Movement and spatial costs of resistance in the Colorado potato beetle, Leptinotarsa decemlineata (say), coleoptera: Chrysomelidae

The Colorado potato beetle, Leptinotarsa decemlineata is infamous for its' ability to develop resistance to insecticides and remains the most important insect defoliator of potatoes today. Long Island populations of the Colorado potato beetle have been at the forefront of developing resistance to every newly developed insecticide. Managing the evolution of resistance requires cultural as well as chemical means. Cultural efforts include field rotation, crop rotation, chemical rotation as well as refugia. Movement plays an integral part of both, the cultural schemes intended to thwart resistance evolution and the life history traits of the Colorado potato beetle. The use of refuges and crop rotation are often promoted to supplement the use of chemical pesticides in an effort to control crop pests. Refuges are untreated areas adjacent to treated crops, where susceptible genes can survive. The efficacy of refuges depends on movement between treated and untreated areas. Differences in movement between resistant and susceptible beetles can play a big role in the success of the refuge or rotation plan. Crop rotation can reduce the amount of insecticide used through dosage levels or frequency of application and slows insects' resistance evolution. Resistance to insecticides often has fitness costs associated with that resistance. I hypothesized that resistance to the insecticide Imidacloprid is correlated with reduced movement capability in Colorado potato beetles, Leptinotarsa decemlineata (Say) (henceforth potato beetles), the primary insect defoliator of potato plants. I examined whether migratory ability or flight propensity have a cost of resistance to imidacloprid in Colorado potato beetles, Leptinotarsa decemlineata (Say) by examining LD50's of flying emergers and walking emergers in the spring. Imidacloprid is the most widely used and in some cases the only effective insecticide for Colorado potato beetle control and there is currently a wide range of variation in resistance. In the spring overwintering potato beetle adults halt diapause and emerge from overwintering sites. For the purposes of this work I will use the definition of diapause presented by Tauber et al (1986): "a neurohormonally mediated, dynamic state of minimal activity that occurs during a genetically determined stage(s) of metamorphosis, usually in response to environmental stimuli that precede unfavorable conditions." Diapause in the Colorado potato beetle begins before the harsh conditions set in (loss of host and cold temperatures). It is an important strategy employed by many temperate zone insects for overwintering. Upon emergence from the overwintering site they emigrate to colonize local and distant fields. Emergence from diapause therefore offers an opportunity to sample genetically diverse groups of beetles. My results indicate that emerging flyers have a higher level of resistance than emerging walkers from overwintering sites. I also examined populations that were under intense selection pressure from one chemical, Spinosad, and largely isolated from other fields or populations. Spinosad is produced by a soil dwelling bacterium called Saccharopolyspora spinosa and it kills by ingestion. Spinosad is currently the only approved chemical available to Organic farmers on Long Island. These results indicate complete failure of Spinosad on that population but less resistance on distant populations and less resistance on populations from conventionally managed fields, all in Suffolk County, Long Island. Additionally early spring colonists of rotated and `non-rotated' fields were evaluated for resistance levels for 3 years. For two of the three years, colonists on long distance rotated fields had high LD50. Assuming long distance colonization is more likely dependent on flight, this is consistent with my results that emerging flyers have a higher LD50 than emerging walkers.