European red miter, Panonychus ulmi (Koch)
and twospotted spider mite, Tetranychus urticae Koch on Pear

I. Introduction: More details are provided in the Apple section on ERM and TSM. Details specific to pear are included here.

II. Injury: These spider mites feed in the same manner as on apple. However, there is an important difference in host response. Pear leaves are damaged at lower mite densities than are apple leaves. Leaves react to feeding by turning dark brown or black (Plate 92), especially if intense feeding occurs during a relatively short period. This blackening is related to reduced water content of leaves. If growers have not observed this damage previously in their orchards, fire blight may be suspected erroneously, but mite foliar injury is more evenly spread through the canopy, and the typical crook-necking associated with fire blight is absent. This injury may result if inappropriate pesticides are used for summer control of pear psylla.

Oregon researchers have documented reduced fruit set in the following year, reduced fruit size, and increased preharvest fruit drop resulting from foliar injury from spider mites. While early summer populations may influence fruit size, fruit finish may be affected by late summer infestations. The following varieties were ranked from highest to lowest susceptibility: `Anjou', `Bosc', `Bartlett', and `Comice'. Australian research has shown that pear trees with leaf scorch and subsequent defoliation by TSM have reduced fruit set per cluster, gross fruit yield, and yield of fruit suitable for canning in the year following infestation. Trees with 80% leaf scorch and defoliation experienced yield reductions of about 2.3 relative to uninfested trees.

III. Monitoring: Follow mite densities during the season by counting mites per leaf with a hand lens or leaf-brushing machine. Take 10-40 leaves from the canopies of 10 trees (use the larger number in spring, when mites are harder to detect). Calculate the % infested leaves; from this mites per leaf can be calculated. This relationship holds until the first acaricide application.

Although a provisional threshold of 5 mites per leaf has been used in earlier literature, a more conservative approach should be used than in apple trees because of differing host response. Use an action threshold of 2 mites per leaf (32% of the leaves infested). If twospotted spider mite is the primary species present, use 1 per leaf (22% of leaves infested).

Back to Pear page
Back to Mid-Atlantic Regional Fruit Loop