Mites feeding on leaves cause injury to the tree by removing leaf tissue. The most serious injury occurs in early summer when trees are producing fruit buds for the following season. Photo by G. Krawczyk.
The mite was introduced into North America from Europe in the early 1900s and is now established in most fruit growing areas, attacking apples, stone fruits, and pears.
Description and life cycle
Eight-legged females are 1/64 inch long, bright red, and have four rows of white hairs on their backs. Males are smaller, lighter in color, and have pointed abdomens. Overwintering eggs are round, bright red, and have a small stalk, approximately the length of the diameter of the egg, arising from the top. Summer eggs are pale and translucent. Six-legged nymphs hatch from the eggs, molt to eight-legged protonymphs, then deutonymphs, and finally adults.
Overwintering eggs are laid in groups on roughened bark, in crevices and cracks, and around bud scales on twigs and branches. Eggs begin to hatch at prepink bud stages and continue throughout bloom. Young mites move to newly opened leaves where they feed, mature, and reproduce. The first generation requires approximately three weeks to develop; summer generations are completed in 10 to 18 days. The rate at which mites develop is primarily temperature dependent. Hot, dry weather favors development, while cool, wet weather delays mite activities. Each female is capable of laying 35 eggs during her average life span of 18 days. Eight to 10 generations occur during the year.
Mites feeding on leaves cause injury to the tree by removing leaf tissue. The most serious injury occurs in early summer when trees are producing fruit buds for the following season. Moderate to heavily infested trees produce fewer and less vigorous fruit buds. Mites feeding on leaves also reduce the ability of leaves to manufacture enough food for desirable sizing of fruit. A characteristic brown foliage that, in severe cases, becomes bronze, results from heavy mite feeding. Left unchecked, mite populations can affect fruit color and result in premature fruit drop.
Our integrated mite management program for apples depends largely on the phytoseiid predatory mites Typholdromus pyri (Scheuten) and Neoseiulus fallacis (Garman) to regulate pest mites to low levels during the cooler spring and late summer/fall months. Once established, T. pyri is capable of maintaining pest mite populations at very low levels if not disrupted due to the use of toxic pesticides (see the Tree Fruit Production Guidefor toxicity ratings). T. pyri is able to regulate pest mites at much lower levels than the black ladybug, Stethorus punctum, and which T. pyri has largely replaced in many grower orchards. For more information on establishing and conserving this predatory mite or Stethorus, refer to the "Natural Enemies" section of the Tree Fruit Production Guide.
European red mite eggs may be evaluated in the dormant period up to prepink. Inspect with a hand lens the bases of twigs and spurs on the 5 to 10 selected trees. Look for clusters of tiny (less than 1/50 inch), red spheres. If overwintering eggs are easily visible, especially to the unaided eye, then a prebloom application of oil or a miticide-ovicide is recommended to prevent mite injury through June.
At pink stage and beyond, before a miticide that kills motile mites is applied in the current season: if no adulticide has been applied in the current season, scan 5 to 10 leaves on each of the selected trees for mites. A leaf is mite infested if it contains one or more motile mite. Record each leaf as either "mite infested" or "mite free." Divide the number of mite-infested leaves by the total number of leaves examined. Multiply the number by 100 to compute the percentage of mite-infested leaves. As mite populations increase, so will the percentage of infested leaves. Refer to Table 2-3 to estimate the expected number of mites per leaf for the given percentage of mite-infested leaves.
Table 2-3. European red mite densities predicted from the percentages of mite-infested leaves.
|Percentage of mite-infested leavesa||Expected density in mites/leaf||Limits of mite populations in mites/leafb|
|aLeaves with at least 1 motile stage.|
b95% confidence interval.
After a miticide has been sprayed to control motile mites in the current season: follow the above procedures except that the number of motiles mites (all stages except eggs) per leaf should be counted. Determine the average number of mites per leaf by dividing the total number of mites found on all leaves by the number of leaves examined.
The impact of beneficial predatory mites N. fallacis, T. pyri, and Zetzellia mali on phytophagous mite population is described under the "Natural Enemies" section of the Tree Fruit Production Guide.
Mite action thresholds and predator/prey ratio calculations
Growers should use the following guidelines for incorporating the economic impact of mite management into the decision-making process. Variables such as time of season, crop load, and miticide costs and efficiency are considered, along with predatory mite and Stethorus counts. Action thresholds have been developed to aid in making mite management decisions. An action threshold is a mite population level at which control measures should be taken to prevent economic damage. Factors that tend to lower action thresholds are damage by other pests (e.g., white apple leafhopper) and severe weather conditions (e.g., extremely dry conditions intensify the losses from mite damage).
The figure below 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 either percent mite-infested leaves or actual counts after an acaricide has been applied. 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 control either by using predatory mites, Stethorus punctum, or by applying 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 5 (Table 2-4).
Table 2-4. Relationship between number of European red mites per leaf at their peak and accumulated number of mite days per leaf.
|Peak number of mites/leaf||Expected accumulated number of mite days||Expected limits of number of mite daysa|
|a95 percent of confidence interval--there is only a 1 in 20 chance of the accumulated mite-day value falling outside of this range.|
If the mites per leaf do not exceed the action threshold, no control action need be taken. Typically, orchards with stable populations of T. pyri never reach these thresholds as they never leave the tree and exert their control over pest mites in the spring and fall. Mite control with N. fallacis is much more variable. N. fallacis do not overwinters on the tree but instead move up from the weeds in the ground cover midseason and sometimes arrive too late to exert their control over pest mites. Alternate row middle sprays with a selective miticide is sometimes required to establish the proper predator-to-pest ratio for biological control. For both T. pyri and N. fallacis, a predator-to-prey ratio of 1 predator to 10 pest mites will almost always ensure biological mite control if not disrupted with toxic pesticides. Z. mali is strictly an egg predator and mostly supplements early and late season control with the other predator mites. For biological control with Stethorus, populations 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.
Biological control with predatory mites
- Avoid pesticides that are toxic to the natural enemies.
- Do not attempt to eliminate the entire mite population and in the process also risk eliminating predatory mites.
- The alternate row middle pesticide application method is sometimes recommended, although certain conditions will dictate the use of complete sprays. These conditions include extremely high mite pressure, use of a pesticide of questionable efficacy, inadequate spray coverage, and weather conditions conducive to a rapid increase in the mite population.
- The population densities of the mites and the beetles must be known in order to determine if the predator populations are sufficient to overcome the mite population or if the aid of a miticide is needed.
- Closely monitor mite populations.
- Encourage natural enemies into the orchard by using insecticides and miticides more selectively.
- Use action thresholds to determine the necessity of spraying.
- Rotate miticides within the same season (i.e., do not use the same chemical or products from the same IRAC group more than once in the same growing season).
- Spray late in the evening or at night for better coverage and contact.
- If using the alternate row middle method to apply miticides, do not allow the number of mites to get too high before applying a miticide (i.e., use one-half the thresholds listed above). If properly timed, only one alternate row middle application may be needed.
- Increase the volume of water to 100 gallons per acre if coverage
is a problem. The efficacy of various acaricides is detailed in the Penn State Tree Fruit Production Guide.
- Specific chemical recommendations for home gardeners are in Fruit Production for the Home Gardener.