Mite Pests and their Predators in Ontario Orchards

Agdex#:	206/624
Publication Date:	12/95
Order#:	95-057
Last Reviewed:
History:
Written by:	Bernt D.Solymar - Horticultural Crop Advisor/OMAF; Gerry Walker - Pest Management Advisor/OMAF

Table of Contents

1. Introduction
2. European Red Mite Biology
3. Two-Spotted Spider Mites (TSSM) Biology
4. Apple Rust Mite (ARM) Biology
5. Pear Rust Mite (PRM) Biology
6. Peach Silver Mite (PSM) Biology
7. Influence of Temperature on Mite Development
8. Effect of Pyrethroids on Mite Populations
9. Monitoring Procedures for Pest Mites
10. Resistance Management
11. Predatory Mite Biology
    

Introduction

The two major mite pests of fruit trees in Ontario are the European red mite, Panonychus ulmi (Koch), and the two-spotted spider mite, Tetranychus urticae (Koch). Three other mite pests that can cause sporadic problems in Ontario orchards are the apple rust mite, Aculus schlechtendali (Nalepa), on apple; the pear rust mite, Epitrimerus pyri (Nalepa) on pear; and the peach silver mite, Aculus cornutus (Banks) on peach and other stone fruit.

When phytophagous, or pest mites, feed on leaves, they cause a characteristic leaf injury referred to as bronzing (Figure 1). This results in reduced photosynthesis 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 color, soluble solids, firmness, size and weight of the fruit are also affected.

Figure 1. Bronzing injury to apple.

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European Red Mite (ERM) Biology

ERM is usually the most serious of the mite pests, especially on apple and peach.

Overwintering ERM eggs are usually found in the inner parts of the tree close to the main truck and branches (Figure 2). These eggs will begin to hatch around the tight cluster stage of apple and during the bloom period of peach. As the eggs hatch, the young 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-colored. Size ranges from 0.15 to 0.40 mm depending on the stage (Figure 3, Figure 4 and Figure 5).

There can be 6 to 8 generations of ERM each year. The first few generations are the most synchronous, but by mid-summer the generations overlap and all stages (eggs, nymphs, adults) are present at the same time. Females begin laying winter eggs in late August on twigs and branches as well as in the calyx end of fruit.

Figure 2. Overwintering European red mite eggs on bark.

Figure 3. Adult female European red mite.

Figure 4. Mixed stages of European red mite (eggs, nymphs, adults).

Figure 5. Heavy ERM infestation on apple leaf.

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Two-Spotted Spider Mites (TSSM) Biology

TSSM is a pest of many crops. TSSM is pale green or straw-colored and easily distinguished from ERM by two dark spots on the back (Figure 6). When high populations are present, these mites construct a characteristic webbing on the undersurface of the leaf (Figure 7). There are 3 to 5 generations each year.

TSSM overwinter as orange-colored adults under bark or on weeds beneath the tree and often build up on broadleaf weeds, brambles and sucker growth beneath the tree in the spring. Although TSSM 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 TSSM occur later than ERM infestations. These mites are able to disperse over wide areas and from orchard to orchard by wind and air currents.

Figure 6. Adult two-spotted spider mite.

Figure 7. Colony of two-spotted spider mites.

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Apple Rust Mite (ARM) Biology

ARM seldom cause a measurable loss in apple orchards. 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 ERM and TSSM numbers are low.

ARM are too small to see without a very strong hand lens (16X). They are beige to tan in color, and wedge- or carrot-shaped (Figure 8). They overwinter beneath scales of buds, emerging to feed on foliage as buds open in spring. They have numerous generations each year.

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

Figure 8. Apple rust mites on apple leaf.

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Pear Rust Mite (PRM) Biology

PRM are very similar in appearance and life history to ARM (Figure 9).

Feeding activity begins on the first green tissue at bud break. Tender foliage is preferred and populations will migrate to more succulent tissue as leaves mature. Later, floral clusters and then fruit may be damaged.

Economic injury can occur to fruit when feeding results in russetting. This is characterized by brown, roughened markings on the fruit skin near the calyx or stem end (Figure 10). This injury may render fruit unsuitable for fresh market or processing.

Figure 9. Pear rust mite.

Figure 10. Russetting injury on pear caused by pear rust mite.

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Peach Silver Mite (PSM) Biology

PSM is primarily found on peach and nectarine, but also occasionally attacks plum and prune. Their appearance and biology are very similar to ARM. High populations of 200 to 300 per leaf can cause leaves to turn a silvery green and may adversely affect fruit quality.

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 ERM development.

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Effect of Pyrethroids on Mite Populations

Populations of ERM 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 ERM numbers in check. Another reason was that pyrethroids act as a repellent or irritant to ERM. 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 TSSM as well.

It is therefore strongly advised that pyrethroids should not be used on tree fruit, but used only when organophosphate-resistant pest populations reach (economic) spray threshold levels in the spring, and then never more than once per season.

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Monitoring Procedures for Pest Mites

1. Cultivar Selection

On apple, Red Delicious, Empire and Gala tend to support the largest populations of mites. These, along with blocks with a history of mite problems, should be sampled. On peach, samples should be taken from those cultivars on which the largest mite populations develop (i.e., Loring, Babygolds and Redhaven).

2. Sampling

Sampling for ERM and TSSM consists of collecting leaves and counting numbers of mite eggs, nymphs and adults, as well as predatory mites, under the microscope (Figure 11). Although mites commonly occur in greater numbers on trees in sheltered areas and next to dusty roadways, it is important to sample equally from all parts of the blocks. Separate samples should be taken for each orchard block or treatable area.

Figure 11. Detecting mites on leaves with the aid of a microscope.

3. Evaluation

Include both ERM and TSSM (add totals of each stage together) in your counts. On apple, counting should be done with the use of a dissecting microscope with a magnification of 25-40X.

On peach it is important to fold the leaf in half along the midrib to accurately assess mite populations, which congregate in the leaf fold along the midrib In peach blocks, mite populations may be estimated in the field with the use of a 10-16X magnification hand lens.

4. Frequency

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

Monitoring procedures, thresholds and recommended controls for mite pests change as the season progresses. Table 2 gives guidelines on monitoring, thresholds and control materials for mites attacking apple. This table also applies to peaches except for the following exceptions.

1. Morestan and Apollo are not registered for use on peach. For up-to-date information on the timing and selection of miticides, consult Fruit Production Recommendations (Publication 360) and the regional agri-phone.
2. An annual oil application is a good IPM strategy for peach to control mites and green peach aphid. This treatment should be primarily considered in problematic blocks or where pyrethroids are used.
3. Key decision times to control mites include before applying the second Oriental fruit moth spray (early July) and/or 3 weeks before expected harvest of fruit. Observe the 14-day-to-harvest interval of Omite and Kelthane on peach.
4. Thresholds for peach are the same as apple from early season to midsummer After harvest, the threshold for mites in peach is 30 mites per leaf through September. This is because the potential for population increase is much lower later in the season, when cooler temperatures prevail.
   

Note: Monitoring procedures and thresholds have not been determined for two of the other mite species (ARM and PSM). Populations are often detected during normal sampling for ERM and TSSM or when first injury is noticed. Neither ARM nor PSM are controlled by oil sprays, but summer miticides applied for ERM and TSSM will control both species.

PRM monitoring consists of collecting 25 fruit spurs per block on a weekly basis beginning at bud break until petal fall. Use a microscope to check under bud scales and rough scaly areas. A spray is warranted when 20% or more of fruit spurs are found to be infested.

From petal fall through to harvest, check two fruit from each of 25 trees (total 50 fruit) in a block weekly for russetting injury. A 5% level of infestation requires a spray.

Morestan is recommended pre-bloom (and will also control pear psylla) and Kelthane can be used as a summer miticide. Mitac applied for pear psylla will also control PRM.

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Resistance Management

Resistance of mites to miticides is a serious concern in Ontario orchards. European red mite resistance to Kelthane is widespread in the province and low levels of resistance to Omite have been documented in southwestern Ontario.

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

1. Use superior oil or Morestan for pre-bloom mite control. Oil is a first line of defense as resistance is unlikely to develop to this material. Effects of oil applications are also minimal on beneficial mites.
2. Apollo should be applied on apple between calyx and two weeks post-calyx, but only when the number of mites exceeds the threshold and oil was not used. Preferably apply Apollo only once every two or more years, to minimize the risk of development of resistance.
3. The potential for ERM and TSSM populations to increase is greatest during the hot summer months of July and August. Overuse of Kelthane and Omite 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. Omite works better at higher temperatures. Where possible, its use should be conserved for the summer rather than the spring.
4. Resistance to Kelthane is widespread, but can be detected using a bioassay. Contact your Pest Management Adviser for details.
   

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Predatory Mite Biology

Before chemical pesticides were widely introduced at the end of the 1940s, mites were rare pests on fruit crops. They were almost completely regulated by their natural enemies, mostly beneficial mites and some insects (e.g., mullein bug).

A recent survey of 112 Ontario apple orchards revealed that over 75% still have beneficial mite populations surviving in them, but often in low numbers. A number of orchard pesticides that are toxic to these beneficials and not compatible with IPM, should be avoided to minimize harmful effects on predatory mites. Some examples are pyrethroids, Benlate, Zolone and Carzol. Another pesticide, Sevin, even used at low rates as a fruit thinner, is also very toxic to some important predator mite species.

The three most common species of predatory mites in Ontario apple orchards are Balaustium putmani, Zetzellia mali and Amblyseius fallacis.

Balaustium putmani is a large velvety-red, tick-shaped mite (Figure 12 and Figure 13) that overwinters as eggs underneath bark. Nymphs hatch in early May and begin feeding on mites, their eggs and other small, soft-bodied insects. They remain in fruit trees all season and are important early-season predators.

Figure 12. Balaustium, also known as red velvet mite.

Figure 13. Balaustium mite on human hand to show size.

Zetzellia Mali is a lemon-yellow mite, with a diamond-like shape (Figure 14). It overwinters under bark and may suffer heavy winter mortality in some parts of the province. It becomes active early in the spring, feeding on overwintering European red mite eggs and rust mites. It can be fairly effective in controlling low-density pest mite populations.

Figure 14. Zetzellia Mali feeding on ERM eggs.

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Amblyseius fallacis is the most important mite predator in Ontario. It is a pale mite, the color varying between off-white to brownish to reddish depending on what the individual mite has been feeding on
(Figure 15). It is teardrop-shaped and extremely fast-moving. Somewhat smaller than an adult female European red mite, it feeds on all stages of all 3 pest species of mites on apples. Alternate food sources include ARM and pollen grains. Adult females overwinter near the base of trees or surrounding ground cover. They become active in the spring, moving into tree canopies in June and July where they feed on European red mite eggs, nymphs and adults and other mite pests (Figure 16). There are 4 to 6 generations per season. Amblyseius can be a very effective natural control agent for pest mite populations. Studies have shown that a ratio of one Amblyseius to 10 to 15 pest mites can give effective biological control. Monitoring of pest mites should always include counts of Amblyseius.

Figure 15. Amblyseius fallacis, Ontario's most important mite predator.

Figure 16. Amblyseius fallacis, feeding on adult TSSM.

Since 1993, a pesticide-resistant strain of Amblyseius fallacis has been commercially available in Ontario. Developed by Agriculture & Agrifood Canada researchers, this strain has a high level of resistance to organophosphate insecticides (e.g., Guthion, Imidan) and moderate resistance to pyrethroids. Currently being used experimentally in small-scale releases by commercial growers, this strain may become an important tool in managing pest mites in the future. For more information on purchasing and releasing this predator into your orchard contact your Tender Fruit & Grape IPM Specialist.

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Photo credits: Agriculture and Agri-Food Canada; Ontario Ministry of Agriculture and Food

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