FEATURE ARTICLE

Classical Biological Control of a Midwestern Insect Pest: In Search of Colorado Potato Beetle Natural Enemies

The Midwest is plagued by insect pests that are not native to the area. Known as introduced pests, these insect invaders take advantage of human activity, prevailing winds and other transport systems to find new areas to live. Worldwide, approximately 35% of insect pests are introduced from another area.

Early biological control specialists recognized that introduced pests are problems because they exist in a new environment without the natural enemies that kept them in check "back home." Logically, these early specialists concluded that if one were to re-unite "old enemies," the pest density would decline and be maintained at lower levels over time. As readers of MBCN know, this "re-uniting of old enemies" is called classical biological control or importation. Worldwide, classical biological control has resulted in the complete control of dozens of pests and has contributed to the general lowering of the intensity of dozens more pest problems. It has been suggested that aside from cultural practices, classical biological control is the most successful form of pest control!

The question remains however, can classical biological control be useful to midwestern growers? To address the issue, we need to review some basic concepts of classical biological control to ask whether there are suitable target pests in the Midwest, and see if the Midwest has the necessary logistical and scientific capabilities to conduct classical biological control. Once we have addressed these questions, I will then describe our current search for Colorado potato beetle natural enemies in Central America to indicate the potential for classical biological control in the Midwest.

Basic Concepts. To do classical biological control, one needs several things. First, although the method does work for "native pests," the largest number of successes come from so-called introduced pests. Unfortunately, like elsewhere, the Midwest has it's share of introduced pests. Familiar foes like Japanese beetle, Mexican bean beetle, western corn rootworm and, yes, Colorado potato beetle, are joined by dozens of other unwelcome visitors. Alas for growers (but not entomologists!), we won't be running out of targets any time soon. Second, proper identification of the pest is an absolute must. Knowing the name of the pest puts you in touch with what is known of the beast and with the people who study it. But names can be deceiving! For example, if we searched in Germany for German cockroach predators, Colorado for Colorado potato beetle parasitoids and "out West" for Western corn rootworm pathogens, we'd be looking in the wrong places "German" cockroaches are from Africa, "Colorado" potato beetle is from Meso-America, and "Western" corn rootworms are from Mexico Since one needs to return to the original home of the pest to find its natural enemies, knowledge of the pest's distribution is critical and not always obvious! Once we properly identify the pest and know where it has come from, we need to go to the pest's home and search for its natural enemies -- this is known as foreign exploration. Besides technical skills, foreign exploration often requires knowledge of local customs and languages ... and the desire to travel to some forbidding places, often far from home. Once in-country, the biological control specialist must find the host plant of the pest (which is often NOT a pest in its native home), collect specimens of the pest, rear its natural enemies and then ship the natural enemies back to the U.S. to a USDA-approved quarantine facility. In quarantine the natural enemies are identified to species (if possible), reared on the pest for at least one generation, and are screened to see if they are carrying any undesirable diseases or attack other beneficial organisms. (The track record of the quarantine system is impeccable no insect that has passed through quarantine has ever been documented to cause environmental damage.) Finally, once through quarantine, the natural enemy is released, and its establishment and effect on the pest evaluated. With some luck, but a lot of hard work, the newly re-united pest and natural enemy return to their "old ways," reducing the pest population and continuously cycling over time.

Safety first! Classical biological control is one type of biological control best left to experts. All along the way from determination that the pest is non-native to the release of natural enemies, one can make a serious mistake -- for instance, releasing a natural enemy that actually attacks other beneficial insects. To do classical biological control, one needs specialized training in insect taxonomy, ecology and biological control, as well as international contacts and savvy of local customs and languages. Fortunately, we have just such things here in the Midwest!

Classical biological control in the Midwest. Simply stated, there is nothing in midwestern cropping practices, or weather conditions that precludes success in classical biological control. Indeed, for every "it won't work here" statement, researchers have shown that "it can work here!" Alfalfa growers are spraying significantly less insecticide because of introduced natural enemies that are combating the alfalfa weevil. In small grains, farmers no longer have to worry about the cereal leaf beetle because of the introduction of an egg parasite. And in potatoes, organic growers can use a combination of natural enemies, including an introduced egg parasite to combat the introduced Colorado (but not from Colorado!) potato beetle.

Ok, how about a quarantine facility, do we have one of those? Yes! A new facility at Ohio State that is currently evaluating natural enemies to control purple loosestrife (a noxious weed and yes we do classical biological control against weeds but that is for another article!). In a nutshell, the growth of biological control in the Midwest makes this the time to think classical biological control. The only thing stopping us is -- to paraphrase Pogo -- us!

Colorado potato beetle. If one wanted an example of what can go wrong by the unilateral use of insecticides, the Colorado potato beetle (CPB) is it. In the years since it moved across the U.S., our single-minded use of chemicals have made it one of the most resistant insects on the planet. Fortunately, with the development of an integrative control approach, we can manage CPB populations. The latest tactic in CPB control is to use plants that have been genetically engineered to kill CPB using a toxin derived from Bacillus thuringiensis ("Bt," a common insect pathogen). Ok, you ask: "Why continue with classical biological control if the "case is solved"? Good question! Given CPB's track record, the engineered potato will probably provide only temporary relief. And while integrated control works, several studies have shown that CPB lacks an effective natural enemy to move from programs dependent upon chemicals to control programs dependent upon natural enemies. The place to find new natural enemies is to go where CPB is from -- MesoAmerica.

Foreign exploration for CPB natural enemies. With a grant from the USDA Office of International Research, foreign exploration for CPB natural enemies began in June 1994. The project is a collaborative effort between scientists at Purdue (yours truly), Iowa State (John Obrycki), the Illinois Natural History Survey (Joe Maddox) and the Panamerican School of Agriculture, Honduras (Ron Cave). Exploration was focused in Honduras, since previous workers have conducted foreign exploration in Mexico and Columbia, but not Central America. We focused on the mountains of Honduras (above 4500 ft.) to try to find CPB populations in the higher/cooler elevations, hoping to find "cold hardy" natural enemies, that would perform better in the cool midwestern spring temperatures. Because CPB are relatively rare, we concentrated on a closely related species, Leptinotarsa undecemlineata (CPB is L. decemlineata). Also, as potatoes are sprayed frequently, we collected beetles on wild Solanum relatives of potato.

All together, we made 5 trips to Honduras, collecting for 2-5 days each trip. We traveled hundreds of miles in-country, but spent most of our time in western Honduras. We found literally thousands of beetles and reared thousands of their various life stages. Like most foreign exploration, there's good news and bad news. The bad news is that we didn't find THE natural enemy we felt could be worthwhile releasing in the US. We found several natural enemies that have been previously reported, including the egg parasite Edovum puttleri, several larval parasitoids (Myiopharus spp.), assorted predators (assassin bugs, stinkbugs, spiders, etc.) and a mite, tentatively identified as Chrysomelobia sp. We found a fungus associated with dead eggs, but it seems to be saprophytic (lives off of dead material) rather than pathogenic. We also found some pathogenic protozoans. Finally, we found a hyperparasite (Exoristobia sp.) of the fly parasitoids something we definitely do not want in the US!

So, what's the good news? In our collections, we noted that there seemed to be a significant decrease in beetle numbers from the pupal stage to the adult stage. The beetle pupates in the soil, suggesting that something in the soil is responsible for the decline in beetle numbers. Hopefully, that something will be a (new) natural enemy we can discover, import and evaluate as a control agent for CPB in the States.

The final piece of good news is that we collected the egg parasite at very high elevations (>5500 ft.), where temperatures drop into the 50's at night. We plan to return to this site, collect more parasites and see if they are indeed "cold hardy." If they can withstand cool temperatures, the egg parasite may be useful in an augmentation program, particularly early in the potato growing season when temperatures are relatively low.

Where do we go from here? We are, of course, disappointed that we have yet to find THE natural enemy for CPB. But we have not lost our resolve. Our collections in Honduras have taught us much about CPB and its natural enemies. In future work we will return to Honduras and determine why beetle populations decline so much between the pupal and adult stages. Could it be a new natural enemy or is it simply the mortality inflicted by the fly parasitoid we have already collected (the fly attacks the lava, but emerges from the pupa)? Are there "cold hardy" egg parasitoids and if so, are they more effective than our current strains? Could the saprophytic egg fungus be related to more pathogenic strains or could the eggs be being killed by a as yet unknown natural enemy, that the fungus is "taking advantage of" to invade the dead eggs? We are committed to further long term study of CPB in Honduras. One day soon, we hope to report in MBCN the discovery and successful testing of a new, efficient natural enemy that controls CPB. One thing is certain: if we never look for it, we will never find it!

- Bob O'Neil, Purdue University


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