Good Parasites

Black Swallowtail Parasite- Barnhart

We recently wrote about parasitic wasps which lay eggs on caterpillars, hatching larvae which feed off the caterpillar and usually kill it in the end.  They are a major cause of caterpillar mortality.  We have special mesh at the Butterfly House sized to prevent most of these small wasps from getting to our cats.  We tend to hover like parents over the cats while wishing the were no parasitic wasps and flies anywhere.  We need to be careful about what we wish for.

Parasite has become a derogatory word.  It is used to describe con men, loan sharks and even a relative who moved into your basement "for a few days" and is still there two years later.  Who ever heard of a good parasite?

Parasites are actually an important part of nature.  Just as coyotes and foxes have cycles in the control the potential hordes of rabbits in our fields, parasites control the reproduction of their specific host species.  If they kill too many caterpillars, they will face extinction.

Variable Oakleaf caterpillar-FIDL

Sometimes the system gets out of balance.  The variable oak leaf caterpillar, Heterocampa manteo, is a common larva of a rather undistinguished moth.  It eats the leaves of many species of trees but especially attacks oaks.  Occasional heavy infestations can affect millions of acres and extend over hundreds of miles.  Twice in the last 30 years a swelling of their numbers has defoliated 7-8 million acres in Missouri. Although they may slow the growth of an individual trees, they rarely causing long lasting damage.*

Why they have big years is unknown but you can be sure that their parasites have greater numbers the next few years.  This is an important factor in controlling a large outbreak.  Fortunately, they have lots of parasites which serve to drastically reduce their numbers back to normal.  The Forest and Insect Disease Leaflet (FIDL) -no government pun intended- gives a lot more information.

"In years following large infestations, the egg parasites Trichogramma sp. and Telenomus sp. may kill 90 percent of the eggs. Nearly all egg masses have some parasitized eggs; only the eggs concealed within a cluster escape. This high level of parasitization, plus the failure of many prepupae to pupate in the spring, appear to be the major reasons for lack of consecutive heavy defoliations.
At least seven species of larval parasites attack variable oakleaf caterpillar larvae. The most important species are Diradops bethunei Cress (Ichneumonidae), Protomicroplitus schizurae (Braconidae), and Lespesia schizurae (Tachinidae). Combined larval parasitization may kill 90 percent of the larvae."

The lesson of all this- sometimes you really need good parasites.  Just imagine what would happen of a destructive caterpillar had no natural parasites.  Stay tuned for the exciting sequel,  Good Parasites II

* See the latest Missouri Natural Areas Newsletter