Simulating effects of environmental factors on biological control of Tetranychus urticae by Typhlodromus pyri in apple orchards.

Successful biological control of mites is possible under various conditions, and identifying what are the requirements for robust control poses a challenge because interacting factors are involved. Process-based modeling can help to explore these interactions and identify under which conditions biological control is likely, and when not. Here, we present a process-based model for population interactions between the phytophagous mite, Tetranychus urticae, and its predator, Typhlodromus pyri, on apple trees. Temperature and leaf nitrogen concentration influence T. urticae rates of development and reproduction, while temperature and rate of ingestion of prey and pollen influence T. pyri rates of survival and reproduction. Predator and prey population dynamics are linked through a stage structured functional response model that accounts for spatial heterogeneity in population density throughout the trees. T. urticae biomass-days (BMD's), which account for sizes of larvae, nymphs and adults, indicate level of mite-induced leaf damage. When BMD's exceed 290 per leaf, there are economic losses. When BMD's exceed 350 per leaf, T. urticae population growth is curbed and eventually the population decreases. Simulations were run to determine which conditions would lead to current year economic loss and increased risk of loss in the following year, i.e. where more T. urticae than T. pyri are present at the end of September. Risk was high with one or more of the following initial conditions: a high prey: predator ratio (10:1 or more); a low to intermediate (0.04-0.2 T. urticae per leaf) initial density; T. urticae with a higher initial proportion of adult females than T. pyri; and a delayed first detection of mites, whether in late July, or sometimes in late June, but not in early June. Warm summer weather, higher leaf nitrogen and T. urticae immigration into trees were also risk factors. Causes for these patterns based on biological characteristics of T. urticae and T. pyri are discussed, as are counter measures which can be taken to reduce risk.

Effects of dispersal, predators (Acari: Phytoseiidae), weather, and ground cover treatments on populations of Tetranychus urticae (Acari: Tetranychidae) in apple orchards.

In a 2-yr study of causes of mite outbreaks in apple (Malus spp.) orchards in Nova Scotia, we monitored immigration of Tetranychus urticae Koch from orchard ground cover into trees populated by the generalist phytoseiid predator Typhlodromus pyri Scheuten. In both years, T. urticae-days in the tree canopy increased with number of T. urticae caught in sticky bands on tree trunks. In 2000, T. urticae-days were negatively correlated with T. pyri-days. Lack of correlation in 2001 was attributed to higher rates of immigration, which would mask the effects of predation. Weather also affected mite dynamics. Rainfall in July and August was less in 2001 than in 2000. Heat units were sufficient for six generations of T. urticae in 2001 but only for five in 2000. Consequently, T. urticae-days in the tree canopy and immigration rates were significantly greater in 2001 than in 2000, despite three-fold greater use of miticides. We also tested the effects of herbicides on T. urticae immigration. Application of selective herbicides in laneways reduced coverage of reproductive hosts of T. urticae, but these changes did not reduce immigration. In 2001, application of a miticidal herbicide, glufosinate, in tree rows reduced captures of T. urticae on sticky bands in high immigration orchards but not in low immigration orchards. We conclude that generalist predators and modified herbicide use are insufficient remedies and that effective biological control of T. urticae in the ground cover by a specialist phytoseiid such as Amblyseius fallacis Garman is essential to prevent outbreaks.

Effects of ground cover treatments and insecticide use on population density and damage caused by Lygus lineolaris (Heteroptera: Miridae) in apple orchards.

We conducted a 2-yr study in commercial apple orchards in Nova Scotia to assess the effects of ground cover treatments and insecticides on population density and fruit injury caused by tarnished plant bug, Lygus lineolaris (Palisot de Beauvois). The design was a split-plot with insecticides applied to whole orchard blocks and ground cover treatments applied to plots nested within orchard blocks. Ground cover treatments were 1) standard herbicide use, 2) enhanced weed control in tree rows, and 3) treatment two plus use of a selective herbicide in laneways. Treatments had few significant effects on vegetation in the tree row, but in laneways, known dicot hosts of L. lineolaris were suppressed and nonhost grasses promoted with treatment 3. Ground cover treatments did not affect cumulative captures of adult tarnished plant bugs on white sticky traps located in the plots but did affect captures in sweep nets. Split-plot ANOVA indicated no significant effect of insecticides on injury in either year, but ground cover treatments were significant in 2001. The lowest ranking rates of injury in both years were in orchards treated before bloom with a pyrethroid insecticide, either cyhalothrin-lambda or cypermethrin. The highest ranking rate of injury occurred in an orchard where insecticide was not applied until after bloom despite a high prebloom capture of L. lineolaris adults on orchard perimeter sticky traps. Fruit injury values for the ground cover treatment 3 were 63.3% (n.s.) and 50.0% (P < 0.05), respectively, of those in the standard treatment in 2000 and 2001.

The relationship between winter egg counts of the European red mite Panonychus ulmi (Acari: Tetranychidae) and its summer abundance in a reduced spray orchard.

We studied the relationship between counts of Panonychus ulmi (Koch) winter eggs per bud and mite-days per leaf accumulated in early to mid-summer on 92-96 apple trees in an orchard in Quebec, Canada. Stepwise regression was used to compute mite-days in the 4 year (1992-1995) data set using winter egg density, cumulative rainfall (R), cumulative degree-days (D), squared values of R and of D, and the product RD as potential predictors. Degree-days were accumulated above 10.1 degrees C, the minimum threshold of development for P. ulmi in Eastern Canada. The 4-year model that gave the best fit included terms for winter eggs, rainfall, R2, and the product RD, and explained 76% of the variation in mite-days. When the 4-year model was applied to each year's data separately, winter eggs, adjusted for current year rainfall and degree-days, were always highly significant predictors of mite-days. These significant effects of weather indicate that estimation of potential economic loss should take account of rainfall and heat units, as well as the density of winter eggs. Predators, including the stigmaeid, Agistemus fleschneri Summers, and occasional low numbers of phytoseiids, did not have any evident within-season effect on mite-days but their appearance in July-September of 1994 was followed by a ten-fold decrease in the mean density of winter eggs in 1995 compared with the previous spring. This reduction contrasted with increasing densities each successive spring from 1992 to 1994, which followed summers when predators were scarce or absent.