Tillage disrupts insect habitats and causes changes in the species and numbers of insects. Reducing or removing tillage as done in conservation tillage profoundly modifies the agroecosystem, which influences insect population and species. The degree of disturbance varies with the type of conservation tillage. Seedbed preparation can range from complete burial of plant residue to no-till. Reduced-tillage systems can be used on a continuous basis, from year to year, or on an intermittent basis. In addition, double cropping is particularly popular in areas of the Southeast and often involves conservation tillage. An example is planting a winter crop followed by a no-till, late-spring or summer crop.
Both conventional and conservation practices modify insect habitat. The differences are described here.
Mechanical Disturbance of Soil
Soil disturbance exposes pests that live in the soil to predators and parasitoids. Parasitoids are insects that spend a portion of their lives in a pest host, ultimately killing the host. In addition, soil disturbance can crush pests in the soil or trap them by sealing exit tunnels. As a result, moldboard plowing or other soil inversion to bury crop residues and restructure the soil is traditionally recommended to control some soil insects. In conservation tillage, the destructive effects of this management tool are reduced or removed.
Crop residue on the soil surface reduces erosion and adds to soil organic matter. With conservation systems, increased crop residue and soil moisture reduce soil temperatures when compared with conventional practices. This favors certain soil-dwelling and litter-dwelling insects.
The soil structure is preserved under no-till, with increased moisture and aeration . Residue covering the soil surface decreases soil temperature, which may slow the growth of plants that are susceptible to insect damage in their seedling stages, such as corn. The soil’s long-term structural integrity may also increase the survival of soil-dwelling insects and other arthropods such as spiders.
Weed Diversity and Abundance
Reduced tillage or no-till can lead to changes in weed species, often fostering an increase in grassy, perennial weeds. Because many insects are associated with non-crop plants such as weeds, conservation tillage can lead to profound changes in insect abundance and diversity through changes in the weed community. Crop regrowth following harvest or termination with herbicides can also create changes in the numbers and types of insects present.
Because many conservation tillage systems involve multiple cropping practices and because shading by residue can slow the warming of soil in the spring, the crop is often planted later than in conventional systems. This may shift the types of pests in the soil and their activity. This risks exposure to more damaging pest populations or pest species.
Cover crops are an important part of conservation tillage systems and contribute to a greater diversity of vegetation. This can affect insect populations because insect outbreaks tend to be more frequent in systems with reduced vegetation diversity . Cover crops can increase the diversity and abundance of beneficial insects by providing additional shelter, nectar, pollen or food sources. Increased beneficial insect populations result in reduced pest populations. However, cover crops can also provide critical resources for generalist pests when the crop cannot support pest populations. The cover crops provide a temporary refuge for the pest until the crop is suitable for attack.