|European red mite|
Panonychus ulmi (Koch)
-- Elizabeth H. Beers and Stanley C. Hoyt
Although McDaniel spider mite was the outbreak mite species during the 1960s, European red mite has become the most common mite pest of apple in recent years. Twospotted spider mite is generally the most typical species on pear, but European red mite has been noted more frequently on this crop, also.
Overwintering eggs of European red mite (J. Brunner)
Eggs: Overwintering eggs are found on twigs and small limbs, especially in the crevices, beginning in the late summer or fall, throughout the winter, and up until they hatch in spring. They are about 1/160 inch (0.15 mm) in diameter, are brick red and have a stipe, or stalk. Summer eggs are laid on the foliage and, if populations are high, on the fruit, especially the calyx end. These are slightly smaller and about the same color as the overwintering eggs. After the egg hatches, the empty shell is transparent.
Immatures: There are three immature forms (the larva, protonymph and deutonymph). The larva is only slightly larger than the egg and is an orange-red. It can be distinguished from other immature stages by having only three pairs of legs. The protonymph and deutonymph are successively larger and have four pairs of legs, as do adults. Immature stages of European red mite are reddish in color but sometimes have a greenish cast. Between each stage is a quiescent stage which precedes a molt. The integument can appear silvery during this stage as it separates from the layer beneath.
European red mite adult female (E. Beers)
Adult: The adult female is about 1/72 inch (0.35 mm) in length, typically a solid brick red, and is characterized by her oval shape and strong white bristles on the back of the abdomen. The bristles, which have white bases, look like white spots on the back. The male is a yellowish red tinged with red. The male is only about 1/80 inch (0.30 mm) long and is more slender than the female. It has a tapered abdomen.
Adult male European red mite (B. Croft)
The larvae move to young leaves to feed. Immature mites often rest and feed more extensively on the bottom side of the leaves, but adult mites inhabit both surfaces. Although mites feed on newly expanded leaves early in the season, they are more likely to be found on hardened-off foliage when it is available.
The sex of the mite can be determined at the deutonymphal stage when females are already larger and more oval than the males, but the difference does not become obvious until the adult stage. The first adults appear around petal fall on apple. Males emerge first. The male waits near a quiescent female until she emerges and mates with her immediately. After about two days, females begin to lay eggs. A female mite lives 15 to 20 days and lays 30 to 35 eggs on average. Eggs from unfertilized females produce only males, by arrhenotokous parthenogenesis, but eggs from fertilized females produce both males and females. Eggs are deposited on both sides of the leaf but are concentrated near veins and midribs. In hot weather, they can complete their life cycle in as little as 10 days, though it can take up to 25 days in cooler weather. There are 6 to 8 generations per year, which overlap more as the season progresses. Populations usually peak in late July or August, although the peak is probably modified by pesticide use patterns.
European red mite dispersal behavior (apple) (E. Beers)
When mite populations reach high levels, they can disperse by a process known as "ballooning." Mites, sometimes in great masses, crawl to a high point or a tip of a branch and raise the anterior part of the body. The mite spins a silken thread, which is caught by air currents, taking the mite along with it. This is one of the primary means of mite dispersal to nearby trees where canopies do not overlap.
Females start laying overwintering eggs in mid-August to September. An over-exploited food resource (i.e., badly bronzed foliage) can lead to earlier deposition of overwintering eggs. Females usually lay eggs on the bark, but when populations are high they sometimes deposit them in the calyx end of the fruit. Although European red mite does spin webbing, it does not do so to the same extent as do twospotted or McDaniel spider mite.
Mites feed by inserting their mouth parts into leaf cells to suck out the contents, including the green pigment chlorophyll. The individual spots initially look white, giving the leaves a stippled appearance. As damage progresses, the infested leaves take on a brown hue, commonly called bronzing (Figure 147). Tree species, and cultivars within species, differ substantially in their reaction to mite feeding. Although Red Delicious trees tend to build higher populations of mites, Golden Delicious is more susceptible to damage by mites. Apples, in general, are more tolerant than pears. Whereas damage to apple foliage will cause bronzing and eventually some premature leaf abscission in August, damage to pear leaves can lead to "transpiration burn," where leaves develop large necrotic areas and are shed (Figure 148). This can occur in midseason or whenever substantial mite damage is combined with some additional types of stress on the trees.
Apple foliage damage (bronzing) by European red mite: left, normal; right, bronzed (L. Hull)
Pear foliage damage (transpiration burn) by twospotted spider mite (E. Beers)
Of the common Pacific Northwest pear varieties, the ranking from most to least susceptible to damage is Anjou, Bosc, Bartlett, and Comice. The red strains of pear cultivars are not prone to transpiration burn or defoliation and are considered tolerant of mite damage. As well as damaging foliage, spider mites feed on the epidermis of pear fruit, causing russeting. High populations late in the season are most likely to cause fruit russet.
Prune and other stone fruits have an intermediate reaction. Although the leaves bronze, as opposed to developing transpiration burn, defoliation is more common than on apple.
The effects of mite damage to foliage have been much studied but are still poorly understood. On apple, studies have examined photosynthesis, transpiration, shoot growth, trunk and limb growth, root growth, fruit size and rate of growth, fruit color (internal and external), soluble solids, firmness and other quality parameters, return bloom and return fruit set. At best, it can be stated that under some circumstances, mite damage affects one or more of these parameters. Similar studies on pears indicate a marked reduction in fruit size and return crop.
As the effects of feeding damage are cumulative over time, mite damage is often expressed as a combination of the population level (intensity) plus an indication of length of time of feeding (duration). The unit is called a mite day, or the damage caused by one mite feeding for one day. Thus, 100 mite days could be the product of 100 mites feeding for one day, or 10 mites for 10 days.
Since mite population curves usually have a single peak, the peak population level is related to the total number of mite days. Some studies have used peak populations as a measure of the economic injury level. The variability in damage makes it difficult to set an economic injury level or an treatment threshold that will be valid for all cultivars, times of season, weather, crop load, etc. For apple, a somewhat conservative level of 800 mite days was set for Washington conditions. This corresponds with a peak population of 30 mites per leaf. On pears, the greater susceptibility is reflected in a lower threshold. The economic injury level is set at 5 mites per leaf, and the treatment threshold used to prevent that from occurring ranges from 0.5 to 2 mites per leaf.
There are several approaches to monitoring mite populations. One is an in situ examination. Take a hand lens and examine 10 leaves from each of 10 trees per block (total of 100 leaves), and count the number of mites. This is laborious and subject to counting error if populations are high. However, if other equipment is lacking, it can give a reasonable assessment of mite populations.
A standard approach is to collect leaves in the field and bring them back to where a leaf brushing machine and a microscope can be used. The leaf brushing machine is a device with opposable rollers covered with soft bristles which brush mites from the leaf surface down to a revolving glass plate coated with a slightly sticky substance (e.g., liquid detergent). The mites are immobilized on the surface of the plate, and the microscope aids in counting individual species and stages. Usually a paper grid with 20 wedges marked on it is placed beneath the glass plate to help keep track during counting. A grid that can be printed for use is available. The total mites on the plate divided by the number of leaves brushed onto that plate yields the estimate of mites per leaf. When populations are high, only a fraction of the plate is counted (one-half or one-quarter). This method is also laborious and subject to error but is probably the most accurate of the methods presented. It has been the standard method on which much of the economic injury evaluations have been made.
A technique called binomial sampling has become more popular recently because it simplifies the estimating procedure with the use of statistics (see the article on Sampling). The binomial sample can be done in the field, and each leaf examined is rated as to the presence or absence of mites. The percentage of leaves infested (i.e., with one or more mites) is statistically related to the number of mites per leaf, and can be looked up on a table or chart made expressly for that species. The primary drawback to binomial sampling in the arid regions of the West is that the number of mites per leaf cannot accurately be estimated when 85% or more of the leaves are infested. This means that the highest level that can be estimated with accuracy is 10 mites per leaf for the twospotted and McDaniel spider mite, which is well below the treatment threshold of 30 mites per leaf. The highest level that can be accurately predicted with the European red mite is 26 mites per leaf. An additional drawback is that rust mites and T. occidentalis, which are important components in decision making, cannot be evaluated in the binomial sample. However, this method may be used as an indication if further sampling is necessary. A binomial sample indicating that 95% or more of the leaves are infested with either European red mite or the Tetranychus species shows the need for a standard brushing machine sample.
Because mites are on the tree from the pre-bloom period through fall, it is important to make repeated assessments for mites. You cannot take a single sample at a critical point in its life cycle and be finished for the season. Mites can build up at different times in different blocks or years. The only time that might be termed a single critical point is for the overwintering eggs. However, delayed-dormant oil applications are made routinely in the Pacific Northwest so this is rarely considered a decision making point. Where biological control occurs, many people do not monitor populations. It is advisable to sample at least once in mid- to late June to insure that rust mites and predators are present in adequate numbers.
Cultural factors play a role in spider mite population build-up. High populations have been associated with dusty foliage, such as occurs next to dirt roads. Overtree irrigation, in comparison to undertree irrigation, tends to reduce twospotted and McDaniel spider mite populations, either through microclimate modification or through washing mites from the leaves. European red mite, on the other hand, is little affected by overtree irrigation.
Temperature and humidity also play a role. In areas or years where many heat units accumulate, mite populations can produce more generations. High mite populations are often associated with hot, dry weather (even though in laboratory tests these conditions appear to cause higher mortality). For example, field studies with twospotted spider mite in Oregon show that its potential for population growth is lower in years with below normal temperatures. If the weather is rainy as well as cooler, this reduces survival of mites and will further suppress populations.
The value of the delayed-dormant oil application (at about half-inch green tissue) aimed at overwintering eggs of European red mite has been proven consistently under Washington conditions. Eggs are most vulnerable just prior to hatching, thus true dormant sprays will not be as effective. Usually European red mite populations are prevented from building up through May and June because of the delayed-dormant oil spray, although by itself it does not provide season long control. It does, however, suppress European red mite populations long enough for predators to become established in the trees. In the past, the addition of an organophosphate insecticide has enhanced the efficacy of the oil, but currently most of the effect comes from the oil alone. Mites are less likely to develop resistance to oil than to other insecticides. No report of resistance to oil has yet been documented.
There is evidence on pear that delayed-dormant or pre-pink sprays suppress the overwintering females of twospotted mites, but this suppression does not prevent build-up later in the season. Overwintering females are physiologically and morphologically different than the summer form, and chemical control of this form is more difficult. Specific miticides should be applied after the females lose their orange coloration and before these forms are produced again in the fall.
Effective specific miticides that are registered for use on tree fruits have grown fewer in recent years. In the past, most major groups of insecticides (e.g., the organophosphates) were initially miticidal, but resistance developed within a few years of commercial use. Similarly, most of the specific miticides were ineffective within a few years. The organotins had a relatively long field life, but resistance to these materials is now widespread. Ovicidal miticides have been notorious for their short field life in the past, and experience from other parts of the world indicates this is an ever present danger with the newer ovicides.
An important consideration in choosing a miticide, besides efficacy, is how selective it is toward beneficial arthropods, especially predatory mites. Use of a selective miticide greatly enhances the chance of establishing biological control, which should be the ultimate aim of any mite control action. Even a fairly selective miticide, if sprayed too often or on a prophylactic basis, will deplete the predator population and increase the chance of resistance in the spider mite population. A second consideration is the toxicity of the material to rust mites. Materials with low or moderate toxicity to rust mites will also enhance the chance of establishing or maintaining biological control.
Caution must be exercised not only in the choice of a miticide but also in the choice of insecticides and fungicides. A number of these materials are highly toxic to predatory mites. Two classes notorious for this are the pyrethroids and the carbamates, although not all materials within a given class are equally toxic. Repeated applications of a material only moderately toxic to predators (e.g., organophosphates) may eventually reduce the predator population below an effective level. In the dry areas of the Pacific Northwest, fungicide use is minimal, and this level of use does not appear to present a problem. However, in areas where apple scab is a chronic and serious problem, the selectivity of the fungicide used should be taken into consideration.
Apple: Before specific miticides are applied, the decision must be made as to whether control is warranted. Factors that need to be considered are:
- the level of direct pests and the pesticides used against them;
- damage incurred by other species of foliage feeders;
- tree age and vigor;
- type of irrigation system;
- crop load;
- the weather.
Although an all inclusive threshold is impossible to quantify, mature, vigorous trees with a light or moderate crop load can withstand a considerable amount of foliar damage without any measurable economic loss. Heavier crop loads require an efficient leaf canopy to mature the fruit properly, whereas if the leaf-to-fruit ratio is high, foliar damage may not be as critical. If the trees are water stressed by extremely hot, dry, or windy weather, a sandy or rocky soil that has poor water retention qualities, or inadequate irrigation, then the effects of mite damage will be exacerbated. If no predators are detected during successive samples and mite populations continue to rise, then miticides may be the only recourse. However, if predator populations are building, but more slowly than the pest mites, a miticide (the most selective one available) may be considered. If the use of a nonselective pesticide is required for another pest, then the chances for biological control are diminished and a miticide is more likely to be needed. In the long run, it may be better to incur some damage from mites initially than to run the risk of reverting to a miticide-based program.
Pear: The heavy use of nonselective programs in pears for pear psylla (pyrethroids, neonicotinyls) has dictated an essentially miticide-based program for mite control. Even before the introduction of pyrethroids, the intensive spray program for psylla was very detrimental to predators. This, coupled with pear foliage's inherently low tolerance to mite damage, has prevented biological control from playing a major role. The alternate prey component of the system, rust mites, is present on pear but can cause fruit russeting. Thus, the tolerance for rust mite damage is less on pear than on apple (with the exception of Bosc, a naturally russeted pear cultivar), and predatory mites are less able to maintain themselves. The replacement of a harsh, nonselective chemical program for pear psylla with softer materials would allow a greater chance for biological control, although it may never approach the success of integrated mite control on apples. An additional consideration on pear is the effect of orchard floor management on mite populations, especially twospotted spider mite. This species can build to high levels on weed hosts, especially bindweed, beneath the trees. When this food resource is depleted or killed with herbicides, mites may be driven up into the trees in large numbers. A survey of the weed composition and its mite population will indicate whether this is a potential problem.
One strategy is to add a miticide labeled for this use to the herbicide. Preferably, to minimize selection for resistance, the miticide should be of a different chemical class than the one used for the tree canopy. A second strategy is to open the lower nozzles on the airblast sprayer when applying a miticide to the canopy in order to cover the orchard floor. The drawback to this strategy is that development of resistance to the orchard miticide may be hastened.
The basic components of integrated mite control, however, remain unchanged:
- Use of a delayed dormant oil against over-wintering European red mite eggs.
- Maintenance of a moderate rust mite population in the orchard as a food supply for mite predators.
- Use of selective pesticides to protect predators.
In addition, where dust on the foliage has been a problem, reducing it has been very helpful to integrated control. Because our pest complex and our pesticide options change over time, maintaining these practices will present an ongoing challenge.