Here’s the buzz on wasps: Some are actually beneficial
Question: I was looking for green cabbage-moth worms on my broccoli and cabbage plants when I saw one of the caterpillars next to two piles of eggs. (I don’t really know what they were, but they looked like eggs.) They were on both sides of the caterpillar, and they look yellow and fuzzy or furry.
I didn’t know that caterpillars could have eggs, so maybe they are from something else. But the caterpillar was still in the same spot hours later. What was this? And should I do anything about it?
Answer: Congratulations for noticing something so small. Keep watching — you will see a crazy natural pest control. You are not looking at eggs; rather, those are the cocoons of the pupa stage of a very small wasp.
The wasp is the parasitoid of the cabbage worm. A parasite stays in a host for a long time, feeding from and living on it, while not killing the host. A parasitoid feeds and lives for a while but eventually kills the host.
This wasp has no common name, but the zoological name is Cotesia glomerata. It was imported to North America back in the 1880s to control the cabbage worm, which eats plants in the cole-crop family, such as cabbage, broccoli, Brussels sprouts and cauliflower.
When people hear the word “wasp,” they usually picture something 1 or 2 inches long at the smallest. Most wasps, however, are less than a quarter-inch long and harmless to people and pets.
The adult stage of Cotesia glomerata is only about a millimeter wide and 2 or 3 mm long. The female lays eggs inside the inch-long caterpillar. In a few days, the new wasps eat the nonvital organs and then bore out the sides, leaving behind the caterpillar to die within a day. They weave a fuzzy cocoon and hatch into adult wasps in about a week.
The female can lay 20 or more eggs in a caterpillar, and she can lay eggs in 15 to 20 caterpillars. The entire life cycle takes about a month.
Early in the season, there are fewer wasps than caterpillars. But as the season progresses, as many as 75 percent of the caterpillars can be infested with wasps. Unfortunately, pesticide sprays that kill cabbage worms kill the wasps, too. Look at the bottom of the host plant’s leaves and look for the chrysalis of the cabbage-worm butterfly. If you see some that are brown instead of green, you are looking at the damage caused by another parasitoid wasp that kills the pupa instead of the caterpillar: Pteromalus puparum.
Making the story even more interesting: There are other wasps that lay their eggs in the cocoons of the Cotesia glomerata. They are hyperparasitoid wasps that kill the parasitoid wasps.
One problem with the introduction back in the 1880s was that not much research had been done into whether or not the wasp would infest native caterpillars. And even if people had done that research, they would probably have thought it would be a good idea to kill any caterpillars. Unfortunately, it does appear that the wasp has lowered the population levels of some native butterflies.
The wasps will be so small that you will probably think they are gnats when you are out working in your garden. If you want, pluck the leaf off and leave it in a safe place in the garden so that the wasps can hatch. Enjoy your natural pest control.
Jeff Rugg is a Creators.com columnist.