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Achieving Biological Control of European Red Mite in Northeast Apples: An Implementation Guide for Growers

IPM Pub. #215, 1998

See also the Insert for IPM Pub. 215: Supplemental information on monitoring European red mite, and relative toxicity of pesticides to the mite predator, Typhlodromus pyri.

European red mites (ERM), Panonychus ulmi, feed on leaves of apple trees and thereby interfere with photosynthesis and production of carbohydrates. At high levels, ERM damage to apple leaves reduces fruit yield and quality. As a general rule, keeping ERM numbers below 2.5 per leaf before July, below 5 per leaf during July, and below 7.5 per leaf in August will prevent economic losses from this pest.

Three strategies can be used to control ERM in apple orchards. First, protectant miticides (e.g., dormant oil or an ovicide) can be applied early in the growing season. Second, pest mite numbers can be monitored and miticides applied if densities exceed threshold levels. Third, natural enemies that feed on ERM can be encouraged and managed to constrain pest mite numbers. Strategies based solely on miticides are relatively expensive and eventually lead to the development of resistance by ERM to the miticides. With the help of natural enemies, the cost of managing ERM in apples can be greatly reduced and resistance delayed.

Insect and mite predators, including several species of phytoseiid mites, stigmaeid mites such as Zetzellia mali, and ladybird beetles, feed on ERM. Phytoseiid mites are the most effective of these predators in the Northeast. Several species of phytoseiid mites, including Amblyseius fallacis, Typhlodromus pyri, T. occidentalis, T. vulgaris and A. cucumeris, can be found in commercial orchards. Species cannot be identified in the field because they are so similar in appearance; they are only distinguishable through microscopic examination of the arrangement of the setae (hairs) on their bodies. Typhlodromus pyri and A. fallacis are the two most common species in Northeast orchards. See Figure 1. Of the two, T. pyri is better able to regulate ERM populations. This is the species that should be established and maintained for biological mite control in Northeast orchards. In this bulletin we answer two questions: First, why is it that T. pyri is such an effective predator? Second, how can you make use of this natural enemy to provide cost-free mite control?

Why is Typhlodromus pyri such an effective predator? For many years A. fallacis was promoted as an effective biological control agent for ERM. In truth, A. fallacis gives sporadic and unreliable ERM control, while T. pyri is highly effective in this capacity. Differences in effectiveness of T. pyri and A. fallacis as biological control agents are rooted in their biologies.

Typhlodromus pyri require approximately 32 days to complete a generation, and have 3-4 generations per. They overwinter as mated adult females on trees wherever they can find a protective site (e.g., bark crevices, branches, spurs). Adult females emerge from overwintering sites on warm spring days before budbreak. The adults live about 20 days and lay an average of 20 eggs starting as early as tight cluster or pink bud growth stages. Eggs are usually laid on the undersides of leaves along the midrib. The eggs hatch in 1-3 days and resulting immatures are nearly transparent and look like smaller versions of the adults. Immatures and adults feed on a wide variety of food sources, including pollen and rust mites, along with ERM and two-spotted spider mites (Tetranychus urticae). An adult female will consume 1-2 ERM adults or 3-4 ERM nymphs per day. These predators do not concentrate on leaves with large numbers of ERM, unlike some other phytoseiids (e.g., A. fallacis). T. pyri are relatively winter hardy and remain in the tree even when ERM are scarce, feeding on alternative food sources. See Figure 2.

Based on generation time, oviposition rate, and prey consumption, it would appear that T. pyri is a less effective biological control agent than A. fallacis. But the advantages T. pyri has over A. fallacis are its greater winter hardiness, its use of alternative food sources when ERM are not present, and its tendency to remain in trees when ERM are scarce. When ERM numbers are low, T. pyri will stay in the tree canopy feeding on pollen and rust mites, and will continue to be a presence as ERM numbers start to rise.

How can Typhlodromus pyri be used to provide cost-free mite control? Achieving biological mite control using T. pyri is minimally a one-step process and may require two steps. First, an environment must be established in the orchard that will allow T. pyri to survive and flourish. This requires that pesticides that are toxic to these beneficial mites not be used. Second, if T. pyri are not already present in the orchard, they must be introduced.

An environment conducive to T. pyri Typhlodromus pyri have acquired resistance to some chemical pesticides used in commercial orchards and are innately tolerant of others. However, some pesticides are quite toxic to T. pyri. If biological mite control is to be achieved using this predator, these toxic materials must be avoided. Because T. pyri are resident in trees year round, and because these predators have a relatively slow growth rate, pesticides toxic to T. pyri cannot be used even intermittently (e.g., every other year) without serious disruption to biological control. A list of pesticides that can be used to control insects and diseases of apple while conserving T. pyri is provided as an insert in this bulletin. Be advised that estimates of toxicity to T. pyri were obtained using predators from western NY, and there may be differences in susceptibility among predator populations indigenous to other regions of the Northeast.

Introducing T. pyri into an orchard There are situations where T. pyri might not be present in an orchard or where they are very scarce. This deficiency can be overcome by moving predators from an orchard where they are known to occur to a recipient site. Because phytoseiid species cannot be identified in the field, it is important that you be sure the source predators are, in fact, T. pyri. The best way of ensuring this is to have someone identify them for you. If this is not possible, you can be reasonably sure the predators are T. pyri if either of the following conditions are met: 1) The predators can be found in the trees either before or just after bloom and the predators are easily found even when ERM are scarce. 2) The predators in the source orchard were themselves introduced as T. pyri one or more years ago, and no pesticides harmful to T. pyri have been used since the introduction.

The second method consists of transferring flower clusters from a source orchard to a recipient site. Typhlodromus pyri move into flower clusters at tight cluster and remain there through bloom, probably to feed on apple pollen. As many as 2 to 3 predators can be found in each flower cluster and surrounding leaves. To transfer predators in this manner, at least 20 flower clusters (and associated wood and leaves) should be placed in each recipient tree. The flower clusters are easily attached with paper clips, staples, or twist ties. Flower clusters may be stored for several days in a cooler before being affixed to receiver trees. See Figure 4.

The third method of transferring T. pyri consists of collecting leaves during the summer from trees where T. pyri are abundant, and placing them into recipient trees. Leaves are easily affixed to the target sites using staples. The number of leaves to use depends on the density of T. pyri in the source orchard. As a guide, at least 50 predators should be released in each target tree.

The fourth method of transferring T. pyri is perhaps the easiest and does not carry the risks of also moving unwanted pests that the three prior methods have. Artificial overwintering sites for T. pyri can be created by gluing burlap to the inside of tree wrap. These composite bands, approximately 12 to 16 inches in length, are then placed on source trees in early to mid-September by stapling them around the tree bole and/or large scaffold branches. In early December, these bands should be collected, tightly rolled with a rubber band used to hold them so, and placed in a sealed plastic bag with a bit (ca. 1 in3) of wet cotton. The bag should be placed in an insulated storage container, which in turn should be placed in a cold, though protected, environment that will buffer large temperature fluctuations. Ideally, temperatures should be maintained right at the freezing point. The following spring, the burlap bands should be placed around recipient trees at around the half-inch green bud growth stage. While the number of predators that overwinter in bands is variable, as many as 400 predators can be transferred in each band. We suggest placing a single band on each recipient tree if the bands were collected from trees that harbored moderate to high numbers of T. pyri (1-2 per leaf) the prior fall, and two bands in each tree otherwise. See Figure 5.

After a receiver orchard is inoculated with T. pyri, it often takes 2 to 3 years for the predator population to become abundant enough to regulate ERM without the need for any miticides. During this time, additional control measures are often needed to keep ERM below damaging levels. There are two key aspects to any strategy designed to do so. First, early season dormant oil sprays should be used to reduce ERM populations in the spring. These oil applications have no deleterious effect on T. pyri. Second, ERM numbers should be monitored, and if densities exceed threshold levels, a miticide that is not toxic to T. pyri should be used to control the pest mites. Note that it is actually desirable to have some pest mites in the trees after inoculation with T. pyri because these plant-feeding mites provide a food source for the predators and foster faster predator population growth.

A commonly asked question is; ‘How do you know when there are enough T. pyri to effect biological control?’ This question is difficult to answer. While predators can be seen in the field, they are easy to miss, especially at low densities, and their impact on ERM is dependent on which species they are. Guidelines have been provided for the ratio of predators to ERM needed to achieve biological control; however, estimating these ratios is not practical. Fortunately, all that is required to determine if biological control is working is to note whether pest mites remain below threshold levels. This can be determined without regard to predator abundance. A procedure for determining whether ERM exceed threshold levels is described in the insert in this bulletin. If pesticide regimes for all orchard pests can be followed that allow T. pyri to survive, these predators will become abundant enough to make miticide applications unnecessary.

Research funded by NYS IPM Program, Cornell University, and USDA.

Publication funded by National Biological Control Institute, APHIS, USDA.

Produced by Media Services at Cornell University, Address, Phone.

Cornell Cooperative Extension provides equal program and employment opportunities.

IPM Pub. 215, 9/98

Authors:
Deborah I. Breth, Cornell Cooperative Extension, Lake Ontario Fruit Program
Jan Nyrop, Dept. of Entomology, NYSAES, Cornell University
Joseph Kovach, NYS IPM Program, Cornell University