By Ron Goldy, Senior Educator
Growers are well aware of the need to rotate pesticide chemistry to minimize the chances of pests developing resistance. However, there is more to resistance management and pest control than just rotating chemistries. Understanding pest life cycles is another important piece to this puzzle. The life cycle characteristic I want to focus on for this article is whether the pest overwinters locally or migrates in each year from other locations.
Whether we appreciate it or not, northern winters are a benefit when it comes to keeping some of our worst pests in check. Some pests simply are unable to survive cold temperatures while others need live tissue to survive and the cold kills the host plant. This is true for some insects and diseases.
For migrating pests, the chemistries they are exposed to at their overwintering site and during migration are important factors when it comes to controlling them on your crops.
A good insect comparison is the difference between European corn borer and corn earworm. European corn borer overwinters locally and the population can be reduced by winter cold, especially in years with little snow cover, but some will still survive to infest subsequent corn plantings. The population of European corn borer that affects your crop this year was primarily exposed to the chemistries you used last year.
In contrast, corn earworm overwinters, starting in southern Ohio, and a number of generations are exposed to products to which they can develop resistance to before they reach your crop. It is possible the products you used last year that worked well may work poorly this year since the population may have developed resistance before reaching your location. Resistance developed in migratory pest populations may be due to no fault of your own.
An example for disease is downy mildew and Alternaria leaf spot of cucumber. Alternaria leaf spot overwinters in plant debris and is in your field waiting for you to plant a susceptible crop. According to Michigan State University Extension recommendations, the first steps in controlling Alternaria leaf spot is practicing good crop rotation followed with good rotation of pesticide chemistries.
Like European corn borer, the Alternaria leaf spot population in your field this year was primarily exposed to the control products you used last year. However, downy mildew is an obligate parasite, which requires live tissue for survival, so it completely dies out on your site and must come back from southern locations or greenhouse situations where it may have been exposed to several applications of a control product. Even if you have not used that particular product at your location to control downy mildew, the pathogen may have already developed resistance through exposure prior to arriving on your farm.
The “take aways” from this article are: Know the life cycles of your pests, continue chemistry rotation, and don’t expect products that worked last year to work this year on pests that have lengthy migrations.