Now that corn silks are out all over Iowa, many people scouting fields will notice a mixture of silk-feeding insects (e.g., corn rootworm, Japanese beetle, grasshoppers, etc.). Sometimes I get asked about how long corn rootworm will emerge. It all comes down to temperature. Like all insects, corn rootworms develop based on temperature and we can estimate the adult emergence period with a few metrics.
First, we need to capture the biofix, which is just a term for the very first one you see. This means fields have to be scouted before the biofix is anticipated. Usually, this is just before silking for most of Iowa. Scouts could walk through cornfields or use sticky traps to monitor for adult activity. Know that males emerge 5-7 days before females. If your biofix is female – likely you are too late.
Western corn rootworm adults, male is left and female is right. Photo by Purdue Extension.
Second, the emergence period is based on accumulated degree days (DD) after confirming the biofix. We can estimate this DD value with a few known variables: max and min temp for the day, and the lower developmental threshold (53 degrees). Use this formula to estimate DD: [(max temp + min temp)/2] – lower developmental threshold (53). For example, if the high was 94 and the low was 60, the DD would be 24. Accumulated DD would be a seasonal estimate of temperatures after the biofix.
Now, peak and duration of corn rootworm emergence can be summarized:
- Western corn rootworm males: 50% (118 DD) and 100% (505 DD)
- Western corn rootworm females: 50% (245 DD) and 100% (629 DD)
- Northern corn rootworm males: 50% (169 DD) and 100% (570 DD)
- Northern corn rootworm females: 50% (268 DD) and 100% (643 DD)
Note western corn rootworms develop faster than northern corn rootworms. Adult emergence could last 3-5 weeks depending on July and August temperatures. Warmer days will accelerate development compared to a cool summer.