Mite Pests and Their Predators

Table of Contents

1. Introduction
2. Damage
3. Biology
   • European Red Mite
   • Two Spotted Spider Mites
   • Apple Rust Mite
   • Influence of Temperature on Mite Development
4. Management and Monitoring
5. Resistance Management
6. Table 2: Guidelines for Monitoring, Threshold and Recommended Controls for Mite Pests of Apples

Introduction

The two major mite pests of apple in Ontario are the European red mite, Panonychus ulmi (Koch), and the two-spotted spider mite, Tetranychus urticae (Koch). The apple rust mite, Aculus schlechtendali (Nalepa) can be a sporadic pest in some orchards in some years.

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Damage

When phytophagous (pest mites), feed on leaves, they cause characteristic leaf injury referred to as bronzing (Figures 1, 2). This can result in reduced photosynthesize and a reduction in the nitrogen content of leaves. Prolonged feeding by mites will stress the tree, leading to a reduction in shoot growth and fruit bud set the following year. Fruit colour, soluble solids, firmness, size and weight of the fruit are also affected.

Figure 1: Heavy European red mite infestation

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Biology

European red mite

European red mite is generally considered the most serious of the mite species attacking apple. Overwintering European red mite eggs are usually found in the inner parts of the tree, close to the main trunk and branches (Figure 3). These eggs will begin to hatch around the tight cluster stage of apple and during the bloom period of peach.

As the eggs hatch, nymphs move from the twigs to developing foliage where they begin feeding. The nymphs eventually become adults that mate and lay the first generation of "summer eggs". Nymphs and adult females are dark orange to brown-red. Males are smaller than females and straw-coloured.

Figure 4. European red mites and eggs (female larger and red)

Size ranges from 0.15-0.40mm depending on the stage (Figure 4).

There can be six to eight generations of European red mite each year. The first few generations and the most synchronous, but by mid-summer the generations overlap and all stages (eggs, nymphs, adults) are present at the same time (Figure 5).

Females begin laying winter eggs in late August on twigs and branches as well as in the calyx end of fruit.

Figure 5. Generations of European red mite indicating overlap of stages in summer.

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Two-Spotted Spider Mite

Two-spotted spider mites are pests of many crops. They are pale green or straw-coloured, and easily distinguished from European red mite by two dark spots on the back (Figure 6).

When high populations are present, these mites construct a characteristic webbing on the under-surface of the leaf (Figure 7). There are three to five generations each year.

Figure 6. Adult two-spotted spider mite and egg.

Figure 7. Colony of two-spotted spider mites.

Figure 8. Overwintering two-spotted spider mites in calyx end of apple.

Two-spotted spider mites overwinter as orange-coloured adults under bark or on weeds beneath the tree (Figure 8), and often build up on broadleaf weeds, brambles and sucker growth beneath the tree in the spring. Although two-spotted spider mites can overwinter successfully on trees, large numbers often migrate into the tree canopy from the orchard floor in mid- to late summer.

Generally, severe infestations of two-spotted spider mites occur later than European red mite infestations. These mites are able to disperse over wide areas and from orchard to orchard by wind and air currents.

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Apple rust mite

Apple rust mites seldom cause a measurable loss in apple orchard. Exceptions do occur, however, and miticides are sometimes needed for control. These mites can be beneficial in low numbers because they are an important food source for predatory mites when European red mite and two-spotted spider mite numbers are low.

To small to see without a very strong hand lens (16X), these mites are beige or tan in colour, and wedge- or carrot-shaped (Figure 9). They overwinter beneath scales of buds, emerging to feed on foliage as buds open in spring. They have numerous generations each year.

Figure 9. Apple rust mite on apple leaf.

Apple rust mites feed on the lower leaf surface and tend to congregate around the leaf mid-rib. Prolonged feeding by unusually high populations of an apple rust mite (200 or more over leaf) can bronze the lower leaf surface, causing a leathery appearance and a characteristic curling of the leaf.

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Influence of Temperature on Mite Development

The development of mites is positively correlated to temperature: the warmer it is, the faster mites hatch, mature and reproduce. Table 1 can be used as a guide to estimate European red mite development. Not that at temperatures above 30°C, mite egg laying and development are negatively affected.

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Management and Monitoring

Sampling for European red mite and two-spotted spider mite consists of collecting leaves and counting number of mite eggs, nymphs and adults, as well as predatory mites, under the microscope (Figure 10).

Although mites commonly occur in greater numbers on trees in sheltered areas and next to dusty roadways, it is important to sample at equally from all parts of the blocks. Separate samples should be taken for each orchard block or treatable area.

Figure 10. Counting mites on leaves with the aid of a microscope

Sampling leaves on a weekly basis is especially critical in June and July, since number of mites can increase rapidly. In mid-July the population may double in a week to 10 days.

Red Delicious, Empire and Gala tend to support the largest population of mites. These cultivars, along with orchard blocks with a history of mite problems, should be sampled.

Monitoring procedures, thresholds and recommended controls for mite pests change as the season progresses. Table 2 gives guidelines on monitoring, thresholds and control material for mites attacking apple.

Resistance Management

Populations of European red mite began to increase in Ontario apple orchards in the 1970s with the advent of pyrethroid use to control organophosphate-resistant pests such as tentiform leafminer on apple and oriental fruit-moth on peach. Part of the reason was that pyrethroids killed off predatory mites that normally kept European red mite in check. Another reason was that pyrethroids act as a repellent or irritant to European red mite.

Contact with pyrethroids causes a decrease in feeding and egg-laying activity, and an increase in dispersal of mites to other areas of the tree - often the outer canopy. Feeding and oviposition activity is then resumed, often at a higher pace, causing increased leaf injury and bronzing. This phenomenon occurs in two-spotted spider mites as well. It is therefore strongly advised that pyrethroids should be avoided whenever possible.

Resistance of mites to miticides is a serious concern in Ontario orchards. European red mite resistance to Kelthane is widespread in the province. Several cases of Apollo resistance have also been recorded in southern Ontario.

Since there are relatively few IPM-compatible miticides available to Ontario orchardists, effective mite management requires advance planning. To make the best use of available products, while placing minimal pressure on any one material, the following guidelines are recommended.

1. Use Superior oil for pre-bloom mite control. Oil is a first line of defense as resistance is unlikely to develop to this material. Detrimental effects of oil applications are minimal on beneficial mites.
2. Apollo or Agri-mek should be applied on apple between calyx and two (Apollo) to three (Agri-mek) weeks post-calyx, but only when the number of mites exceeds the threshold and oil was not used. Preferable apply Apollo or Agri-mek only once every two or more years, to minimize the risk of development of resistance. Several cases of Apollo resistance have been confirmed in Ontario.
3. The potential for European red mite and two-spotted spider mite populations to increase is greatest during the hot summer months of July and August. Overuse of Kelthane and Pyramite at these times could lead to rapid development of resistance in spider mites. Always alternate summer miticides and never apply the same material twice in a row.
4. Resistance to Kelthane is widespread, but can be detected using a bioassay. Contact your pest Management Adviser for details.

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Table 2: Guidelines for Monitoring, Threshold and Recommended Control for Mite Pests of Apples

Monitoring
Spray Threshold and Timing
Control Material and Notes on Control Materials

Monitoring

Spray Threshold and Timing

Control Material and Notes on Control Material

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