Status, distribution and seasonal variation of filbert aphid resistance to selected insecticides in the Willamette Valley, Oregon Public Deposited


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  • Levels of resistance to six selected insecticides of eleven field populations of the filbert aphid Mvzocallis coryli (Goetze) were determined by using the leaf-dip residue technique. Test insecticides included compounds widely used in commercial filbert orchards, namely, carbaryl, diazinon, endosulfan, phosalone, fenvalerate and oxydemetonmethyl. Aphid samples were collected within a 100-mile range from Eugene in the south to Wilsonville in the north of the Willamette Valley, Oregon. The filbert orchards were characterized by different levels of insecticide exposure, age and management conditions. The tests were conducted to detect seasonality of resistance during the different phases of the population in the summer, fall and early spring months of 1985-1986. Toxicological responses of various filbert aphid populations exhibited significantly different resistance to tested insecticides. All populations exhibited lethal concentration (Lc₅₀) values of 0.0013 g AI/1 to 0.1507 g AI/1 of endosulfan which, when compared to the susceptible OSU population, were categorized as zero to moderate levels of resistance. With the exception of a high Lc₅₀ value of 1.7853 g AI/1 exhibited by the Harnisch population, resistance to diazinon is still at low levels or non existent in most populations of M. coryli in the Willamette Valley. Lc₅₀ values of carbaryl varied from 0.0075 g AI/1 to more than 1.2 g AI/1, indicating significant differences in tolerance among the filbert aphid populations to this insecticide. The majority of the populations were moderately resistant, but extremely high levels (>1000-fold) of resistance were evident in populations collected from two orchards. Lc₅₀ values for these populations were well above the maximum range of recommended field dosages. Highest resistance levels of more than 1000-fold of fenvalerate also were noticed in filbert aphid populations from these orchards; Lc₅₀'s were more than the maximum field rate. Populations of M. coryli from other orchards were non-resistant (Lc₅₀ value of 0.0003 g AI/1) to highly resistant (Lc₅₀ value of 0.0989 g AI/1) to fenvalerate. Selection for high resistance to fenvalerate after just a few seasons of use in commercial orchards was not expected. Although failure of field control of filbert aphids by phosalone has not been reported, several populations have developed high resistance to this insecticide. Filbert aphid populations from three orchards had Lc₅₀'s above the maximum recomended field dosage of 0.563 g AI/1 of phosalone. The maximum Lc₅₀ values for the rest of the populations ranged from 0.0012 g AI/1 to 0.2499 g AI/1 and were categorized respectively, as non-resistant to highly resistant strains. From one series of experiments in early spring of 1986, the majority of filbert aphid populations indicated zero to moderate levels of resistance to oxydemetonmethyl. One population with Lc₅₀ value of 0.2135 g AI/1 showed the highest tolerance to this insecticide. The shallow slopes of the log dosage mortality curves indicated heterogeneity of responses of the various filbert aphid populations to the insecticides. These responses could be explained by the widespread use and rotational spraying patterns of the insecticides in commercial orchards. The distribution of resistance was not a regional phenomenon. It was associated with, 1) the pattern of insecticide usage and 2) the proximity of the source of aphid population to more intensively managed commercial orchards. The tendency for increased resistance to all insecticides in summer and fall populations of M. coryli was evident. However, as an exception, phosalone resistance of Lemert population was also high when treated in early spring. Seasonal variations in susceptibility of less than 10-fold to endosulfan, and 237-fold to diazinon were measured. Extrapolation of Lc₅₀ values beyond the range of tested concentrations resulted in variations in tolerance of more than 1000-fold among some populations treated with carbaryl, phosalone and fenvalerate. Several factors which may influence the widespread expression and the seasonality of insecticide resistance have been discussed without giving a generalized explanation. Rather than considering these results quantitatively, what is vital from the present studies is the information on changes in susceptibility of M. coryli to those insecticides recommended to filbert growers in this area. Resistance monitoring is considered critical to effective insect control programs in commercial filbert orchards of the Willamette Valley, Oregon.
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