From the Viette's Views Gardening Blog
Pickleworms (Diaphania nitidalis) are pests of pumpkins as well as various types of summer squash, cucumbers, and cantaloupes. They literally devour the fruit from the inside.
These destructive pests are the larvae of a moth that doesn’t even survive in this area over the winter. They are subtropical insects and spend the winter in areas like Florida and Texas. In the spring, they migrate northward to invade vegetable gardens as far north as southern New England and parts of the Midwest. The only blessing is that it takes them a little while to make it up here so one way to avoid them is to plant squash crops as early as possible and/or plant early maturing varieties so you can harvest a nice crop before they become prevalent. That is easier to do with summer squash than with cantaloupe and pumpkins, though.
Unfortunately, pickleworms have a very short life cycle (often less than 30 days) and can produce up to 4 generations per year depending on the climate. In our area, I doubt they have more than two or three generations but this is still enough to wreak havoc in the garden.
It’s the larval stage that does all the damage. The young pickleworm larvae/caterpillars begin feeding as soon as they hatch from their tiny eggs. At first, they bore into and feed on the flower buds and the tender young stems of the plants. As these caterpillars grow, they molt several times going through 5 larval instars before they mature and pupate.
The major damage comes when the older pickleworm larvae bore into the fruit. The small entrance holes they make are tell-tale signs of the invasion. Sometimes you will see little piles of frass (insect “poop”) on the fruit just outside the holes. Inside, these caterpillars are voraciously consuming all the good stuff – hollowing out the interior and leaving the fruit inedible.
Normally pickleworms are difficult to control with pesticides because once they bore into the fruit, they are protected from sprays. Timing is critical; you have to control them when they are first hatching out.
When the buds first form and the flowers begin to open, spray the plants with Bonide Bt Thuricide or Bonide Captain Jack’s Deadbug Brew which contains spinosad. These products are normally less harmful to beneficial insects when used according to the label directions.
Pickleworms can also be controlled with Bonide Eight sprayed according to the label directions.
Always spray the plants early in the morning or in the early evening when the bees are less active. Pay close attention to the pre-harvest interval (PHI) when spraying vegetables.
In our area where pickleworms can’t overwinter, the best bet is to plant resistant varieties and early maturing varieties of squash and to plant these crops as early as possible. Starting the seeds indoors and planting hardened off plants can help you get a jump on the season. That way you can usually get a nice harvest before these caterpillars become prevalent. For us, they are normally a problem later in the season.
Pickleworms can do major damage to cucumber crops. Because summer squash is their food of choice, one suggestion to help protect cucumbers is to plant a trap crop of summer squash beside the cucumbers so that the pickleworms attack the squash and leave the cucumbers alone – this is supposed to work pretty well.