The dose-independent lethal time of 20-hydroxyedysone ingested by Formosan subterranean termites

Subterranean termite baiting systems containing chitin synthesis inhibitors (CSI), such as noviflumuron, can provide colony elimination.  Commercial baits contain 0.5% noviflumuron, which is lethal, non-deterrent, slow-acting, and has a dose-independent lethal time.  The dose-independent lethal time allows large numbers of termites that have ingested a range of lethal doses to continue with normal behavior for several weeks before exhibiting mortality.  This dose-independent lethal time is unique to CSIs, and may be partly responsible for the colony elimination that is achieved using these compounds.  However, the time required for colony elimination is relatively long, and there is potential to improve baiting systems by shortening the lethal time.  The insect molting hormone, 20-hydroxyecdysone (20E), causes subterranean termite mortality within 12 days of exposure.  Using 20E in place of, or in combination with noviflumuron may result in a shortened lethal time.  The focus of this study was to evaluate the dose-independent lethal time of several concentrations of 20E and noviflumuron, alone and combined.  Groups of 30 Coptotermes formosanus Shiraki workers and 3 soldiers were placed into Petri dishes containing a paper disk containing one of the aforementioned treatments.  Survivorship was monitored daily for 14 days.  Termites ingesting 20E survived for around 6 days before exhibiting mortality, whereas termites ingesting only noviflumuron survived to the end of the experiment.  20E does seem to have a brief dose-independent lethal time, but additional studies involving whole colonies will be necessary to determine if 20E can eliminate colonies.