Some Like it Hot: Summer Monitoring for Mites

Two common mite species affecting fruit and vegetable crops include the European red mite (ERM) (Figure 1) and the two-spotted spider mite (TSSM) (Figure 2). Both thrive in hot, dry weather, although temperatures above 30°C can negatively affect ERM egg laying and development. TSSM have an upper developmental threshold of 40°C. Mites pierce plant tissue with their mouth-parts and remove plant fluids. Affected leaves have mild chlorotic spots (stippling) and become bronzed if populations are sufficiently high. (Figures 3 & 4) Severe infestations may result in necrosis, deformations and defoliation. If you are starting to notice discoloured or bronzed leaves during scouting activities, make sure to check plants for the presence of mites.

Mites are tiny, so unless you are actively looking for them, you may mistake the earlier signs of damage they cause with environmental stress (hot, dry conditions). Sampling protocols vary by the crop, but in general, you should be checking for eggs, nymphs and adult mites on the underside of leaves. In some cases, tapping leaves over a light-coloured dish can be used to detect mobiles. Because mite numbers can increase very rapidly, it is necessary to monitor frequently during hot weather. Populations are often higher along dusty roadways and field edges.

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