Potential utility of novaluron in Colorado potato beetle, Leptinotarsa decemlineata (Say), management
The Colorado potato beetle (CPB), 'Leptinotarsa decemlineata' (Say), is the most destructive insect defoliator of potato. Heavy reliance on insecticides to manage CPB has resulted in rapid development of resistance to virtually all compounds used against it. Better control could be achieved with integrated pest management (IPM), but most insecticides are non-selective, impeding the full development of CPB IPM. This research evaluates the potential of novaluron, a novel chitin synthesis inhibitor with demonstrated selectivity in favor of beneficial insects, in CPB management. In laboratory studies, novaluron exhibited excellent residual activity against larvae, and good direct contact activity against larvae and eggs. However, larvae from eggs treated with 1 ppm novaluron weighed significantly more than those from untreated eggs, suggesting novaluron can have a hormetic effect on CPB larval development. Adults fed foliage treated with field rates of novaluron produced fewer eggs, and hatch of eggs from adults feeding on treated foliage was almost completely suppressed. Studies were conducted to determine the potential of CPB to develop resistance to novaluron. Second instars (L2) of an imidacloprid-resistant CPB strain exhibited reduced susceptibility to novaluron. Toxicity was synergized by S, S, S-tributyl phosphorotrithioate (DEF) but not by piperonyl butoxide, suggesting that esterase-based detoxification caused novaluron resistance. Bioassays with treated potato foliage found that novaluron was highly persistent under field conditions, resulting in up to 85% mortality of L2 5 weeks after treatment. The susceptibility of L2 to a novaluron diagnostic dose (DD) was determined for 27 different populations collected from 6 Canadian provinces. Despite no previous exposure to novaluron, mortalities at the DD ranged from 55-100%. Laboratory experiments were conducted to determine the susceptibility of the CPB predator 'Podisus maculiventris' (Say) to novaluron. L2 were susceptible by direct contact and through exposure to potato foliage treated with field rates. Although eggs dipped in field rate solutions hatched, there was subsequently a sharp decrease in moult of emerged nymphs. Similarly, fifth instars were unable to moult after consuming treated CPB larvae. ' P. maculiventris' adults caged with CPB larvae and novaluron treated potato plants had reduced oviposition, and hatch of eggs from adults on novaluron treated plants was significantly reduced. In field trials, although foliar applications of novaluron caused no significant reductions in numbers of CPB adults, egg masses, or first instars, numbers of second-fourth instars were greatly suppressed. Novaluron application was most effective when applied soon after L2 were first observed on plants. At 50 g a.i./ha, novaluron provided prolonged protection, whether applied twice during the season, or once in alternation with imidacloprid.