Web site - Biological Control of Mites

Biological Control of Mites

The most successful biological control programs in eastern tree fruits have centered on the conservation of native species of mite predators to control the European red mite and twospotted spider mite. After 40 years of use, some of these predators have developed resistance to organophosphate insecticides (e.g., Stethorus), but are suppressed or eliminated when broad-spectrum carbamate and pyrethroid insecticides are used. The use of pheromone mating disruption, horticultural oils, and some of the more selective reduced-risk insecticides and miticides will allow a natural increase of predators capable of regulating pest mite populations to tolerable levels without the use of miticides. Mite control through biological control has the additional advantage of stopping the development of miticide resistance and, once established, is sustainable long-term if the use of certain harmful pesticides is avoided. The routine use of carbamates and pyrethroids in stone fruits, pears, grapes, and small fruits currently prevents reliable biological mite control agents even though many of the same predators found in apples can be present.

Listed below are descriptions of the main biological mite predators found in Pennsylvania apple orchards:

Typhlodromus pyri (Phytoseiidae)
Discovered in Pennsylvania for the first time in 2003, this predatory mite is currently the most reliable and effective mite predator in eastern U.S. apple orchards. Pear shaped and slightly larger than a European red mite adult, they are white/translucent until they feed. When feeding on adult red mites or apple rust mites, its abdomen may appear reddish. It is very similar in appearance to Neoseiulus fallacis, also commonly found in apple orchards, but the predatory mite is an omnivore and much more closely associated with its apple host. T. pyri is very active and moves rapidly to consume up to 350 mite prey in a lifespan of about 75 days. Females may lay up to 70 eggs each and have several generations per season. Populations, therefore, can build rapidly in response to pest mite populations. Most effective in the cooler weather of the spring and fall, T. pyri is somewhat less effective in the summer months. It overwinters on the apple tree under the bark where it is less susceptible to dormant oil applications and is very tolerant of Pennsylvania's winters.

Preferring spider mites, T. pyri is able to regulate pest mite populations well below injury thresholds of less than five pest mites per leaf. It is able to reproduce well on relatively harmless apple rust mite populations when spider mites are absent and can subsist for long periods on other predatory or scavenger mite species, and on pollen and fungal spores when pest mite populations are low. It is also known to feed on immature thrips and scale crawlers. Well adapted to living in apple, T. pyri does not leave the tree during the season and once populations are established, sustainable mite control is virtually assured when the predator-to-prey ratio is at least 1:5 and is highly probable at a ratio of 1:10. For apple varieties less susceptible to spider mites than Delicious, predator-to-prey ratios as low as 1:20 may still result in successful biological control if they occur during the cooler spring and fall months. This seasonal association with its apple host, however, makes them very susceptible to toxic pesticides. Because they do not disperse quickly, they may take several growing seasons to reestablish after elimination by harmful pesticides unless artificially reintroduced. Once populations are identified or artificially established, conservation is therefore very important and applications of certain pesticides have to be avoided (see Table 4-5). Natural populations are most likely to be found in grower orchards that rely primarily on organophosphate and reduced-risk insecticides and where pheromone mating disruption is being used. T. pyri can sometimes be found in orchards with large, standard-sized trees despite harmful pesticide applications because inadequate spray coverage may leave refuge areas for populations to persist. Establishment of T. pyri into orchards where it is absent is relatively simple and can be accomplished in one to two seasons once “donor” orchards with abundant T. pyri populations have been identified as a source. Transfers of T. pyri from these orchards can be successful by physically moving blossom clusters or shoots in May and June. (See orchard transfer methodology sections below.)

Conservation and augmentation of Typhlodromus pyri
While a number of mite predators such as Stethorus punctum, Neoseiulus fallacis, and Zetzellia mali may contribute to the biological control of European red mites and twospotted spider mitess in apples, only the conservation of native populations of Typhlodromus pyri have proven to give consistent, long-term control. Once established, T. pyri can almost completely regulate pest mite populations without the need for miticides, if the use of certain toxic pesticides is avoided.

1. The first step for apple growers in establishing mite control with T. pyri is to determine if it exists in significant numbers in their orchards. The most likely sites are:

Those that have not received pyrethroid or methomyl applications for several seasons.
Older orchards with large trees where spray coverage is not complete.
Abandoned orchards.
Reduced-risk pesticide orchards or those relying mostly on pheromone mating disruption to control codling moth and Oriental fruit moth.

Sample several trees in each block by examining with a hand lens (10 to 15X) the underside, mid-veins of 25 leaves per tree for fast-moving, teardrop-shaped mites. They will appear to be clear or slightly reddish, but not red or bright yellow in color or have spots. The best time to sample orchards would be midseason (June or July) or when pest mites are beginning to build. Samples taken early in the spring and in the fall may have relatively low populations that are difficult to detect.

2. If T. pyri is present, do not use pyrethroids or carbamate insecticides after bloom (with the exception of carbaryl for fruit thinning).

T. pyri begins to emerge from overwintering sites deep in bark crevices at the beginning of bloom, so prebloom pesticides have little effect on them. In addition, dormant and summer oil applications have little effect on T. pyri, but help suppress pest mite populations. Applications of pyrethroids and methomyl after bloom cause near complete elimination of populations, and may require two to three seasons to return naturally. If a ratio of at least one predator to every ten pest mites is not reached, it may be necessary to suppress the pest populations with a selective miticide (see Table 4-5).

3. If T. pyri is not present in particular orchards, they can be introduced from shoots or blossom clusters cut from identified "donor" sites.

In order to have the best chance of establishing T. pyri populations in a single season, transfers of shoots and leaf spurs are best made early season after petal fall (and June), but before the hot weather of summer (July and August). Transfers after July appear to be less likely to establish populations. Also effective are transfers of flower clusters during bloom when T. pyri are concentrated in order to feed on pollen. Transfers should be made at two shoots or clusters to every sixth tree in high-density plantings and every third tree in normal plantings. Cutting with hand pruners from a T. pyri donor orchard and placing the shoots or flower clusters in the tree canopy of a new orchard takes approximately 1.5 hours per person/acre.

Neoseiulus fallacis (Phyoseiidae)
Almost indistinguishable from T. pyri except under a microscope, this predator is currently more widespread in distribution in Pennsylvania apple orchards than T. pyri, due to a higher tolerance for some pesticides and the use of alternative plant hosts. Like T. pyri, N. fallacis is also very active, but is able to build populations three times faster during the hotter summer months. This predator lives only about 20 days with each female laying 40 to 60 eggs and may have 6 to 7 generations/year. Like T. pyri, N. fallacis is resistant to organophosphate insecticides, but it is very susceptible to pyrethroids and carbamates.

This predator is not as tolerant of cool weather in the spring and fall and is susceptible to winter kill in Pennsylvania. Purely a predator, N. fallacis is not able to coexist on apple trees without pest spider mite populations to consume and will often leave the tree to feed on mites in the orchard groundcover. Apple rust mites are not an attractive alternative prey for this predator. Because its association with the apple host is not nearly as close as that of T. pyri, N. fallacis populations often do not build until mid- to late summer, leaving trees susceptible to early season mite injury. Because it can also survive in the orchard groundcover, however, N. fallacis is not as susceptible to elimination in the orchard due to applications of toxic pesticides applied to the tree. If conserved using selective pesticides, T. pyri gradually replaces N. fallacis after several seasons. The predator-to-prey ratio of T. pyri also applies to N. fallacis and distinguishing between the two species is not important as long as this ratio is reached.

Zetzellia mali (Stigmaeidae)
An omnivore like T. pyri that is able to exist on pollen, fungi, and rust mites when spider mite populations are absent, Z. mali is very slow moving and feeds only on the eggs of pest and predatory mites. Its diamond shape and bright yellow coloration (turning more reddish after feeding) make it easy to distinguish this predator from other predatory mites. It is smaller in size than the European red mite. Because it is less active, it is able to exist on pest mite populations even lower than T. pyri. Like T. pyri, it is also more active in the cooler spring and fall months. However, with only a couple of generations each season and a consumption rate of only two to three eggs per day, it cannot usually be relied on to control mite pests alone. It is a valuable supplement to control by other mite predators and is much more tolerant of most pesticides, including carbamates and pyrethroids. Generally, populations of more than one per leaf are necessary to exert significant control of spider mite populations.

Strethorus Punctum (Cocinellidae)
Once the cornerstone of biological mite control in Pennsylvania apple orchards, this small, black ladybeetle predator has greatly declined in importance over the last five years. Although one of the smallest of all ladybird beetles, S. punctum was the most important beneficial insect in Pennsylvania apple orchards starting in the mid-1970s and conservation of this predator reduced miticide use by 50 percent for over 30 years. While tolerant of many organophosphate insecticides, the decline of this predator was mainly due to the greater use of pyrethroids and the introduction of several new neonicotinoid and IGR insecticides that are toxic to various life stages of this predator. Reproducing only when populations of pest mites exceed eight to ten mites per leaf, relying on S. punctum alone requires grower tolerance of some foliar mite injury. With the registration of newer, more effective miticides in recent years, most growers are not willing to tolerate this injury, despite the high cost of miticides. S. punctum is now much less common in orchards and generally in small localized “hot spots” of mites. The main advantage of this predator is its ability to fly and quickly colonize areas of high mite populations.

Description and life cycle
Stethorus adults are tiny (1/20 inch long), oval, convex, uniformly shiny black, and covered with sparse, fine, yellowish to white hairs. Eggs are very small (1/50 inch long), pale white, and oval. They become blackish just before the larva emerges. Larvae are gray to blackish and have many long-branched hairs and black patches. As the larva matures it becomes reddish, at first on the edges; just prior to pupation the entire larva turns reddish. Pupae are uniformly black, small, and flattened. The wing pads are prominent and the entire body is covered with yellow hairs. For a short period after it is formed, the pupa is orange.

Stethorus produces three generations per year in south-central Pennsylvania. The average period from the time the egg is laid to the appearance of the adult is 23 days. The adults feed for an average of 25 days before beginning to lay eggs. This time lag between emergence and egg-laying is of little consequence because there is such an overlapping of active adults in the trees at all times. Adults overwinter beneath the trash cover under fruit trees and in other protected habitats near the orchard. The distribution of Stethorus in the orchard groundcover is closely associated with the leaf litter, especially around root suckers. About 70 percent of the adults are located immediately around the trunks, 20 percent are located in the border of the herbicide strip and the row middles, and the remaining 10 percent reside in the rest of the herbicide strip. The overwintering adults emerge from these sites and move into the trees between tight cluster and petal fall.

Stethorus adults are very active when in fruit trees and if disturbed will often fall to the ground. They are good fliers and thus tend to concentrate where prey is plentiful and to disappear when the mite population becomes low. Indications are that areas in the orchard having at least five mites per leaf are necessary to keep Stethorus active, and that from eight to ten mites per leaf are needed in these “pockets” to encourage the beetle to reproduce. The beetles feed on all stages of mites, and the adult can consume approximately nine mites per hour, or about 75 to 100 mites per day.

During early May Stethorus females begin to lay eggs on the leaves of fruit trees. The eggs are laid singly on their sides, with one to ten per leaf depending on mite density. Most of the eggs are laid close to the primary veins of the leaf and adhere tightly, with 95 percent on the under surface of the leaf and 5 percent on the upper surface. Egg-laying continues through mid-August.

After a 5-day development period, larvae hatch and begin feeding on all stages of mites. Larvae go through four stages in an average of 12 days. The peak periods of larval activity in south-central Pennsylvania are mid-May, mid-June, and mid-August, but this is highly dependent on mite populations. The larva eats an average of approximately 10 mites per hour. After feeding for the 12-day period the fourth-stage larva fastens itself to the leaf and remains there in a motionless state for 24 to 48 hours before pupation. The pupal stage lasts an average of 5 days. Although pupae are constantly in the trees, the peak pupal periods are late May, late June, and late August, again dependent on the availability of mites to consume.

Monitoring and management
It is advisable not to disturb the area in the herbicide strip near the trunk of the tree from November 1 to mid-April. Adults are active in the orchard from mid-April to late October. See “European red mite” for determining the predator-to-prey ratio for making decisions about mite management.

Stethorus punctum, Black Ladybird Beetle Life Stages

Stethorus punctum (Coccinellidae)
Once the cornerstone of biological mite control in Pennsylvania apple orchards, this small, black ladybeetle predator has greatly declined in importance over the last five years. Although one of the smallest of all ladybird beetles, S. punctum was the most important beneficial insect in Pennsylvania apple orchards starting in the mid-1970s and conservation of this predator reduced miticide use by 50 percent for over 30 years. While tolerant of many organophosphate insecticides, the decline of this predator was mainly due to the greater use of pyrethroids and the introduction of several new neonicotinoid and IGR insecticides that are toxic to various life stages of this predator. Reproducing only when populations of pest mites exceed eight to ten mites per leaf, relying on S. punctum alone requires grower tolerance of some foliar mite injury. With the registration of newer, more effective miticides in recent years, most growers are not willing to tolerate this injury, despite the high cost of miticides. S. punctum is now much less common in orchards and generally in small localized "hot spots" of mites. The main advantage of this predator is its ability to fly and quickly colonize areas of high mite populations.

Description and life cycle
Stethorus adults are tiny (1/20 inch long), oval, convex, uniformly shiny black, and covered with sparse, fine, yellowish to white hairs. Eggs are very small (1/50 inch long), pale white, and oval. They become blackish just before the larva emerges. Larvae are gray to blackish and have many long-branched hairs and black patches. As the larva matures it becomes reddish, at first on the edges; just prior to pupation the entire larva turns reddish. Pupae are uniformly black, small, and flattened. The wing pads are prominent and the entire body is covered with yellow hairs. For a short period after it is formed, the pupa is orange.

Stethorus produces three generations per year in south-central Pennsylvania. The average period from the time the egg is laid to the appearance of the adult is 23 days. The adults feed for an average of 25 days before beginning to lay eggs. This time lag between emergence and egg-laying is of little consequence because there is such an overlapping of active adults in the trees at all times. Adults overwinter beneath the trash cover under fruit trees and in other protected habitats near the orchard. The distribution of Stethorus in the orchard groundcover is closely associated with the leaf litter, especially around root suckers. About 70 percent of the adults are located immediately around the trunks, 20 percent are located in the border of the herbicide strip and the row middles, and the remaining 10 percent reside in the rest of the herbicide strip. The overwintering adults emerge from these sites and move into the trees between tight cluster and petal fall.

Stethorus adults are very active when in fruit trees and if disturbed will often fall to the ground. They are good fliers and thus tend to concentrate where prey is plentiful and to disappear when the mite population becomes low. Indications are that areas in the orchard having at least five mites per leaf are necessary to keep Stethorus active, and that from eight to ten mites per leaf are needed in these "pockets" to encourage the beetle to reproduce. The beetles feed on all stages of mites, and the adult can consume approximately nine mites per hour, or about 75 to 100 mites per day.

During early May Stethorus females begin to lay eggs on the leaves of fruit trees. The eggs are laid singly on their sides, with one to ten per leaf depending on mite density. Most of the eggs are laid close to the primary veins of the leaf and adhere tightly, with 95 percent on the under surface of the leaf and 5 percent on the upper surface. Egg-laying continues through mid-August.

Monitoring and management
It is advisable not to disturb the area in the herbicide strip near the trunk of the tree from November 1 to mid-April. Adults are active in the orchard from mid-April to late October. See European red mite for determining the predator-to-prey ratio for making decisions about mite management.