Population patterns changing for field crop pests

Ray Mueller

KIMBERLY – Historic population lows for some of the common and well-known pests in Wisconsin's major crops may suggest that “I might have to find another field to work in,” University of Wisconsin Extension Service entomologist Bryan Jensen quipped to attendees during the annual pest management update.

University of Wisconsin Extension Service entomologist Bryan Jensen explains how to count corn rootworm beetles during a video shoot.

Jensen cited the season-end report by the Wisconsin Department of Agriculture, Trade and Consumer Protection that the population of European corn borer larvae hit a 77-year low of .01 per corn plant, the corn rootworm counts in 2017 and 2018 for carryover to the next year that were thelowest in the 40 yearsof the collection of data, and that, once again in 2018, soybean aphid infestations remained well below the thresholds for yield and economic losses virtually everywhere in the state.

Remaining challenges

But those findings don't mean that the coast is clear for attention to both the damaging and beneficial insects which are linked to the state's field crops, Jensen cautioned. He referred to the threats posed by a few newly emerging insects, to the role of technologies in coping with pests and protecting beneficial insects, and to how farm management is linked to those concerns.

These western corn rootworm adult beetles emerge late June through
August and lay eggs almost exclusively in corn fields from early August through
early September.

Based on the .2 count of corn rootworm beetles per corn plant in the past two years and infestations in only 10 percent of surveyed fields compared to .4 to .6 beetles per plant and 16 to 24 percent of fields in the previous three years, Jensen suggested that corn growers could save on seed costs by not having the biotechnology trait for controlling the pest, particularly on fields where corn wasn't grown the previous year, or by using seed or soil treatment as a method of controlling the pest.

What bothers Jensen on that point is that surveys indicate that only 50 percent of corn growers scout for the presence of corn rootworm beetles in their fields. Without knowing that, it is difficult to make a good decision on crop input for the next year in fields with continuous corn, he advised.

An additional concern with the corn rootworm is the finding that Wisconsin has now joined several adjacent states in documenting instances of non-performance by all four of the proteins that were inserted in the biotech corn hybrids to control the pest, Jensen reported. For that reason, he recommends scouting, crop rotation, growing hybrids without the special trait, soil-applied insecticide, or seed treatment when corn rootworm pressure is known to be minimal.

Soybean strategies

If soybean aphid populations should rebuild, Jensen is proposing a new approach on how to decide if an insecticide treatment is needed to prevent an economic loss. The traditional point of concern was reached when populations topped an average of 250 per plant, he noted.

Jensen introduced the term “damage boundary” to refer to per plant populations of 400 to 600 which would probably lead to yield loss while a population of 675 or more is likely to cause an economic loss. But rather than relying on the numbers at any given time, he suggests creating a cumulative number by multiplying days of infestation by how many aphids there are per plant.

Entomologist Bryan Jensen is proposing a new approach on how to decide if an insecticide treatment is needed to prevent an economic loss.

For example, a relatively consistent number of 200 aphids per plant for 20 days would result in a “damage boundary” total of 4,000 while an accumulation of more 5,500 from that formula would probably constitute an economic loss without a treatment, Jensen indicated. “Late in the season, it's a tough decision on aphid control but there's less yield to protect by then.”

Japanese beetles

Japanese beetles, which feed on field, garden, and yard plants, seem to be most prevalent in soybean fields close to urban areas, Jensen observed. He noted that its populations tend to spike and then level off.

Don't be misled by the visuals when seeing Japanese beetles in soybeans, Jensen advised. He pointed out that feeding tends to be at the top of soybean plants, that 15 percent defoliation of the entire plant is needed for significant damage, that rushing to apply an insecticide would endanger the beneficial insects that frequent soybean plants, and that late season rains are likely to support the survival of Japanese beetle grubs through the winter.

Japanese beetles chew away at soybean leaves.

There's much less history and data about the two-spotted spider mites which have been mainly an isolated problem, Jensen added. Outbreaks tend to be associated with hot and dry weather.

Regarding true armyworms, Jensen observed “a surprise in the summer generation” with some severe outbreaks in corn, wheat, and pastures after a rather low first generation presence. Treatment will be most beneficial if made when the larvae are less than one inch long, which is when they feed most heavily, he explained.

Protecting pollinators

Because of their estimated $20 billion annual value in pollinating crops in the United States and the effects that a number of insecticides for controlling pests have on pollinator insects, another element of concern for Jensen is for the welfare and survival of those species, many of which are already coping with population losses. He noted that this applies to both the native species and the imported European honeybees.

Pest management also needs to take into account the welfare and survival of pollinators, many of which are already coping with population losses.

Among the native species Jensen mentioned are bumble bees (a social group with hives), sweat bees (solitary with nests in the ground), and leaf cutter bees (pollinators for alfalfa seed). The pollinators include other insects, birds, mammals, some butterflies, flower flies, and the hover flies which feed on soybean aphids, he added.

Social type bees live in colonies or hives – up to 50,000 for honeybees and 50 for bumblebees, Jensen pointed out. For most species, the nectar obtained from plant flowers is used by the adults while the pollen, which is protein, provides the nourishment for larvae, he explained.

Insecticide observations

While the active ingredients and application methods and timing (not always proper) of many insecticides can be toxic to beneficial insects, Jensen said one bright spot is the introduction of Sivanto Prime, a less toxic insecticide from Bayer which is labeled to control soybean aphids.

The overuse of insecticides also has the potential to harm beneficial insects.

On the regulatory front, Jensen is frustrated by the slow pace of federal level assessment and decision making on the use and/or rules on a number of popular products, including seed treatments, that can have a detrimental effect on pollinators. He referred to the issuance of “a proposed interim decision” and wondered how long it would for a final decision to be rendered.

Despite an August ruling by a federal appeals court ruling that the Environmental Protection Agency (EPA) should ban the use of Lorsban (active ingredient chlorpyrifos), the agency has not done so, Jensen reported.

“So it is still legal to use Lorsban,” which is a popular insecticide that has the potential to kill beneficial insects, he observed.

Jensen emphasized, however, that insecticides are only one cause of pollinator population losses. He mentioned mites, pathogens, poor nutrition, loss of habitat, reduction of genetic diversity, and inadequate management practices by some owners of honeybees.

Mitigation methods

To protect the pollinators, Jensen outlined a number of mitigation practices that apply to the use of insecticides on agricultural crops. Before any such applications, be sure to “practice integrated pest management and be aware of economic thresholds,” he emphasized.

“Spray late in the day to reduce the direct exposure to the pollinators,” Jensen advised. “Don't spray on crops that are not labeled for the product.”

Farmers should take measures to avoid liquid or particle drift to bee colonies or to their sources of pollen such as corn. The BeeCheck app for smartphones shows nearby locations of beehives.

Take measures to avoid liquid or particle drift to bee colonies or to their sources of pollen such as corn, Jensen stressed. “There's a BeeCheck app to find those locations.” It's also crucial to prevent drift to the few remaining sources of water during dry periods, he added.

Bees and other pollinators are attracted to clovers, alfalfa, soybeans, and corn as well as many wildflowers and weed species, Jensen pointed out.

“Be sure to look first and then apply the right product” within the confines of the EPA restrictions, he remarked.

Although Wisconsin does not have an apiary registry program, Jensen observed that beekeepers can request a 24-hour notice for any intended application within 1.5 miles of their colonies. In cases of perceived imminent crop damage, owners should give a 48-hour notice, he advised. “All involved and affected parties must communicate.”