I. Introduction: Worldwide,
twospotted spider mite (TSM) is the most economically important
mite species. However on pome and stone fruits, and grapes, in
our region (humid temperate), it is of secondary importance to
ERM. Under some conditions, however, it can be of importance.
II. Hosts: TSM occurs on more than 150 economically important host plants, including all of the tree fruit crops, as well as small fruits, vegetables, and ornamental crops.
III. Description: Adult females are oval, dome-shaped mites, similar in shape and size to ERM. Adults are pale straw-colored, with two black spots (caused by the dark gut contents). Eggs are clear and spherical, deposited on leaves. First stage immature mites (larvae) have six legs; the next stages (protonymphs and deutonymphs) have eight legs, as do the adults. Overwintering females are bright orange, without the dark spots.
IV. Biology: TSM overwinters as adult females in the orchard ground cover. These mites feed on various weed species. Populations are fostered by some ground covers, such as vetch. During the late spring and summer, mites may disperse upward into the orchard canopy. The developmental threshold is 54 degrees F (12 degrees C). The optimal temperature for development is 85-90 degrees F (30-32 degrees C). At such optimum temperatures, the egg incubation period is 3-5 days, the developmental stage of the female requiring 4-5 days, the preoviposition period 1-2 days; the total life cycle therefore requires only 8-12 days. The average fecundity of females is 90-110 eggs, but it can reach 200 eggs.
Males are generally a minority in a population; however, each male fertilizes about 70 females. If females are fertilized, they give rise to a mix of males and females. Unfertilized give rise to only males. Such reproduction without mating is partly responsible for the rapid rate of population increase. Females attract males by a pheromone released shortly before the female deutonymphs emerge as adults. Males are arrested and stand by the female waiting for the appearance of the female, whereupon mating occurs. Mites are assisted in their spread among host plants by their tendency to become airborne by facing into a breeze.
V. Injury: Feeding injury is similar to that caused by ERM, a diffuse stippling followed by bronzing. However, Tetranychus species feeding generally has a greater impact of host leaf function than Panonychus species. Approximately half as many mite-days are required to reduce photosynthesis of apple as is the case with ERM. Infestations can reduce yield and return bloom.
VI. Monitoring: Follow mite densities during the season by counting mites per leaf with a hand lens or leaf-brushing machine. Take 10-40 leaves from the canopies of 10 trees (use the larger number in spring, when mites are harder to detect). Calculate the % infested leaves; from this mites per leaf can be calculated. This relationship holds until the first acaricide application.
TSM may be counted along with ERM during routine sampling. Under most conditions, TSM may be summed with ERM. In those situations where TSM is the predominant species, use an action threshold of half the recommended value for ERM.
VII. Chemical control: When
biological control has been disrupted, chemical control of TSM
may be required. See the following links for Virginia
Spray Bulletin for Commercial Tree Fruit Growers (html)
Pest Management Guide for Commercial Vineyards (html)
Pest Management Guide for Commercial Small Fruit (html)
Pest Management Guide for Home Fruit (html)