Progress Report to the Maine Potato Board Research Subcommittee

Project Title: Field Performance of the Resistance Management Plan for the Colorado Potato Beetle

Executive Summary: Detection of at least eleven Colorado potato beetle populations that are either tolerant or resistant to imidacloprid confirms that imidacloprid resistance in the Colorado potato beetle is a growing threat to Maine potato industry. To prepare for this challenge, we completed the second field season fine-tuning management plan for the Colorado potato beetles in the area already affected by resistance. The project was conducted in close collaboration with commercial growers, other university researchers, and industry scientists. Rotating applications of SpinTor and Vydate successfully controlled imidacloprid-resistant Colorado potato beetle populations for the second year in a row. Di-Syston applications were shown to be unnecessary for achieving desirable levels of crop protection. Crop rotation helped to reduce beetle numbers and the number of sprays required to achieve satisfactory beetle control.

Investigators: Andrei Alyokhin and Gary Sewell, Department of Biological Sciences

Grant received: $9,000

Background: High imidacloprid efficacy has led to its wide acceptance by Maine potato farmers, with most non-organic growers currently using this compound. Potato growers usually apply systemic formulation of imidacloprid (AdmireÒ) at planting to the whole field. This maximizes plant coverage and significantly increases insecticide persistence in potato foliage. However, whole-field systemic applications also create strong selection pressure on insect populations, eventually resulting in their resistance to this compound. This year’s survey confirmed the existence of at least eleven Colorado potato beetle populations that are either tolerant or resistant to imidacloprid. This is a ten-fold increase in the number of resistant populations compared to just one year ago.

In 2004, we started a project developing an approach to controlling imidacloprid-resistant beetles. Application of Di-Syston in seed furrow at planting followed by alternating foliar sprays of SpinTor and Vydate provided excellent control on commercial potato fields with imidacloprid-resistant beetle populations. Small plot trials also confirmed good insecticidal activity of both compounds. They also revealed that Di-Syston contributed to beetle suppression early in the season, but for a fairly short period of time. Not surprisingly, we detected high level of cross-resistance between Admire and Platinum. Rimon provided poor beetle control.

During the past growing season, we focused on further improvement of the developed approach. We replicated last-year’s tests of SpinTor and Vydate. We also conducted large-scale tests to determine if Di-Syston was necessary for beetle control, and compared beetle pressure on rotated and non-rotated potato fields.

Accomplishments: We scouted potato fields with a history of imidacloprid resistance at weekly intervals using Cooperative Extension guidelines. Our data confirmed that selected control approach was successful. Populations of adults and small larvae exceeded economic threshold at least once during the season at only 2 fields each (10% of fields surveyed), and populations of large larvae exceeded threshold at seven fields (35%). Only one field (5%) had beetle problems on more than 1 sampling date, and none of the fields had problems on more than 2 sampling dates.

A non-rotated field received 4.78±0.40 (mean±SE) insecticide applications per season, while rotated field received 2.73±0.30. Despite that, rotated fields still had lower beetle populations than non-rotated fields (Fig. 1). The difference was especially pronounced for adults, presumably because they had difficulty locating fields following diapause termination.

Di-Syston had a negative impact on the Colorado potato beetle (Fig. 2), which is consistent with the results of last year’s replicated field trials. Because densities of overwintered adults were low, and foliar insecticides provided excellent control later in the season, Di-Syston applications were not necessary in the 2005 growing season. However, they might be a valuable option in the situation when feeding by overwintered adults presents more of a threat to recently emerged potato plants.

Fig. 1. Overall seasonal densities of the Colorado potato beetle life stages on rotated and non-rotated fields. Data from all scouted fields are pooled together.

Fig. 4.1. Weekly densities of the Colorado potato beetles on commercial potato fields partially treated with Di-Syston. Treated and untreated parts were scouted separately throughout the growing season.

A. Adults	B. Egg masses
C. Small larvae	D. Large larvae