Economic injury levels and feeding studies for the potato flea beetle, Epitrix cucumeris (Harris), in Manitoba
Pernal, Stephen Francis
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In 1989 and 1990, cv. Russet Burbank potato plants were grown in cages in field plots, and densities of potato flea beetles, Epitrix cucumeris (Harris), and Colorado potato beetles, Leptinotarsa decemlineata (Say), were introduced in different multiples of naturally occurring field densities. Colorado potato beetles were introduced only in the early part of the growing season, but potato flea beetles were introduced for the duration of the season. Numbers of feeding punctures per leaflet were counted in the lower, middle, upper and upper non-terminal strata of plants. The yield of tubers was weighed, graded and examined for common scab, Streptomyces scabies (Thaxter). Without early season injury by Colorado potato beetle, plant had no yield loss up to 290 flea beetles per plant; above this density, yield was inversely proportional to flea beetle density. The level of economic damage for these plants ranged between 0.43-1.87%, equivalent to a peak density of 300-335 flea beetles per plant. For plants which had sustained early season Colorado potato beetle injury, yield loss was linear and inversely proportional to flea beetle density over the entire response. For these plants, the level of economic damage ranged between 0.40-1.88%, equivalent to a peak density of only 4-19 potato flea beetles per plant. Behavioral models were used to describe the spatial and temporal switching pattern of flea beetle feeding. Beetles changed their preferred site of feeding from lower parts of the plant to upper regions as the growing season progressed. High densities of flea beetles had an inhibitory effect on the amount of defoliation caused per individual, and this may alter the pattern of feeding preference by causing beetles to disperse more evenly throughout the plant. A population model was also derived which established a relationship between feeding punctures per leaflet and average potato flea beetle weeks. This relationship enables the estimation of flea beetle numbers per plant, but is very sensitive to the accuracy of feeding puncture counts. Very few tubers exhibited signs of common scab disease, but there was a positive correlation between number of feeding punctures and numbers of immature potato flea beetles. The absence of scab disease suggests that its incidence is less affected by flea beetle densities, than by abiotic factors such as soil moisture, and varietal differences in susceptibility. A preliminary examination of feeding by Melanoplus bivittatus (Say) and Melanoplus sanguinipes (Fabricius) on potato plants showed no significant yield response to occur at varying densities of these species.