Invertebrate pests and their natural enemies in conservation tillage cropping systems
Prepared by Kristen Devlin
The most widely known benefits of cover cropping and conservation tillage -- like erosion control and nutrient retention -- have to do with the soil. Less well known is how these practices affect soil-dwelling animals. Penn State Professor of Entomology Mary Barbercheck studies how different tillage and cover-cropping systems affect invertebrate pests and their natural enemies. She shared some results from her research in the third installment of the 2013 Cover Crop Innovations Webinar Series.
“Cover crops can have a tremendous number of benefits in both conventional-till and in conservation-till systems,” said Barbercheck. Cover crops add diversity to a cropping system, which can help disrupt pest cycles. They also can serve as important food sources to predatory insects, especially in field crop situations where there aren’t usually many flowering plants. “Most beneficial insects that prey upon or parasitize pest insects also need pollen and nectar in their diets to complete their life cycles and to lay eggs,” Barbercheck explained. “And cover crops, especially flowering cover crops, can provide those.”
Cover crop residue can serve as habitat for insect predators, too. But, warned Barbercheck, that same residue can create a “green bridge” for pests, providing a food source that sustains pest populations before the cash crop emerges, making that cash crop more vulnerable to attack.
The choice of tillage practice also strongly affects the pest population, and the effect of different tillage choices varies from pest to pest. Black cutworm, true armyworm, and slugs seem to be favored by the heavy residue that may be present in cover cropped and no-till plots. These early season pests are of special concern, since they pose such a threat during the vulnerable germination and seedling stages of cash crops.
Barbercheck shared information about the life cycles of each of these three pests, as well as tips for how to prevent outbreaks and how to determine when rescue treatments are needed. She also shared a table from the Penn State Agronomy Guide; it shows which pests are favored by different tillage systems and is available as a free download <http://extension.psu.edu/agronomy-guide>.
Despite evidence that conservation tillage and cover crop use can favor some pest populations, Barbercheck emphasized that pests are not a guaranteed consequence of these production practices. “Pests in no-till are not a sure thing,” she said. “They are very sporadic and we can go years without pests. The bottom line is that scouting is recommended.”
Scouting, or monitoring fields for pest populations, helps growers determine which pests are present, whether rescue treatment is needed, and whether they should delay planting their cash crops to avoid the damage incurred by early season pests. Barbercheck shared a web-based tool <http://pa-pipe.zedxinc.com/cgi-bin/index.cgi?> that can help growers in their scouting efforts.
Where pests are a problem, a healthy population of naturally occurring enemies is one of the most important tools in combating them. “In our annual field-crop systems, the generalist predators are really the most important,” explained Barbercheck. She and her colleagues have found that reducing tillage and incorporating cover crops enhances and conserves common generalist predators in field crops. These generalists include spiders and harvestmen, ground beetles, predatory mites, above-ground predatory insects, and centipedes.
Of the generalist predators, ground beetles are the most important. They eat aphids, fly and beetle eggs and larvae, moth larvae, snails and slugs, and weed seeds. But, these voracious predators are also the most sensitive to management. A growing body of evidence shows that pesticides used in conventional no-till systems, especially neonicitinoid seed treatments, affect beetle populations both directly and indirectly. Barbercheck explained that exposure and distribution of these chemicals in the agricultural environment is very prevalent, and raised the question of whether losses of predator populations are outweighing the pest-suppression benefits through use of these seed treatments.
Webinar Recording and Further Resources
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