it is said that the squash vine borer lays its eggs when the blue chicory is in bloom and a drive along any of our major interstates will confirm that it is indeed blooming now. (image by Chris Evans, University of Illinois, Bugwood.org)
The eggs, which are very small, are laid singly at the base of the stalks near the soil. This will make it easier for the newly-hatched larvae to enter the stalk. Seven to ten days later the larvae, which are white with a brown head, will emerge from the reddish-brown eggs and within hours instinctively burrow into the stem to begin feeding.(image by Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org)
At this point symptoms will begin to appear starting with a wilting of the plant that recovers in the evening but progressing to a plant that does not revive in the evening or after watering. There may also be small entry holes visible at the base of the stem and sawdust-looking frass (waste). The larvae feed inside the stem for a little over two weeks, reaching 1” in length, at which time they exit the plant, burrowing 1-6” into the soil where they will pupate until next spring. I plant my cucurbits in upside-down coco coir liners that have a 2″ diameter hole in the bottom (now the top).
The small opening and the protective coco coir make it easier to cover the base of the plant with row cover cloth and harder for the larvae to get to the soil to pupate. In warmer climates there may be two generations per year so we are fortunate that Connecticut only experiences one generation each summer.
It is almost impossible to control the larvae once they have entered the stems. If Bt (Bacillus thuringiensis) is applied to the plant tissue that is near the area where the larvae will hatch then they will feed on the residues prior to entering the stalk. Bt is a common soil-dwelling bacterial organism that forms crystals of insecticidal toxins called Cry proteins or crystal proteins. When consumed by the larvae, the Cry proteins undergo a series of chemical changes to the point that they paralyze the intestinal tract and the insect starves to death. Also good to know is that mammals have no toxic or allergic reactions to Bt, it only affects species in the orders Coleoptera, (beetles), Diptera (flies), Hymenoptera (ants, bees, sawflies, and wasps), Lepidoptera (moths and butterflies, and nematodes. Bt can also be injected into the stem where squash vine borer activity is suspected making it the only treatment that may work once the borer is inside. Additionally, normal exposure rates of Bt will not harm bees so that is good news for our pollinators.
Butternut squash, cucumbers, and melons are not as susceptible to the squash vine borer as summer squash, pumpkins, and Hubbard squash, so plant the former varieties if you don’t want to deal with the borer. There are some practices that can be used if, like me, you can’t imagine a summer without freshly picked and grilled summer squash or a winter without home-canned ratatouille.
The best protection is to prevent the clearwing moth from laying its eggs in the first place. Row covers placed during the egg-laying period starting in mid-June can be highly effective, just be sure to remove when the blossoms are ready for pollination (or leave them on and hand-pollinate). If possible, don’t plant in the same location as the prior year. If it’s not possible to rotate, at least turn over the soil at the end of the season to expose the pupae to the freezing temperatures of winter.
For more information and control measures please check out our: Squash Vine Borer fact sheet.