The recent Arctic-like weather has some people wondering what this means for insects during the upcoming growing season, particularly for the western corn rootworm. Will this severe cold snap kill eggs that are currently lying dormant in the field? Before we answer that, it is important to understand a little bit about rootworm egg laying behavior.
Female egg laying occurs over several months, approximately from July to October. The beetles require moist soil for egg laying and will not lay the eggs on the soil surface. Instead, females utilize various cracks and fissures in the field to deposit eggs. How deep into the soil the eggs are laid depends on the depth of the cracks, but most importantly where the moisture is located. Past studies1,2 have shown eggs can be recovered from as little as a half an inch below the surface of the soil and down to 8 inches or more. However, a majority of eggs are typically found 4 to 6 inches below the soil surface. In some cases, when soil cracks do not form, corn rootworms have been seen to utilize earthworm burrows for egg laying.
In regards to egg survival, temperature has been shown to have an impact. Results from a 1967 study3 showed that six or more weeks at a temperature between 14°F and -9°F prevented rootworm eggs from hatching. Egg survival significantly decreased in 3 to 6 inches of soil when the ground temperature remained below 18°F4. Additionally, a lab study showed that 97% of western corn rootworm eggs held at 14°F for three weeks failed to hatch5. In 2004, Ellsbury and Lee examined how sudden, brief cooling of western corn rootworm eggs affected hatchability6. Only around 40% of eggs hatched after a sudden hour-long exposure to 10°F. Eggs exposed to sudden temperature drops to .5°F and -7°F failed to hatch. Interestingly, in the same study, they showed that dry conditions did not negatively affect egg hatchability for western corn rootworm. Given these studies, it appears that prolonged low and/or sudden, intense temperature drops can kill eggs.
Given that Kansas has experienced record breaking air temperatures and wind chills this week, what does that mean for our soil temperatures? Using data from Kansas Mesonet, the average high and low air temperatures as well as average high and low soil temperatures at two locations in western Kansas were calculated for the current period of intense cold (Table 1).
Table 1. Average air and soil temperature for period between February 11, 2021 and February 17, 2021 at two locations in western Kansas during intense arctic weather. Note that snow, grass cover thickness, and soil type greatly influence soil temperatures. Data from Kansas Mesonet.
|
Average Air Temperature (F) |
Average Soil Temperature (F) |
|||||
|
High |
Low |
2" High |
2" Low |
4" High |
4” Low |
|
|
Garden City |
11.5 |
-6.2 |
25.6 |
17.3 |
25.4 |
19.4 |
|
Colby |
7.2 |
-9.7 |
26.8 |
23.7 |
27.1 |
24.9 |
Actual air temperatures, coupled with their sudden arrival at these locations, appear to be within the range of those temperatures that negatively affect rootworm egg hatchability. However, average soil temperatures at these locations (up to the writing of this article) are not as harsh as the air temperatures and had a much more gradual decrease over the time examined. The upcoming warming trend will also serve to bring soil temperatures back up. It is possible that rootworm eggs laid very shallowly or those that have become unprotected will not hatch in the spring, but eggs deeper below the soil surface are much more protected from the extreme cold temperatures in Kansas over the last few days.
References
1Kirk, V. (1979). Drought Cracks as Oviposition Sites for Western and Northern Corn Rootworms (Diabrotica: Coleoptera). Journal of the Kansas Entomological Society, 52(4), 769-776.
2Kirk, V. (1981). Earthworm Burrows as Oviposition Sites for Western and Northern Corn Rootworms (Diabrotica: Coleoptera). Journal of the Kansas Entomological Society, 54(1), 68-74.
3Patel KK & Apple JW (1967) Ecological studies on eggs of the northern corn rootworm. Journal of Economic Entomology 60:496-500
4Gustin RD (1981) Soil temperature environment of overwintering western corn rootworm eggs. Environmental Entomology 10:483-487.
5Gustin RD (1983) Diabortica longicornis barberi (Coleoptera: Chrysomelidae): Cold hardiness of eggs. Environmental Entomology 12:633-634
6Ellsbury, M.M. and Lee, R.E., Jr (2004), Supercooling and cold?hardiness in eggs of western and northern corn rootworms. Entomologia Experimentalis et Applicata, 111: 159-163.
Anthony Zukoff, Extension Entomologist – Southwest Research and Extension Center, Garden City
azukoff@ksu.edu