[Annual Cockroach Control Issue] Horizontal Transfer & Cockroaches

We’ve seen it work with termites: In their daily foraging activities, the insects feed on baits and take the insecticide back to the colony, effectively spreading the insecticide and multiplying its impact. Over the past couple of years, researchers at Purdue University have been testing baits to see if they can get the same kind of horizontal transfer results among cockroaches. In the lab, they’re seeing great success.

“We’ve been looking at some of the newer products coming into play, and examining how these insecticides can be moved by foraging cockroaches — how they might distribute these insecticides to their brothers, sisters and other family members in the hiding areas where they aggregate,” says Dr. Gary Bennett, a professor at Purdue’s Urban Pest Management Center. “Specifically, we have done a lot of work testing an oxadiazine pesticide with German cockroaches. In the lab setting, we have achieved some remarkable results.”

How it Works. The concept of horizontal transfer is simple: Insects that eat baits or crawl over residual sprays pick up residues and carry them back to their colony, where the insecticide is passed from one individual to another. When enough insecticide is transferred, the result is secondary kill (primary kill is the mortality of the insect that fed directly on the bait or came into direct contact with the spray, because it eventually dies as well). This movement through the colony distributes the insecticide from individual to individual and into cracks and crevices where other insects are likely to hide.

Horizontal transfer shows promise in managing cockroach populations because it plays into the insect’s behaviors in a couple of important ways:

• Gravid females (those carrying egg cases) and young nymphs tend to stay in hiding almost continuously. Horizontal transfer provides access to these important components of the pest population.
• Nymphs are attracted to dying insects and will feed on their oral and anal excretions, particularly when no other sources of food or moisture are present. When a dying insect has been poisoned by insecticide, the nymphs feeding on its cadaver become poisoned as well.

In the lab, Research Assistant Professor Dr. Grzegorz Buczkowski and his team placed 50 German cockroach nymphs into a Petri dish and introduced an adult male that had fed on an oxadiazine bait. After 72 hours, 38 of the 50 nymphs (76 percent) were dead either through direct contact with the live male or through ingestion of his secretions once he had died.

Next, the research team created a situation to find out if the nymphs that had fed on the dying donor could transfer the insecticide to other individuals (tertiary mortality). Affected nymphs were placed into a Petri dish with 20 adult males, and 16 of the 20 adult males (80 percent) died.

Bennett warns that these results must be considered in the context of the laboratory setting, explaining, “We are working under controlled conditions in the lab. Out in the field, where cockroaches are widely scattered and have access to a variety of food sources as well as moisture, PMPs are not likely to achieve these high levels of secondary and tertiary mortality. However, the overall implications related to the potential for increasing management effectiveness through horizontal transfer, particularly in getting to populations in hiding, are positive.”

Another positive finding is that the cockroaches that feed on insecticide baits, effectively becoming “walking bait stations,” bring about secondary and tertiary mortality even after their own death. Purdue’s research revealed that the toxicity of the cadavers didn’t wane even after several days.

Minimizing Resistance. Of course one of the issues that always comes up in a discussion of managing cockroaches is resistance. “We need to use insecticides judiciously, because overuse leads to resistance,” says Bennett. “If you look back at synthetic insecticides dating back to the chlorinated hydrocarbons of the 1940s, you see that we’ve developed resistance to every category of insecticide introduced to the market. Most recently, we’re seeing cockroach populations becoming resistant to newer materials on the market, including spray and bait active ingredients. We need to use presently available chemistries in a more managed way so that they don’t end up losing their long-term effectiveness.”

Purdue Associate Professor Dr. Michael Scharf, an urban entomologist heavily involved in both lab and field experimentation, is studying the genetic makeup of cockroaches to learn more about the activities controlled by genes within the insect and seeking ways to modify those activities to discourage insecticide resistance.

Scharf’s research includes administering insecticides to test subjects in very small doses to watch the progress of resistance in the insect. After a period of time, these subjects are tested against a population collected in the field to see how each group responds to various insecticides. In many cases, the test subjects exhibit a rapid buildup of tolerance to the pesticides, reflective of how resistance evolves in the field.

“We have to be smart, using new formulations and different formulations,” concludes Bennett. “If you depend on using the same material over and over again, it may work initially, but eventually, the cockroach will win. Don’t let that happen. Minimize the evolution of resistance by staying informed about new products and using them wisely in conjunction with other baits, sprays and dust.”
 

The author is a Cleveland-based freelance writer. E-mail her at ddefranco@giemedia.com.