Organosilicone molecules are important surfactant ingredients used in formulating pesticides. These methylated silicones are considered inert ingredients, but their superior surfactant properties allow them to wet, and either suffocate or disrupt important physiological processes in mites and insects. Aqueous solutions of the tri-siloxane surfactants Silwet L-77, Silwet 408, and Silwet 806 were bioassayed against adult female two-spotted spider mites, Tetranychus urticae Koch, with leaf dip methods to compare their toxicity with organosilicone molecules containing bulkier hydrophobic components. All three tri-siloxanes in aqueous solutions were equivalently toxic (LC50 = 5.5-8.9 ppm), whereas Silwet L-7607 solutions were less toxic (LC50 = 4,800 ppm) and Silwet L-7200 was nontoxic to mites. In another experiment, the toxicity of Silwet L-77 was affected by the wettability of leaf surfaces. The LC50 shifted from 22 to 84 ppm when mites were tested on bean and strawberry leaf disks, respectively. Droplet spreading on paraffin and surface tension were both related to the toxicity of surfactant solutions. Surface tensions of solutions below 23 mN/m caused > 90% mite mortality in leaf dip bioassays. A field test of Conserve SC and its formulation blank, with and without Dyne-Amic adjuvant (a vegetable oil-organosilicone surfactant mixture) revealed that Dyne-Amic had the greatest miticidal contribution, reducing mite populations by 70%, followed by formulation inactive ingredients. Spinosad, the listed active ingredient in Conserve, only contributed miticidal activity when synergized by Dyne-Amic. Researchers should include appropriate surfactant or formulation blank controls when testing insecticides or miticides, especially when using high spray volumes.

Topical application bioassays of bifenthrin and lambda-cyhalothrin were conducted on field-collected populations of Listronotus maculicollis Kirby (Coleoptera: Curculionidae) from eight southern New England golf courses, six in Connecticut, one in Massachusetts, and one in Rhode Island. Concentrations versus mortality regression lines were estimated to compare the LD50 values and resistance ratios for each insecticide and each population. The LD50 ranges for bifenthrin and lambda-cyhalothrin were 1.80-244.67 ng per insect and 0.52-159.53 ng per insect, respectively. The field-collected strains showed low to high levels of resistance to bifenthrin (6.1-135.9-fold) and lambda-cyhalothrin (28.7-306.8-fold). This is the first report of insecticide resistance in this species. The loss of pyrethroid efficacy is making control of Listronotus maculicollis more challenging.

In 2007-2008, the "annual bluegrass weevil," Listronotus maculicollis Kirby (Coleoptera: Curculionidae), a serious pest of Poa annua L. (Poales: Poaceae) on U.S. golf courses, was shown to be resistant to two pyrethroids, bifenthrin and lambda-cyhalothrin. In 2008, we showed that bifenthrin resistance was principally mediated by oxidase detoxification (cytochrome P450 [P450]). P450s can be inhibited by demethylation inhibitor fungicides and gibberellin inhibitor plant growth regulators, both of which are commonly used on golf courses. We tested these compounds for synergistic activity with bifenthin against a pyrethroid-resistant population of L. maculicollis. The LD50 value for bifenthrin was significantly reduced from 87 ng per insect (without synergists) to 9.6-40 ng per insect after exposure to the fungicides fenarimol, fenpropimorph, prochloraz, propiconazole, and pyrifenox and the plant growth regulators flurprimidol, paclobutrazol, and trinexapac-ethyl. Simulated field exposure with formulated products registered for use on turf revealed enhanced mortality when adult weevils were exposed to bifenthrin (25% mortality, presented alone) combined with field dosages of propiconizole, fenarimol, flurprimidol, or trinexapac-ethyl (range, 49-70% mortality).

In 2009, pyrethroid resistance was confirmed for seven "annual bluegrass weevil" Listronotus maculicollis Kirby (Coleoptera: Curculionidae) adult populations from southern New England. The mechanisms responsible for conferring this resistance were unknown. In this study, topical application bioassays with bifenthrin and bifenthrin combined with synergists affecting three detoxification systems were conducted on four field-collected adult populations of L. maculicollis from Connecticut to determined whether cytochrome P450 monooxgenases (P450s), glutathione S-transferases (GSTs), and/or carboxyl-esterases (COEs) mediated metabolic detoxification. Because a susceptible L. maculicollis laboratory strain does not exist, the most susceptible field-collected population (New Haven) provided a baseline against which all other populations were compared. In the population with the lowest resistance (Norwich), only detoxification by P450s was significant. Detoxification in the population with the second highest level of resistance (Stamford) involved both P450s and GSTs. Detoxification in the population with the highest level of resistance (Hartford) involved P450s, GSTs, and COEs. This study suggests that enzyme-mediated metabolic detoxification plays an important role in annual bluegrass weevil pyrethroid resistance.

No abstract