by Sonja Heyck-Merlin
“Insecticides are very popular in the United States,” said Penn State entomologist John Tooker. At the 2022 Northeast Cover Crop Council conference, Tooker focused on the overuse of insecticides in no-till corn and soy crops that incorporate cover crops. “They have been overused since they were introduced … and they continue to be overused. Even though we have had major checks on the system – the publication of ‘Silent Spring’ and the formation of the Environmental Protection Agency – from the time that insecticides were introduced shortly after World War II, the amount being applied has continued to increase every year.”
According to Tooker, the indiscriminate use of insecticide negatively influences beneficial predator and decomposer populations in no-till cropping systems that incorporate cover crops. Planting a fall cover crop and then no-till seeding a crop of corn or soybeans is popular, but Tooker said growers need to consider how their insecticide applications may be killing beneficial predators and decomposers.
Affected beneficial predators include ground beetles (both adults and larvae), rove beetles, ants, spiders and firefly larvae. Impacted decomposers include springtails (also called Collembola).
For example, growers often include insecticides in tank mixes of pesticides to try to control true armyworm and black cutworm populations, common pests of corn in Pennsylvania no-till, cover cropped systems. Both are migratory moth species which come from the south each spring, laying eggs on weeds (black cutworm) or cereal crops like rye and wheat (true armyworm). Typically, growers put insecticides out for these pests when they’re applying their pre- or post-plant herbicides. “This rarely provides a benefit because it is poorly timed. It doesn’t relate to when the moths are arriving and when the caterpillars are active, so it usually doesn’t do a whole lot,” Tooker said.
Another example of the overuse of insecticides, to the detriment of beneficial predators and decomposers, is planting seeds treated with neonicotinoid insecticides. When these seeds are planted, the water-soluble insecticide enters the soil where some of it is taken up by plant roots. The chemical then runs systemically through the plant, protecting young seedlings from insect pests. Tooker cited Penn State research showing that seeds treated with neonicotinoids reduced populations of natural enemies by 10% – 20% in North American and European farming systems.
Neonic seed treatments exacerbate slug problems, another perennial problem in no-till, cover cropped systems, Tooker’s research shows. “Where we have seed coatings, we have fewer predators that eat slugs because the neonicotinoids from the plants kill insect predators. Where insecticide-coated seeds are planted, we also have fewer Collembola, which play important roles in decomposition. With fewer Collembolans, residue breaks down more slowly and stays around longer,” Tooker said. Slugs thrive in residue-rich environments that provide them moist places to hide.
To boost beneficial predators and decomposers in these farming systems, Tooker wants growers to consider the original premise of integrated pest management (IPM), which was introduced by entomologists in the 1950s. “If you’re using IPM, you’re using a combination of biological, cultural and chemical tactics. But most importantly, the chemical tactic is the last resort, and I don’t think that is emphasized enough,” Tooker said. One of the original goals of IPM was to boost beneficial arthropod populations (or natural enemy populations, as Tooker calls them).
Tooker’s research reveals that no-till fields are great habitats for natural enemies. If cover crops are added to the system, it provides even more habitat for these natural enemies and extends their season. This provides an ideal starting point for IPM because the predators are already there.
According to Tooker, current crop production practices ignore IPM strategies – particularly corn, but also soybeans. “If we continue down this path,” he said, “we’re going to continue to decrease the abundance of good insects, leaving the fields more vulnerable to pests.” Research shows that the most effective way to deal with pests such as true armyworm and black cutworm is to scout and find infestations and then apply insecticides when their populations exceed economic thresholds. In the years when treatment is unnecessary, growers will be building a predator-rich system.
For example, to manage black cutworm and keep with the original spirit of IPM, a grower must diligently scout their corn fields. The larvae feed on young corn plants, making little holes in the seedlings. Older larvae will cut off plants at this base. “We have a good grasp on how to manage this pest species and good economic thresholds,” Tooker said. “So we know if we have two or three or five or seven cut plants for various growth stages, we know we should do something.” For the true armyworm, infestation to 25% of the corn plants warrants spraying an insecticide.
There is also increasing evidence, Tooker stated, that planting green can provide benefits for slug control. Planting green refers to the process of establishing corn or soybeans into a standing cover crop, and then spraying the cover crop with an herbicide, such as glyphosate, one to seven days after planting, so the cover crop dies slowly. Research shows that the slow death of the cover crop releases protein, attracting the slugs who are protein seekers.
Scouting, applying economic thresholds and planting green are IPM strategies that can help control pests in corn and soybean crops. In the long run, practices like these will protect natural enemies and save farmers money. “The large increase of insecticides in corn and soybean crops has nothing to do with pests becoming more abundant in these crops, and it has nothing to do with yield. It has everything to do with companies producing more of the stuff and selling it to farmers,” Tooker said.