Mite pests in apples

Excerpt from Publication 310, Integrated Pest Management for Apples.
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Table of Contents

  1. Introduction
  2. European red mite
  3. Twospotted spider mites
  4. Apple rust mite

Introduction

The two major mite pests of apples in Ontario are the European red mite, Panonychus ulmi (Koch), and the twospotted spider mite, Tetranychus urticae (Koch). Both species belong to the phytophagous (plant feeding) mite family Tetranychidae, or spider mites, and are pests of many crops including tree fruit, small fruit, vegetable, ornamental and field crops. The apple rust mite, Aculus schlechtendali (Nalepa), is a sporadic pest belonging to the mite family Eriophyidae.

Apple rust mites seldom cause a measurable loss in apple orchards. There are exceptions and miticides are sometimes needed for management of populations. Twospotted spider mites tend to be more of a problem in eastern Ontario than southwestern Ontario. Growing season temperature is the most important abiotic factor affecting population buildup. The development of mites is positively correlated to temperature - the warmer the temperature, the faster mites hatch, mature and reproduce. Both European red mite and twospotted spider mite do best under hot, dry conditions.

European red mite

European red mites are the most common mite found in Ontario apple orchards, and are found from early in the spring through to harvest.

Description

European red mite eggs are red, slightly flattened (onion-shaped) and have a hair-like stalk protruding from the top. Newly hatched nymphs (larvae) have three pairs of legs. Older nymphs have four pairs of legs as do the adults. Immature mites are typically reddish, but may appear green following molting (the red colour develops with feeding). European red mites range in size from 0.15-0.4 mm, depending on life stage. Males and females are distinct from one another. The adult female is a deep brown-red, about 0.40 mm in length and has rows of spots on her back with raised "spines" (Figure 4-1). The male is smaller (0.28 mm), lighter or drabber in colour with a pointed abdomen, and legs proportionately longer than the female. Immature mites feed primarily on the lower surface of the leaf near the veins and midrib. Adults feed on both the upper and lower surfaces of the leaf. Although they belong to the same family as twospotted spider mites, European red mites produce less obvious webbing.

Figure 4-1. Female European red mite and eggs

Figure 4-1. Female European red mite and eggs

Biology

Overwintering European red mite eggs are usually found on roughened bark around the bases of buds and spurs, and in the inner parts of the tree close to the main trunk and branches (Figure 4-2). Eggs begin to hatch around the tight cluster stage of apple.

Figure 2. Overwintering European red mite eggs on spur

Figure 2. Overwintering European red mite eggs on spur

As European red mite eggs hatch, nymphs move from the twigs to developing foliage where they begin feeding. Nymphs eventually become adults that mate and lay the first generation of "summer eggs." There can be six to eight generations of European red mite each year. The first few generations are generally synchronous in development, but by mid summer generations overlap and all stages (eggs, nymphs, adults) are present at the same time. Females begin laying winter eggs in late August on twigs, branches and in the calyx end of fruit.

Damage

European red mites feeding on leaves cause characteristic leaf injury referred to as bronzing (Figure 4-3). Mites insert their needle-like mouth-parts into leaf cells and suck out cell contents, including chlorophyll. Affected leaves appear stippled and may become bronzed if populations are sufficiently high. Severe infestations may result in defoliation.

Figure 4-3. Bronzing due to mite infestation

Figure 4-3. Bronzing due to mite infestation

Prolonged feeding by unmanaged mite populations stresses the tree, leading to reduced shoot growth and fruit bud set the following year. Fruit colour, soluble solids, firmness, size and weight of the fruit are also affected. In severe cases, mite-induced tree stress may result in death during harsh winters.

Monitoring and threshold

Begin weekly monitoring programs for European red mites in the dormant/tight cluster stage. Examine fruit spurs and twigs for overwintering mite eggs. From tight cluster through to petal fall, collect 2 fruit spurs from 25 random trees per block and examine the underside of the leaves using a dissecting microscope with a magnification of 25-40X for the presence of mite eggs, nymphs, adults and beneficial mites. Using a hand lens in the field may help experienced consultants and scouts obtain quick estimate of numbers, but does not provide accurate counts required for threshold numbers.

After petal fall, collect 2 leaves from each of 25 well-spaced trees per block (50 leaves total). Pick leaves at arm's length into the canopy. Include European red mite and twospotted spider mites in counts - add together totals of each life stage, eggs, nymphs and adults. Miticides vary in performance against different life stages and species.

Sample leaves on a weekly basis, especially during hot summer months when numbers can increase and exceed thresholds very quickly. Although mites commonly occur in greater numbers on trees in sheltered areas and next to dusty roadways, always sample equally from all parts of blocks.

Take separate samples for each orchard block or treatable area. Red Delicious, Empire and Gala tend to support the largest mite populations. Sample these cultivars, along with orchard blocks with a history of mite problems.

Beneficial mites can delay or prevent the need for a miticide application, so be sure to note their presence during monitoring. For thresholds refer to Table 1. Thresholds for European red mites and twospotted spider mites.

Table 1. Thresholds for European red mites and twospotted spider mites
Stage of growth Spray threshold and timing*
Dormant to tight cluster Eggs present
Petal fall to 21 days after calyx 1st egg hatch and before more than 3 nymphs/leaf
Petal fall to 21 days after calyx 5-7 active mites/leaf timed for 50% egg hatch
June to mid July 7-10 active mites/leaf, when approximately 50% of population are nymphs
Mid July to August 10-15 active mites/leaf, when approximately 50% of population are nymphs

*Some recently registered miticides are applied slightly earlier than conventional products, at 5 mites/leaf. For information on the timing of specific miticides, see OMAFRA Publication 360, Fruit Production Recommendations.

Management

Mite outbreaks in orchards are caused by:

  • weather conditions favourable to development
  • poor groundcover management (providing overwintering sites for twospotted spider mites)
  • high nitrogen levels in leaves (resulting in higher mite fecundity)
  • loss of shelter and habitat used by predators
  • use of pesticides toxic to natural enemies

Natural enemies of mites include predatory mites such as Typhlodromus pyri (Scheuten), T. caudiglans (Schuster), Amblyseius(=Neoseiulus) fallacis (Garman), Zetzellia mali (Ewing), Agistemus fleschneri (Summers) and Balaustium spp. Other important predators of mites include Stethorus punctillum (LeConte), minute pirate bugs (Orius spp.), predatory thrips, lacewings and mullein bugs Campylomma verbasci (Meyer). Beneficial mites (refer to Predatory mites) can provide biological control of pest mites. Research in Michigan shows a ratio of 1 beneficial mite/insect to 10 pest mites provides a sufficient level of biological control.

Use a selective pesticide program to preserve mite predators. Some pesticides are toxic to beneficial mites and act as a repellent or irritant to European red mite (pyrethroids), while others can increase egg laying (some neonicotinoids). A selective pesticide program may be less harmful to beneficial mites and prevent or delay the need for miticide applications.

Heavy rain can physically remove and kill many mites, and remove dust that collects on foliage and interferes with mite predators. Extreme winter weather or adverse conditions during hatch can negatively affect survival of overwintering European red mite (eggs), and twospotted spider mite and apple rust mite (adults). Temperatures above 30°C negatively affect European red mite egg laying and development. Twospotted spider mites tolerate higher temperatures before suffering detrimental effects.

Use well-managed cover crops between rows to limit dusty conditions that favour a buildup of pest mites. If ground cover dries, pest mites such as twospotted spider mites may move into trees.

Use a delayed dormant oil (from tight cluster through pink) to effectively manage European red mite by smothering its eggs. These oils have little or no impact on twospotted spider mite and apple rust mite that overwinter as adults on the orchard floor. Dormant oils are an important part of any apple integrated pest management (IPM) program.

Resistance to miticides is a serious concern in Ontario orchards. The long-term sustainability of mite resistance management programs requires judicious use of available products. Fortunately, Ontario growers have many different tools to manage mites and provide excellent options for a resistance management strategy. Consider the following information when managing mites.

  • Miticides are often highly selective. Some may only affect certain types of mites at certain life stages. Be aware of this when selecting a product.
  • Miticides are most effective when applied alone, using recommended rates and high water volumes (>1,000 L of water/ha).
  • Apply each miticide only once per season to delay the development of resistance. If miticides are needed two years in a row, use products from different chemical families to avoid the development of pesticide resistance. For more information on the chemical families of miticides, see OMAFRA Publication 360, Fruit Production Recommendations.
  • Some newer miticides are slower acting - remember this when applying products and for follow up assessments. For products with slow knockdown, wait one to two weeks after spraying to determine if the product was effective.
  • Research from the United States and British Columbia in apple and other crops indicates increased numbers of phytophagous mites are found where multiple sprays (three or more) of some neonicotinoids are used to manage other pests.

Twospotted spider mites

Twospotted spider mites are usually in lower numbers in orchards, and appear later in the season than European red mites. Production areas including Georgian Bay and east of Toronto often have more problems with twospotted spider mites than European red mite.

Description

Twospotted spider mite eggs are clear and spherical, becoming milky-white over time. Newly hatched individuals are colourless, with spots appear following feeding. As with European red mite, twospotted spider mites progress through several immature stages including a six-legged larva and eight-legged nymph. Twospotted spider mite adults and nymphs are easily distinguished from European red mites. They are pale green or straw-coloured with two dark spots on the back (Figure 4-4) and more elongated than European red mite. Twospotted spider mites produce dense webbing to coat the surface of leaves and accumulate dust. Overwintering females usually turn reddish-orange, and are often found near the calyx and stem of fruit near harvest (Figure 4-5).

Figure 4-4. Twospotted spider mite adult

Figure 4-4. Twospotted spider mite adult

Figure 4-5. Overwintering female twospotted spider mite

Figure 4-5. Overwintering female twospotted spider mites

Biology

Twospotted spider mites overwinter as orange-coloured adults under bark or on weeds beneath the tree. Populations often build up on broadleaf weeds, brambles and sucker growth beneath the tree and adjacent areas in the spring. Although twospotted spider mites can overwinter successfully on trees, large numbers often migrate into the tree canopy from the orchard floor in mid to late summer when weeds and other plants on the orchard floor dry up. Twospotted spider mite populations continue to thrive until cool, late summer weather reduces population activity. There are three to five generations each year. These mites are able to disperse over wide areas and from orchard to orchard by wind and air currents. Severe infestations of twospotted spider mite generally occur later than European red mite infestations.

Damage

Twospotted spider mites cause bronzing on the leaves. Spider mites create a characteristic webbing on the under surface of the leaf. Webbing gives mites and their eggs protection from natural enemies and environmental fluctuations. Prolonged feeding by unmanaged mite populations stresses the tree, and reduces shoot growth and fruit bud set the following year. Fruit colour, soluble solids, firmness, size and weight of the fruit are also affected. In severe cases, mite-induced tree stress may result in death during harsh winters.

Monitoring and threshold

For more information on monitoring twospotted spider mite and thresholds see the section on monitoring European red mite.

Management

More information on managing twospotted spider mite is in the management section for European red mite. Note that some miticides are more effective against European red mite than twospotted spider mite.

Apple rust mite

Apple rust mites are commonly found in orchards, and generally are not a concern unless numbers are extremely high.

Description

Apple rust mites are carrot or wedge-shaped, with cream to light brown colouration (Figure 4-6). In contrast to European red mite and twospotted spider mite, adults have only two pairs of legs. A dissecting microscope with a 100X magnification is required for identification and counts. Their small size makes apple rust mite easy to overlook.

Figure 4-6. Apple rust mite

Figure 4-6. Apple rust mite

Biology

Female apple rust mites overwinter beneath bud scales, emerging to feed on foliage as buds open in spring. Eggs are deposited on the underside of leaves. There are numerous generations each year. These mites can be beneficial in low numbers because they are an important alternate food source for predatory mites when European red mite and twospotted spider mite numbers are low. Apple rust mite presence encourages predatory mites and other natural enemies to remain in the tree canopy rather than migrate to other areas.

Damage

Apple rust mite injury is variable and includes yellowed and distorted foliage. Low populations of apple rust mites are not a concern, but prolonged feeding by high populations (200 or more per leaf) damages the lower leaf surface and causes a leathery silver appearance and characteristic curling of the leaf. Feeding on developing fruit may damage the epidermis and cause russeting.

Monitoring and threshold

Do not use miticides unless apple rust mite populations are greater than 200-500 mites per leaf. Low populations of apple rust mites provide valuable prey for predatory mites.

Management

Some miticides are not effective against apple rust mites. For a list of products with activity against apple rust mites, see OMAFRA Publication 360, Fruit Production Recommendations.

 


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Author: OMAFRA Staff
Creation Date: 21 July 2011
Last Reviewed: 21 July 2011