Toxic and sub-lethal effects of selected pyrethroid insecticides on spider mites

Spider mite (Tetranychidae) outbreaks often occur following pyrethroid insecticide applications, although there is considerable variation between the response to different pyrethroids. Recent reviews suggested that differences in toxic and sub-lethal effects may be responsible and that recently developed pyrethroids, such as fluvalinate, show greater miticidal activity and cause fewer outbreaks. A comprehensive study of the toxic and sub-lethal effects of fluvalinate revealed this pyrethroid caused a range of beneficial sub-lethal effects, which esfenvalerate did not cause. These were discussed in relation to the lack of reported spider mite outbreaks with fluvalinate. At the recommended field concentration (0.1 g a.i. 1⁻¹) for spider mites the toxicity of fluvalinate was minimal to two-spotted spider mite, Tetranychus urticae Koch, but was moderately toxic to European red mite, Panonychus ulmi Koch. Larvae of both species were the more susceptible stage and fluvalinate persisted long enough in laboratory experiments to control emerging larvae, even when applied to newly laid eggs. Differences between the toxicity of four fluvalinate formulations were only detected above field concentration. The emulsifiable concentrate formulation was the most toxic to two-spotted spider mite while the wettable powder formulation was most toxic to European red mite. The main response to fluvalinate was rapid dispersal from treated areas. Both spider mite species were able to accurately detect and avoid fluvalinate residues within and between leaves. There was a high survival rate for mites which dispersed from treated areas, although their fecundity was reduced. This reduction depended on the initial amount of exposure. European red mite were comparatively more effected than two-spotted mite. The emulsifiable concentrate formulation caused the largest reductions in fecundity. Fecundity was reduced because fluvalinate prevented feeding. Two-spotted spider mite mating was inhibited on fluvalinate-treated surfaces, probably because of male intoxication, and this resulted in the production of mostly male offspring. The development rate of juvenile two-spotted spider and European red mites which remained on fluvalinate-treated surfaces was severely reduced and at the field concentration few survived to adulthood. Development rate of larvae was severely reduced by three hours exposure to fluvalinate. Prevention of feeding was also responsible for reductions in development rate. Physiological/biochemical resistance did not develop after 12 field applications or 20 applications of fluvalinate to a laboratory colony of two-spotted spider mite, but a low level of behavioural resistance developed in the laboratory colony. Fluvalinate was less active against the pyrethroid-resistant predator; Typhlodromus pyri, compared to two-spotted spider mite. But the predator showed little inclination to follow dispersing two-spotted spider mites and predation rates were lower on fluvalinate-treated surfaces and this may disrupt integrated mite control. Esfenvalerate was also relatively non-toxic to two-spotted spider mite adults, but caused more dispersal at equivalent concentrations compared to fluvalinate. Esfenvalerate residues were more easily and rapidly avoided. Larvae were also less affected by esfenvalerate compared to fluvalinate. Esfenvalerate had no effect on two-spotted spider mite fecundity and two times the concentration was required to cause the same reductions in development rate compared to fluvalinate. Esfenvalerate also prevented two-spotted spider mite mating. The greater toxicity to juveniles, reductions in oviposition, development rate, feeding and mating, in conjunction with relatively long persistence and dispersal effects were considered to prevent spider mite outbreaks with fluvalinate. Esfenvalerate had less effect on these sub-lethal parameters and combined with greater repellency and loss of density-dependent suppression, was more likely to cause outbreaks.