Issue No. 3, Article 2/April 11, 2008
Baiting Recommendations and Toxicity Evaluations of Thianicotinoid Insecticides (Poncho and Cruiser) for Wireworms
The recommended scouting approach for wireworms is to establish bait stations a few weeks prior to planting. It is common knowledge that this pest management recommendation is rarely implemented in most commercial corn fields. However, in many areas of Illinois, planting this season will be delayed compared with previous years, so why not grab a shovel, get rid of some nervous energy, and give this approach a chance this year? According to the Entomological Society of America's Handbook of Corn Insects, the following procedures should be used in setting up bait stations:
- Establish stations 2 to 3 weeks before the targeted planting date.
- Target fields with a history of grass infestations or with small-grain stubble.
- Consider 5 to 10 bait stations per field.
- Bait stations should be 2 to 3 inches deep and 6 to 9 inches wide.
- Within each soil depression, place one-half cup of an equal mix of untreated corn and wheat seeds and replace the soil.
- Place a sheet of black plastic directly over the mounded soil area, then overlay a sheet of clear plastic on top of the entire baited area.
- Secure plastic with loose soil and flag each bait station.
- Prior to planting, check each bait station for wireworms. As few as one wireworm per bait station indicates a potential economic infestation.
Figure 1. Wireworm larvae.
Figure 2. Establishing wireworm bait stations.
Because of the escalating use of Bt hybrids, little regard is paid to potential wireworm infestations, the underlying assumption being that the thianicotinoid seed treatments (Poncho or Cruiser) will effectively control these secondary soil insects. But relatively few peer-reviewed articles have been published on the efficacy of these products against wireworm species. This is not surprising given the difficulty in establishing field plots with economic infestations. In the most recent issue of the Journal of Economic Entomology (Volume 101, issue 2), two papers were published by scientists who evaluated the lethal and sublethal effects of several insecticides, including Poncho (clothianidin) and Cruiser (thiamethoxam), against several economically important species of wireworms:
Vernon, R.S., W. Van Herk, J. Tolman, H. Ortiz Saavedra, M. Clodius, and B. Gage. 2008. Transitional sublethal and lethal effects of insecticides after dermal exposures to five economic species of wireworms (Coleoptera: Elateridae) [pp. 365-374].
Van Herk, W.G., R.S. Vernon, J.H. Tolman, and H. Ortiz Saavedra. 2008. Mortality of a wireworm, Agriotes obscurus (Coleoptera: Elateridae), after topical application of various insecticides [pp. 375-383].
I include here key points from each paper that shed new light on the performance of various insecticides used for wireworm control.
- The toxicity of various soil insecticides on wireworms and their behavior in the soil is poorly understood.
- Many field studies of soil insecticide efficacy on wireworms have been based primarily on stand counts. Using stand counts as the only measurement of efficacy may be inadequate and provide a poor assessment of product performance.
- Insecticide efficacy studies against wireworms should evaluate the influence of products on morbidity.
- Wireworms characterized as morbid (as indicated by writhing or leg or mouthpart movement) following an insecticide treatment may recover or eventually die.
- In some instances, morbid wireworms have recovered 100 days after treatment with certain insecticides. This potentially has important long-term management implications for wireworm populations in producers' fields. For instance, certain insecticides may prevent damage to seedling corn in the current growing season; however, if wireworms recover, densities could build to very significant levels across an area over time. Recovery of moribund larvae has been reported previously for several insecticides, including tefluthrin (active ingredient of Force). Neonicotinoid insecticides induced morbidity in wireworms when applied at concentrations close to the LC50 level (lethal concentration required to kill 50% of the exposed population). The morbidity of these wireworms was extended and required considerable time before they could be classified as dead or alive. Conversely, wireworms treated with organophosphate insecticides either died or recovered in a much shorter period.
- Mortality of wireworms exposed to clothianidin and thiamethoxam may occur, at least in part, as a consequence of wireworms' dispersing nearer to the soil surface and succumbing to dessication. Also, wireworms may become moribund and more susceptible to predation. Wireworm responses to these thianicotinoids require further field investigations.
- Greater concentrations of thiamethoxam as compared with clothianidin were required to kill one species of wireworm Agriotes obscurus. Thiamethoxam is metabolized to clothianidin within insects.
To learn more about the efficacy of these novel insecticides against wireworms, I encourage you to read the articles. Abstracts are available on-line.--Mike Gray