I was recently asked if adult corn rootworms are done emerging for 2019. Of course, I respond by saying – it depends! A publication by Nowatzki et al. (2002), developed an emergence model for northern and western corn rootworm in Iowa. It all starts with the biofix, which is just fancy way of saying “first catch.” After the biofix is reached, the duration of emergence is based on heat units, or accumulating degree days (ADD). The upper developmental threshold is 95°F and the lower developmental threshold is 53°F. In general, males develop faster than females and westerns develop faster than northerns.
* WCR Males: 50% (118 ADD), 90% (278 ADD), 100% (505 ADD)
* WCR Females: 50% (245 ADD), 90% (429 ADD), 100% (629 ADD)
* NCR Males: 50% (169 ADD), 90% (348 ADD), 100% (570 ADD)
* NCR Females: 50% (268 ADD), 90% (449 ADD), 100% (643 ADD)
For example, I saw my first male western corn rootworm at FEEL (demo farm between Ames and Boone) on 10 July 2019 (biofix!). The ADD since the biofix is 246, likely reaching 90% emergence by next week. You can make your own Iowa ADD map by visiting the ISU Mesonet.
More recently, research from ISU demonstrated those western corn rootworm populations exposed to Bt took longer to develop than those populations feeding on non-Bt corn (Petzold-Maxwell et al. 2013). What these finding mean is the emergence window may actually be longer than the 2002 model predicted. The implications for this delayed emergence of Bt-resistant rootworm may create asynchrony of mating with adults completing development on non-Bt corn.