Cucurbit feeding: A bean leaf beetle feeding on a pumpkin (var. "Magic Lantern"). Photo is figure 2 from Koch et al. (2004).
The bean leaf beetle is native to North America and may be common anywhere soybeans are grown. It has a large host range (Bradshaw et al. 2007) that mostly includes leguminous plants such as soybean, green bean, and clover; however, it will occasionally feed on stinging nettle (Helm et al. 1983) and cucurbits such as pumpkin and cucumber (Koch et al. 2004).
This beetle is multivoltine (three generations per year) in the southeastern U.S. but is bivoltine in Iowa [Smelser and Pedigo 1991] and Illinois, uni- or bivoltine (two generations per year) in Wisconsin, and univoltine (one generation per year) in Ontario, Canada. As an adult the bean leaf beelte mostly overwinters within leaf debris in woodlots (approximately 80%) (Lam and Pedigo 2000); however, some do overwinter is soybean field residue (approximately 20%).
Predicted mortality of overwintering bean leaf beetles: Predicted cumulative mortality of bean leaf beetles from the nine crop reporting districts in Iowa from October 1, 2006 to April 14, 2007. Estimated mortalities are derived from the predictive model by Lam and Pedigo (2000). Note the variability in the cumulative mortality between the different regions in Iowa.
Survival for overwintering beetles is governed by the number of days accumulated below the temperature of 14oF (-10oC). Studies by Lam and Pedigo (2000) found that > 50% of bean leaf beetles can survive for hundreds of hours at 23oF (-5oC); however, most beetles died by 15 minutes at 14oF (-10oC). In southern latitudes it is not clear what key factors are responsible for mortality during the winter; however, weather is thought to be important.
Seasonal cycle of the bean leaf beetle: The bean leaf beetle has three populations that produce two generations of beetles annually in Iowa. The first population are actually the second generation adults from the previous year. This figure is based on weekly 50-sweep samples averaged from three alfalfa (overwintered adults) and soybean (first and second generation adults) fields (4 sampling units per field) from Ames, Iowa (Johnson Farm, Iowa State University, 2002 and 2005).
In the spring, adult beetles emerge from overwintering habitat and migrate to available host plants -- i.e., they are commonly found in alfalfa and other legumes such as tick trefoil and various clovers in late April or early May (Smelser and Pedigo 1991, Krell et al. 2003, Bradshaw et al. 2007). However, as the season progresses bean leaf beetles are found on more preferred hosts (e.g., soybean or green bean). Note that, although these first beetles begin to appear well before soybean emergence, peak abundance can coincide with the emergence of soybean (depending on the date the soybean was planted).
Adult beetles that colonize crops such as soybean will also reproduce in those fields and deposit eggs in the soil near the bases of the plants. In about one week (at 82.4oF or 28oC), the eggs will hatch and the larvae will disperse into the soil to feed on soybean roots and root nodules (McConnel 1915, Leonard and Turner 1918, Pedigo 1994). The larvae will develop through 6 instars and for about [X days] pupate in the soil. In total, it takes beetles about 3 weeks to develop from the time the egg hatches until eclosion (emergence from the pupal stage) of the adult at 82.4oF or 28oC. However, larval survival requires highly organic soils ( approximately 66% organic matter) and is inversely proportional to clay content (Marrone and Stinner 1983). During their time in the soil, bean leaf beetles are most susceptible to heavy rainfall (Lam et al. 2001). Once the adult beetles emerge from the soil, they remain soft and probably flightless (these beetles are termed "teneral" or "callow" adults) (Chapman 1998) for a short time until their exoskeleton hardens. In Iowa, the time required for the development of these first-generation from egg laying to adult emergence is 1,212 Degree Days with a developmental threshold of 46oF or 7.8oC (Zeiss et al. 1996). Depending on the length of the growing season, these adults either produce another generation or move to overwintering sites when the host plants begin to senesce.