Insect problems vary in state's corn

Ray Mueller

Fond du Lac — Experiences with pests in Wisconsin's corn fields during 2016 and heading into 2017 are variable across the state, according to a summary given by Extension Service entomologist Bryan Jensen at the 2016 round of pest management updates.

Jensen noted that corn rootworm beetle population counts were down compared to a year ago in all but the north central and northwest agricultural districts among the nine in the state. As a whole, the number of adult beetles has been relatively low for the past three years, he observed.

Rather that focusing on areas or averages, Jensen cited “a lot of variability” between particular fields – some of which were over the threshold for likely economic losses for corn while other fields have no rootworm population. For that reason, he urges good scouting to identify infestations.

Pesticide resistance

What bothers Jensen is the development of resistance by the rootworms to all four of the biotech control protein traits in corn, most recently to the Herculex Cry 34/35 Ab1 trait in a few fields in Iowa during the spring of 2016.

“My question to you is 'what is the new corn rootworm silver bullet?',” Jensen remarked. “I don't know,” he confessed.

At the moment, Jensen's prescriptions to corn growers are to rotate the selection of the proteins in the hybrids they grow, to switch between granular and liquid soil applied insecticides if possible, to apply the insecticide only if beetle numbers were high in the field, to continue seed treatments, to know the past performance of each protein by checking corn plant roots in July, and to consider using two protein types at the same time though not in combination with a soil applied insecticide.

Western bean cutworm

Not only did western bean cutworm populations increase a bit in 2016 but there are also indications of a breakdown in the effectiveness of control by some insecticides, Jensen reported. He noted that 34 of the 229 corn fields sampled this year had moderate to high populations. Those fields were in Chippewa, Dunn, Green Lake, Juneau, and Marquette counties.

The particular concern with insecticide is with the Herculex Cry1F protein, Jensen pointed out. He said it is still effective for the European corn borer.

Similar findings with that protein have also occurred in Michigan, Indiana, Ohio, New York, and Pennsylvania, Jensen reported. At a minimum, do not count on the Cry1F protein to control moderate to high western bean cutworm infestations, he cautioned.

As alternatives, Jensen advised considering the Vip3A protein along with foliar application of the products in the pyrethroid, chlorpyrifos, diamide, or spinosad classes if scouting shows a need for treatment.

European corn borer

Perhaps due to the increase in the planting of conventional corn hybrids in order to limit crop input costs, there's been a bit of a resumption in European corn borer (ECB) populations in some areas after the state survey in 2015 indicated the lowest population in 74 years, Jensen observed.

That changed in 2016 with a few localized heavy ECB outbreaks, Jensen reported. His advice to farmers who will be growing corn without ECB control biotech traits in 2017 is to scout their earliest planted corn for ECB infestation and act accordingly. He noted that, through its first 18 inches of growth, corn is fatal to ECB feeding on it.

When there is a 50 percent ECB infestation (or an average of one borer for every two corn plants), Jensen calculates the possibility of a 4.6 bushel per acre yield loss on corn that would yield 185 bushels per acre.

At a corn selling price of $3.50 per bushel, Jensen indicates there would be a per acre loss of $16.10, $12.88 of which could be protected with the 80 percent control provided by an pesticide application to control an ECB infestation. He noted that this situation leaves the grower facing the question of whether the cost of an application would result in an economic gain or loss.