European Red Mite, Panonychus ulmi (Koch)

I. Introduction: The European red mite (ERM) (Plate 44) is a major tree fruit pest, especially in the eastern U. S, and is considered by many growers to be their most important and sometimes most difficult pest to control. The mite was introduced to North America from Europe in the early l900's and is now established in most fruit growing areas.

II. Hosts: The ERM will attack a number of different fruit crops and can cause extensive injury if left uncontrolled, but apple is usually the most important host. It will also attack cherry, nectarine, peach, pear, plum, and prune.

III. Description: The ERM overwinters as a fertilized egg (Plate 45). It is oval, bright red, and has a small stalk arising from the top, approximately the length of the diameter of the egg. Overwintering eggs are deposited in groups on the roughened bark area, especially around buds and fruit spurs. The summer eggs are globular, somewhat flattened, and pale green, almost white to greenish amber when first deposited, but slowly change to a reddish-orange just before hatch. Six-legged larvae hatch from the eggs which are initially a pale orange, darkening to a pale green as they feed. The larvae molt to eight-legged protonymphs which are variable in color from a pale green to reddish brown with the dark green predominating. Pale spots at the base of the bristles are very faint, if present at all. The deutonymphs are variable in color from an amber color to a dark green, with a dark-green color predominating. Pale spots are distinct at the bases of the bristles. The eight-legged adult female mite has a globular body and is approximately 1/64th inch (0.4 mm) long, bright red to velvety brown in color, and has four rows of white hairs on its back. The adult male mite is smaller, has a pointed abdomen, and the color is straw-yellow to reddish-yellow.



IV. Biology: Overwintering egg hatch in the spring is closely correlated with apple bud development and first occurs when 'Delicious' buds are in the prepink stage; hatch continues throughout the bloom period. The larvae immediately move to the young foliage and begin to feed. Before the mites transform to each succeeding stage, they go through a short quiescent period. Shortly before the adult female emerges, she releases a pheromone which attracts a male. If mating ensues, female young are produced; inf a female is unmated, she produces male offspring. Adult mites usually appear by petal fall. Female mites live up to an average of 18 days and each female is capable of laying approximately 35 eggs during her life span. The rate of development is temperature dependent. Under ideal hot conditions (average 80*F), a life cycle may be completed in 10 to 12 days, however, a generation normally takes 20 to 25 days to complete. Eight to 10 generations can occur during a normal growing season. ERM populations usually peak in July. Females begin laying overwintering eggs in early to mid-August.



V. Injury: The mites injure the tree by feeding on leaves, destroying chlorophyll, and increasing respiration. This is accomplished by the insertion of the mite's mouth parts into the leaf cells to withdraw the contents. All motile stages feed on the foliage. The lower leaf surface is preferred, but both leaf surfaces are attacked when populations are high. All apple varieties are susceptible to attack, but the mites appear to increase faster and to higher densities on 'Delicious' and 'Yorking'. A characteristic brown foliage, starting as a subtle cast to the green leaf, but in severe cases becomes bronze, results from heavy mite feeding. The most serious injury occurs in late spring and early summer when trees are producing fruit buds for the following season. Moderate to heavily injured trees (i.e. with 750 mite days or with 43 motile mites per leaf) produce fewer and less vigorous fruit buds. Mites feeding on leaves also reduce their ability to manufacture enough photosynthates for desirable sizing of fruit. Injury effects from mite feeding are more severe during periods of drought stress. Late season high populations of mites can cause further indirect loss of fruit by depositing overwintering eggs, especially in the calyx end.

VI. Monitoring: From the dormant period up to early pink, overwintering ERM eggs may be evaluated. Select at random 5-10 trees in an orchard block and inspect small branches using a magnifier. Close attention should be given to the bases of twigs and spurs.

If no oil application or other acaricide were applied prior to bloom, then monitoring should begin during the bloom period. Select 5-10 trees of the same cultivar (e.g. `Delicious', `Stayman' or `Yorking') randomly scattered throughout the block. Collect 5-10 leaves of middle age (i.e., from the middle of a fruit spur) and examine each leaf for the presence or absence of one or more motile mites. After examining all leaves, determine the percentage of mite-infested leaves. Refer to Table 2 and select the expected number of mites per leaf for a given percentage of mite-infested leaves. If an oil application were made prior to bloom, the first sample for motile mites can usually be delayed until early to mid June. The same procedure should be followed. After an acaricide application is made, this method of monitoring mites is not as reliable. A direct count of the motile mite population is necessary. Examine the same cultivar and number of trees as before, but count the total number of motile mites on each leaf. Mite populations should be monitored on a weekly basis, especially during periods of rapid buildup. See the Univ. Maine link on sequential sampling for ERM.

Once pest mites [e.g., ERM and twospotted spider mite (TSM)] and natural enemy populations are determined, it is important to incorporate the economic impact of mite management into the decision-making process. Variables such as time of season, crop load, and miticide costs and efficiency should be considered, along with mite, Stethorus and predatory mite counts. The degree of mite injury to a leaf is affected not only by the density of mites per leaf but also by the number of days a given density of mites is allowed to feed. Thus, the term "mite days" is defined as the product of the number of mites per leaf multiplied by the time they are present (e.g., 5 mites per leaf for 5 days equals 25 mite days per leaf). Since growers think in terms of mites per leaf rather than mite days per leaf, a mathematical relationship has been determined between mite days and motile mites per leaf. Therefore, mites per leaf may be used to make decisions about mite control (see second Table). Based on the relationship of mites per leaf and mite-days, action thresholds have been developed by Penn State University to aid in making mite management decisions. The following graph includes mite action thresholds for various crop loads at different times of the growing season. To use this figure, determine the number of mites per leaf based on the previous monitoring instructions for both pest mites and Stethorus populations. Next, estimate the projected production per acre (harvested bushels) for the affected block. Select the threshold line on the figure for the appropriate time of the growing season. For a given time of the growing season and a given estimated crop load, if the mites per leaf exceed the threshold then some control mechanism is needed-either control by Stethorus or by application of miticides. If you are using the alternate row-middle system of spraying to make your miticide applications, reduce the action threshold to one-half the figure value since you are only spraying one-half of the tree. These levels apply to healthy, vigorous trees with mite damage occurring only after June 1. If the mite population does not exceed the action threshold, the mite population should be reassessed within 1-2 weeks.

Table 2. European red mite densities predicted from the percentage of mite-infested leaves.

Percentage of...............Expected density....... .Limits of mite
infested leaves1............in mites/leaf...............pop. in mites/leaf2

40...............................0.7...........................0.25 - 1.20
45...............................0.9...........................0.35 - 1.45
50...............................1.1...........................0.45 - 1.75
55...............................1.3...........................0.60 - 2.15
60...............................1.6...........................0.80 - 2.65
65...............................2.0...........................1.05 - 3.25
70...............................2.6...........................1.35 - 4.10
75...............................3.4...........................1.85 - 5.35
80...............................4.7...........................2.55 - 7.25
85...............................6.8...........................3.85 - 10.55
90..............................11.4...........................6.50 - 17.55
95..............................26.4..........................15.30 - 40.30
1 Leaves with at least one motile stage. 2 95% confidence interval.

Table 3. Relationship between number of European red mites per leaf at their peak and accumulated number of mite days per leaf.

Peak number......Expected number.........Expected limits of
of mites/leaf......no. of mite days..........no. of mite days1
5......................88........................0 - 332
10....................176........................0 - 420
15....................263.......................10 - 507
20....................351......................107 - 595
25....................439......................195 - 683
30....................527......................283 - 771
35....................615......................371 - 859
40....................702......................458 - 946
45....................790......................546 - 1034
50....................878......................634 - 1122

1 95% confidence interval - there is only a 1 in 20 chance of the accumulated mite-day value falling outside this range.

If the mites per leaf exceed the action threshold, the Stethorus population should be assessed by determining a predator-to-mite ratio. To calculate predator-to-mite ratios, divide the number of Stethorus adults and larvae counted in 3 minutes by the number of motile mites per leaf. Example: 25 Stethorus adults and larvae divided by 10 motile mites per leaf equals a predator-to-mite ratio of 2.5. If the predator-to-mite ratio is less than 2.5 and the action threshold has been reached, then a miticide application is justified. The orchard should be checked again in 5 to 7 days.

Amblyseius populations can be monitored at the same time growers are scouting for spider mites since they occupy the same habitat. Initial populations in the spring may be assessed by selecting 10 apple leaves from suckers beneath each of 10 randomly selected trees in a block. Examine the surface for Amblyseius moving across the leaf surface. Research in Michigan on this mite system has yielded tentative thresholds for predicting success of biological control by Amblyseius. A ratios of predators to prey of at least 1:10 presents a good probability of biological control; higher ratios increase the probability of success. Lower predator:prey ratios (e.g. 1:20) may result in successful control on some apple varieties less conducive to spider mite reproduction than `Red Delicious'. If high populations are found in late season, ground cover management, particularly herbicide selection in the fall and the following spring, should take Amblyseius into consideration. There are no thresholds available for ground cover/sucker counts.

Zetzellia populations can also be monitored at the same time growers are scouting for the European red mite and twospotted spider mite. There are no validated management thresholds for Zetzellia; however, populations averaging 2-3 motile Zetzellia per leaf can cause phytophagous mite populations to decline.

See New Zealand photos.

This is taken primarily from a chapter by D.G. Pfeiffer, L.A. Hull, D.J. Biddinger, & J.C. Killian on apple indirect pests, reprinted with permission from Mid-Atlantic Orchard Monitoring Guide, published by NRAES, 152 Riley-Robb Hall, Ithaca, New York 14853-5701.
Additional Reading:
Back to Virginia Apple Page
Back to Home Page for "Arthropod Management in Fruit Crops" course
Back to Virginia Fruit Page
Maintained by: Douglas G. Pfeiffer
Department of Entomology
Virginia Tech
Blacksburg