Field Crops

Do Surrounding Habitats Influence Parasitism in Field Crops?

Every farmer knows that certain pests can invade their crops from surrounding habitats. But we also know that these habitats can be important reservoirs for natural enemies as well. Current research is examining how natural enemies exploit non-crop habitats with the goal of using this knowledge to help improve biological control.

One recent study examined the alternate hosts and habitats of Diaeretiella rapae, a parasitoid attacking various aphids including Russian wheat aphid, corn leaf aphid, English grain aphid, and greenbug. The study found that plants in the families of Chenopodiaceae (e.g. beets, lambsquarter), Cruciferae (e.g. mustards and cole crops), Solanaceae (e.g. horsenettle, potato) and Liliaceae (e.g. onions, iris), tended to harbor other aphid species that were attacked by D. rapae. The authors suggest that having such plants in the farm environment may help to stabilize the population of D. rapae and therefore aid it in controlling crop pests.

In another study, researchers asked if agricultural landscapes that contain more non-crop habitats support more diverse, and therefore more effective, communities of parasitoids. To assess this, they released armyworm larvae into cornfields in areas of high and low habitat diversity and recovered the larvae to determine the parasitoids that emerged. In one case the complex landscape had significantly more parasitism than the simple landscape (29.3% versus 3.7%), but there were no significant differences in the other two comparisons. In all cases, parasitoid diversity was about the same and dominated by just two species, Meteorus communis and Glyptapanteles militaris. The authors concluded that while landscape structure can influence parasitoid success, more needs to be learned about the mechanism of these interactions before they can be manipulated.

Sources:

Pike, K.S, P. Stary, T. Miller, D. Allison, G. Graf, L. Boydston, R. Miller and R. Gillespie. 1999. Host range and habitats of the aphid parasitoid Diaeretiella rapae (Hymenoptera: Aphidiidae) in Washington state. Environ. Entomol. 28: 61-71.

Menalled, F.D., P.C. Marino, S. Gage, and D.A. Landis. 1999. Does agricultural landscape structure affect parasitism and parasitoid diversity? Ecological Applications 9: 634-641.

Increasing Ground Beetle Numbers Results in Increased Predation

Frequent readers of MBCN are familiar with the predatory ground beetles in the family Carabidae. Many species of ground beetles occur in Midwestern cropping systems and are thought to be important predators of many pest species. A logical assumption is that if we could increase the overall number of carabids in an area we would also increase the rate of pest predation. However, in some cases this may not occur. Take for example, the case where one carabid species increases at the expense of another species that itself may be the more effective predator.

Researching the role of increased carabid populations on pest populations has proven to be a tricky business. Recently, researchers have tried another method to tackle this problem. By using trenches lined with plastic and varying the orientation and slope of the trench wall, researchers were able to create plots in cornfields that experimentally increased, decreased or left unchanged the overall population of carabid beetles. Increase plots had a vertical wall on the outside of the trench that allowed the ground dwelling carabids to fall into plots but not to escape. Decrease plots were reversed and the controls contained no trenches. Ground beetle populations were increased 54.2% and decreased 83.1% in comparison to the controls. Prey removal was significantly higher in the increase plots and was highly correlated with the abundance of the major carabid species (Pterostichus melanarius, P. permundus, P. lucublandus and Poecilus chalcites).

Source:

Menalled, F., J. Lee, and D. Landis. 1999. Manipulating carabid beetle abundance alters prey removal rates in corn fields. Biocontrol. 43: 441-456.


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