By James Tansey, PhD. Provincial Insect/Pest Management Specialist, Regina
As in most years, we have seen heavy insect pressure in some crops this year. These range from the perennial issue of flea beetles in canola, localized wheat midge pressures and high grasshopper numbers in several crops. Can localized insect pressures be influenced by crop rotation? That is, can last year's crop affect the numbers of insect pests in subsequent years? The short answer is, it depends.
Many insect pests we deal with in Saskatchewan are specialists. That is, through co-evolution with specific plant families and biochemical defences that evolve to prevent insect feeding, their food choices are limited to closely-related plants that share a similar defence strategy. An example of this is the crucifer flea beetles, Phyllotreta cruciferae. This insect shared a co-evolutionary history with members of the Brassicaceae plant family, like canola. Defence compounds such as the breakdown of products in glucosinolates actually act as attractants to these beetles. They have evolved to get around potent defences to take advantage of these hosts. Physical and biochemical properties of potential host plants influence the choices specific insects make.
A "host" is a term to describe the suitability of a plant as food or a resource for the development of young insects. A "true host" allows the development of juvenile insects; a "food host" can be used by adult insects for nourishment, but will not support development. An example of this is the relationship of cabbage seedpod weevil (CSW) and members of the Brassicaceae. Adult weevils will feed on a number of species within this plant family, but egg laying typically occurs and larvae can complete development on only a few member species. True hosts for CSW include canola and brown mustard; food hosts include brassicaceous weeds like flixweed. This means that contributions to the population associated with larval development can contribute to next year's problems in another food host planted in the same area.
Although this seems like a good general approach to predicting problems, it can be confounded by two major factors: generalist feeding and insect movement, including to and from overwintering sites.
Generalist feeding is not limited to one or two species, but can feed and develop on multiple plants within a family or even among members of multiple plant families. Examples of insects that employ this feeding strategy include the Lygus species and several of the grasshopper species that can be problematic in crops in Saskatchewan. Although generalists demonstrate preferences among plant species, they are capable of exploiting a broad group of potential hosts. Predictions based on last year’s pressures can be problematic in these cases.
Many insects overwinter as adults and move into crops once activity begins in the spring. Some are also excellent flyers and can cover great distances to feed or lay eggs. This reduces the predictability of pressures based on last year's incidence. However, given a choice, many insects will limit their movement as much as possible, making preferred hosts near sites of adult emergence or deposition of eggs (in the case of those insects that overwinter as eggs or larvae), more susceptible.
Some local insect pressures are also strongly influenced by climate. For example, wheat midge overwinters in the ground as larvae and pupae. Survival and emergence of adults in the summer is poor if spring conditions are dry. Heavy local populations, followed by moist conditions the following spring, coupled with the presence of a suitable host like wheat can lead to problems. Grasshoppers are another example of populations influenced significantly by weather. In the case of most pest grasshopper species, moist conditions contribute to outbreaks of fungal and bacterial diseases and are detrimental to population growth.
Crop rotation can affect insect pressure year to year but many caveats need to be considered. These include: the host range of specific pests, climatic effects and mobility of pests. In general, rotation away from preferred hosts in subsequent years can be beneficial for controlling damage by many specialists. Work continues on increasing our understanding of the interaction of these factors and predictive power will certainly increase. If you have questions on crop rotations and its impact on insects, contact…