Butterfly and moth eggs turn dark all over and never hatch? That’s trichogramma wasps infection, the miniature (almost microscopic to microscopic) parasitoid.
First, let’s look at a normal egg so we have a standard in our minds. Before a butterfly or moth egg hatches, it normally (depending on species) turns dark in one spot first. That spot is the head of the caterpillar.
But sometimes eggs turn all dark and never hatch. They aren’t infertile. Infertile eggs collapse upon themselves. No, the problem wasn’t a predator. Predators either eat the entire egg or drain it, leaving an empty shell. It’s trichogramma wasps, a parasitoid.
Trichogramma wasps are tiny, sometimes microscopic.
Trichogramma wasps often ride around on female butterfly’s body. When she lays eggs, they hop off and lay eggs in her eggs. We’ve photographed (not a good photo by any stretch of the imagination) a trichogramma wasp on the antenna of a Gulf Fritillary butterfly. We brought in Eastern Tiger Swallowtails for a few of their eggs. (Females can be released again after they lay a few eggs.) As we photographed the eggs laid, we found trichogramma wasps with them. They rode in on the female. We spoke with a Lepidopterist who collected butterflies for science. He found as many as 31 trichogramma wasps on one female butterfly.
The only way to totally avoid trichogramma wasps is to emerge adults away from eggs, in a closed room. Allow the adults to pair indoors and lay eggs indoors. Once they are outdoors, trichogramma wasps can find them and infect the eggs.
If you suspect that butterfly eggs may be infected by trichogramma wasps, place them in a clear SEALED container and date it with the date the eggs were laid. If you aren’t sure when the eggs were laid, date the container with the date you bring in the eggs. If a caterpillar emerges from any of the eggs, remove it and place it on a host plant. Before opening the container to remove hatchling caterpillars, hold the cup under a bright light, checking the lid for trichogramma wasps underneath it, crawling around on the underside of the lid. After a week in cooler weather or five days for warmer weather, destroy the contents by freezing for a week, flushing the eggs, or throwing away the sealed container in the garbage. Trichogramma wasps take a week to ten days to emerge.
The fine mesh popup habitats that are sold by Butterfly Conservation Supplies and Shady Oak Butterfly Farm have a mesh so fine that most trichogramma wasps cannot fit through the screen. Check out the photo to the left that has trichogramma and chalcid wasps as well as a tachinid fly lying on top of both window screen and the fine mesh used by Butterfly Farming Supplies.
Trichogramma wasps are available from many beneficial insect companies. Although butterfly enthusiasts consider them an enemy, farmers and (non-butterfly) gardeners consider them beneficial. To farmers and most gardeners, caterpillars are considered ‘worms’ that eat their plants. They are the most used biological control agent. Farmers began using trichogramma wasps as biological control agents in the early 1900s. There are over 200 species of trichogramma wasps.
Some of our photos were taken with a phone camera and are not good quality. Without a good camera in hand, photographing these miniature critters is difficult to impossible.