Jeff Burbrink with a tomato hornworm covered with parasitic wasp larva. Such larva should be left alone in the garden so the wasps can continue to control the hornworms. These wasps do not sting people.

By JEFF BURBRINK
Extension Educator, Purdue Extension Elkhart County

ELKHART COUNTY — When I have the opportunity to talk about insects with farmers or Master Gardeners or the public, one of the things you will probably hear me say is that insects often die horrible deaths, aside from splatting on the windshield. Predators, parasites and pathogens are a reality in the world of insects. 

There are many predators that love to eat insects. Spiders, rodents, birds, mites, beetles and bugs are often seen prowling about looking for their next six-legged meal. Ladybugs and praying mantis are the two creatures that come to mind. 

And there are parasites of all sorts too. There are many types of wasps and flies that lay eggs in or on the bodies of other insects. When these eggs hatch, the larvae consume the targeted insect, even as it continues to live out its life. 

A familiar example for many people would be tomato hornworm larvae parasitized by a small wasp, Cotesia congregatus. Larvae that hatch from wasp eggs laid on the hornworm feed on the inside of the hornworm until the wasp is ready to pupate.

The cocoons appear as white projections protruding from the hornworms body. If such projections are observed, the hornworms should be left in the garden to conserve the beneficial parasitoids. The wasps will kill the hornworms when they emerge from the cocoons and will seek out other hornworms to parasitize.

Pathogens, or diseases, are probably the least understood of the common ways that insects die. Yet pathogens often play a huge role in suppressing insect populations when things get out of control. 

An example of how diseases can keep insects in check occurred here a few years ago. The Indiana Department of Natural Resources had been trying to slow the spread of gypsy moth, an insect known to defoliate millions of acres of trees. The wave of insects was generally moving from the northern portions of Indiana to the south. IDNR was using a series of targeted sprays containing bacillus thuringiensis, a natural organism that can kill larva once it is consumed. More than 14,000 acres of Elkhart County were sprayed to get the pest under control.

Interestingly, however, a few years later, we had a cool, wet spring and summer, similar to 2015. IDNR was planning to spray again that year, but the weather encouraged the development of a fungal disease in the gypsy moth population, entomophaga, which very efficiently wiped out most of the population. It took years before we saw gypsy moth populations to come back in the county. 

The idea to commercialize predicators, parasites and diseases has been around for a while, and there are a few successful examples, such as BT and Milky Spore. In general, it is often difficult in practice to develop a biological spray, in part because you are dealing with a living organism, and in part, because you need to be sure to reduce the risk to non-target species. 

We all have been affected by a predator introduction that did not work out so well. The Asian lady beetle, which invaded our homes beginning in the 1990s, was a species introduced to control pests in gardens. Predator introductions, whether it be ladybugs or lions, really need to be thought out carefully.