The looming potential for spider mites in parts of Michigan raises another issue: the impact of preventative or insurance pesticide applications on insect or mite control. In the last few years, there has been a lot of research on the benefits of fungicide use, especially on corn and wheat. This season, I am not surprised to see a similar push for fungicide sprays on alfalfa and soybeans. Bulletins that discuss fungicide application focus on yield, and some of these yield gains have gotten the attention of growers and crop managers, especially given commodity prices.
The use of Roundup Ready corn and soybean has encouraged insurance applications because it is relatively easy and cheap to throw a second pesticide into the spray tank. Perhaps we are also more receptive to spending money on pesticide applications to field crops after our experiences during soybean aphid outbreaks, when single applications of insecticide kicked aphid butt and usually resulted in big yield differences. Many also spent money on new equipment to spray aphids and soybean rust, and the tendency is to put equipment to use. (I have referred to soybean aphids as a "gateway drug" that opened the door for more pesticide sprays on soybeans.)
Whatever the reason, more people are considering or doing "preventative" applications of fungicides and insecticides to field crops.
I have no problem with pesticide applications to protect yield based on valid scouting, trapping or weather data, but I am concerned with the concept of preventative or insurance sprays that focus on yield.
Bulletins showing reams of data on yield gain don't mention the potential, longer-term negative impacts of insurance sprays, specifically disruption of biocontrol that increases the potential for secondary pest outbreaks. For example, right now I am concerned that with current, dry conditions in parts of the state, fungicide or fungicide/insecticide tank-mix applications to soybean will cause spider mite outbreaks a few weeks later, because mites are usually held in check by predators and pathogenic fungi.
At a minimum, growers applying or paying for fungicide sprays should understand the risks (especially in dry weather) and pay special attention to sprayed fields for mite outbreaks.
As more and more acres are treated for yield gain, if insurance applications really are a problem, we would expect landscape-level changes in insect populations. I believe we are beginning to see evidence of this in the field, at least in Michigan. Cereal leaf beetles are at levels not seen for years. Reports of alfalfa weevils have increased. Odd, secondary insects such as grass sawflies and corn leafminers are popping up. In Michigan's armyworm outbreak in wheat, there seemed to be fewer parasitoids compared to previous years. The common theme is that these insects are mostly held in check by biocontrol, and biocontrol is disrupted by conventional pesticides.
Yield increases are an easy thing to measure, and that by necessity growers and companies focus on short term gains. However, routine insurance pesticide applications to major acreage crops may be having longer-term consequences, specifically the loss of free, community-wide biocontrol. Although difficult to quantify, this is a hidden financial cost for farmers that may balance out short term yield gain.
Dr. DiFonzo's writes from Michigan State University Extension, Department of Entomology, and her work is funded in part by MSU's AgBioResearch.