Adult Corn Rootworm Sampling and Economic Thresholds in Continuous Corn
Eileen Cullen, Extension Entomologist UW Entomology Department
Damage to corn roots by corn rootworm larvae in continuous corn fields during the 2006 growing season will depend on adult corn rootworm beetle populations in corn fields over the next 5 to 6 weeks. The only way to determine whether a field is likely to have an economic rootworm infestation next year is to scout for adults this August-September.
Corn rootworm beetles typically appear in corn fields beginning in mid-July. At that time a large distribution of the population has completed larval development (3 instars; finished feeding on corn roots), pupated and emerged from the soil. Adult corn rootworm beetles are pollen feeders primarily, but will also feed on silks and leaves. Western corn rootworm adults appear in corn fields slightly before those of the northern corn rootworm. Western corn rootworm male beetles begin to emerge before the females. Females begin laying eggs in the upper soil layers approximately two weeks after mating. In Wisconsin, corn rootworm beetle egg laying begins in August and continues well into September.
Rootworm beetle scouting should be initiated no later than early August, conducted three times per field at 7-10 day intervals, and continued through mid September. I discuss examples here of both UW Extension and Nebraska Cooperative Extension corn rootworm scouting recommendations and treatment guidelines. Both sources are cited at the end of this article and provide essentially the same example of how to use adult corn rootworm beetle scouting to make treatment decisions for next year’s continuous corn crop. The Nebraska information (Wright et al. 2003) is included because these authors have constructed a table by which you can adjust the adult beetle threshold by plant population (plants per acre).
Although no studies have developed thresholds specifically for northern corn rootworm beetles, it is accepted by field crop entomologists that thresholds developed in studies using western corn rootworms are acceptable for both species. Therefore, both western and northern corn rootworm beetles should be counted in the total beetles per plant tabulation on each field visit examining 50 plants per field.
The whole corn plant is examined for corn rootworm beetles. When you check the ear, grasp the ear tip tightly, enclosing the silks in the palm of your hand and count all other areas of the plant first. The silks typically have the most beetles on the plant, so a firm hold on the ear tip prevents the beetles from dropping. Open your hand slowly and count the beetles that come out of the
silks as you strip the husk away from the ear tip. Pull leaves away from the stalk a bit to examine the leaf axils.
The UW Extension sampling pattern is to examine 5 plants at each of 10 areas in the field. Based on a review of the research, Nebraska Cooperative Extension (2003) refines the recommendation, suggesting that the most efficient way to reliably scout for corn rootworm beetles and attain the 50 plant minimum sample is to: examine 2 plants per site and to sample 54 corn plants per field (27 sites). Plants should be sampled in such a way that some samples are taken from each quarter of the field (e.g., U-, V-shaped pattern). Sampled plants should be several paces apart so as not to rustle and disturb beetles
on the second plant while you are counting the first plant.
Determining the level of infestation this year will help you determine whether a soil insecticide, seed applied insecticide, or Bt CRW hybrid is necessary the following year. Importantly, it will also help you
identify fields that do not justify the cost of treatment. The standard economic threshold is to go ahead and recommend or apply corn rootworm control measure next year if you find an average of 0.75 beetles per plant during any of the three field samplings this August-mid September. The 0.75 beetles per plant threshold is based on 24,000 plants per acre (18,000 rootworm beetles per acre) and assumes a 50:50 ratio of males to females (Wright et al. 2003).
In Table III above, Nebraska Cooperative Extension expresses corn rootworm beetle thresholds as number of beetles per plant for various plant populations, ranging from 14,000 plants per acre to 32,000 plants per acre. If beetle numbers in a field exceed the threshold, there is a potential for economic damage if corn is planted back into the field the next year.
References used in preparation of this article:
Integrated Pest Management Program – University of Wisconsin-Extension, Cooperative Extension Service. Field Crop, Crop Scouting Manual. 2001.