Performance of a pyrethroid-resistant strain of the predator mite Typhlodromus pyri (Acari: Phytoseiidae) under different insecticide regimes.

An organophosphate pyrethroid-resistant strain of Typhlodromus pyri Scheuten imported from New Zealand was reared on potted apple trees in an outdoor insectary. From 1988 to 1995, the population was selected one to three times per year with a dilute solution (1.7 ppm) of the pyrethroid cypermethrin. Petri dish bioassays with cypermethrin in 1995 indicated that the insectary-reared T. pyri had an LC50 of 81 ppm versus 0.006 ppm for native T. pyri taken from a research orchard. The bioassays suggested that recommended orchard rates of cypermethrin would cause heavy mortality in native populations of T. pyri but only moderate losses in the imported New Zealand strain. Bioassays in 1996 with the organophosphate insecticide dimethoate indicated both New Zealand and native T. pyri were susceptible and that recommended orchard rates of dimethoate likely would cause high mortality of T. pyri in apple orchards. These findings from bioassays were supported by data from orchard trials. In June and July 1993, insectary-reared New Zealand T. pyri were placed on five apple trees in each of eight 38-tree plots in the research orchard. In late August 1994, New Zealand T. pyri from orchard trees that had been sprayed twice by airblast sprayer with the full recommended rate of 50 g (AI)/ha (83 ppm) cypermethrin were placed on the other 33 trees in each of six plots. In the summers of 1994-1996, plots were treated with one of the following insecticide regimes: (1) conventional integrated pest management (IPM) (registered neurotoxic insecticides considered harmless or slightly toxic to T. pyri); (2) advanced IPM (use of newer, more selective insecticides); (3) pyrethroid (at least one full-rate application of cypermethrin); (4) dimethoate; and (5) dimethoate plus pyrethroid. Densities of European red mite, Panonychus ulmi (Koch), were highest in all plots treated with dimethoate and in pyrethroid plots not yet inoculated with New Zealand T. pyri. Densities of apple rust mite, Aculus schlechtendali (Nalepa), and of the stigmaeid predator Zetzellia mali (Ewing) were highest in plots treated with dimethoate and were nearly absent in the IPM plots. Densities of T. pyri were high enough for effective biocontrol in the IPM plots and in the pyrethroid plots 1-2 yr after release of the New Zealand strain, provided pyrethroid was applied just before the resistant strain was released in the orchard. A recurring theme of this study was the generally negative association between densities of phytophagous mites and those of T. pyri, suggesting the ability of this predator to suppress their prey. In contrast, the positive association between phytophagous mites and Z. mali suggests the inability of this predator to regulate their prey at least under the conditions of this study.

Effects of temperature and glycerides on the enhancement of Agkistrodon piscivorus piscivorus phospholipase A2 activity by lysolecithin and palmitic acid.

The effect of temperature and various glycerides to modulate the ability of lysolecithin and fatty acid to promote high phospholipase A2 activity was studied using dipalmitoylphosphatidylcholine large unilamellar vesicles as substrate. The length of the lag phase prior to the accumulation of sufficient hydrolysis products (lysolecithin and fatty acid) to support high phospholipase activity was shortest at temperatures near the thermotropic phase transition of the phospholipid substrate. A reduction in the lag phase correlated with a reduction in the requirement for hydrolysis products at the phase transition temperature, where the bilayer exists in a state of fluctuating domains of gel and liquid crystal. Dipalmitoylglycerol and tripalmitoylglycerol also reduced the length of the lag phase. This reduction was both concentration-dependent and temperature-dependent relative to the phase transition in the presence of the glycerides. As with the effect of temperature, the ability of di- and triglycerides to decrease the lag time correlated with a decrease in the amount of reaction products necessary to promote high phospholipase activity. This effect coincided with the tendency of the glycerides to form domains in the bilayer. Glycerides that did not form domains either had no effect (monopalmitoylglycerol) or increased the length of the lag phase (dicaprylglycerol). These data suggest that the effect of the reaction products to increase phospholipase A2 activity is aided by the presence of fluctuations in lipid domains within the bilayer.

Quantification of the interactions among fatty acid, lysophosphatidylcholine, calcium, dimyristoylphosphatidylcholine vesicles, and phospholipase A2.

The rate of hydrolysis of phosphatidylcholine bilayers by soluble phospholipase A2 (PLA2) is greatly enhanced by the presence in the bilayer of a threshold mole fraction of the reaction products: fatty acid and lysophosphatidylcholine (lyso-PC). The threshold requirement of these products appears to vary as a function of vesicle and calcium concentration. To further identify the roles of myristic acid, lyso-PC, and calcium in promoting optimal PLA2 activity, we have quantified the various interactions among these components and dimyristoylphosphatidylcholine large unilamellar vesicles. The bilayer/water partition coefficient for myristic acid was obtained by competition of vesicles for the binding of the fatty acid to an acrylodan conjugate of an intestinal fatty acid binding protein as monitored by the acrylodan fluorescence emission spectrum. The partition coefficient for lyso-PC was obtained by a similar procedure using the tryptophan emission spectrum of bovine serum albumin. The effect of calcium concentration on these interactions was also quantified. These results were incorporated into an empirical model to describe the threshold requirements for these products in the bilayer. This information is vital for elucidating the mechanism of activation of PLA2 by the hydrolysis products.