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Southeastern Apple ProductionENTOMOLOGY |
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Two-Spotted Spider Mite
Tetranychus urticae (Koch)
Description
The summer color pattern of the adult female twospotted spider mite (TSSM) varies, but most are greenish yellow with a prominent dark spot on each side near the middle of the body. These spots may enlarge to cover most of each side of the body as the mite feeds. The body is somewhat egg shaped, broadest toward the head, and not much larger than a period. Eggs are spherically shaped, shiny, and light or clear to pale green.
 Adults and eggs |
 Adult |
Life History
TSSM overwinter as orange-colored females that congregate under debris on the orchard floor or under bark scales at the base of trees. In spring TSSM feed on vegetation (especially vetch and other legumes) underneath the trees. As hot weather causes these hosts to dry out, mites move onto apple leaves. Females lay approximately one egg per day. If the female has mated, the fertilized eggs develop into both male and female mites; if not, unfertilized eggs develop into males. Eggs hatch into six-legged larvae. The larvae then pass through eight-legged protonymph and deutonymph stages before becoming adults.
 Overwintering females |
Damage
TSSM feed on leaves. Severe mite injury produces browning and loss of color in the leaves (commonly referred to as bronzing). Extensive foliage injury may reduce the quality and quantity of fruit and the following year's return bloom.
The extent of mite injury is influenced by numerous factors: (1) time of the growing season when injury occurs, (2) duration of feeding when injury occurs, (3) vigor and tree cultivar, (4) crop load, and (5) weather conditions.
 "Bronzed" trees (on left) |
Monitoring and Control
TSSM is a sporadic problem in southeastern apple orchards. Economically damaging populations generally develop during the latter part of the season, and hot, dry weather can increase populations further. Unlike the European red mite, which feeds only on deciduous plants, the TSSM feeds on a very wide variety of hosts (including garden, greenhouse, nursery and ornamental plants and weeds).
Several beneficial arthropods can help keep TSSM populations below damaging levels. The most common in the Southeast are the phytoseiid mite (Neoseiulus fallacis) and the complex of generalist predators (e.g., black lady beetles (Stethorus punctum) and lacewings). However, recent research in North Carolina suggests that neither of these predators overwinters to any significant degree within orchards, so they must be reestablished in orchards in the spring. Hence, practices that delay the buildup of TSSM and enable predators to increase before mites become a problem will favor biological control. The two most effective practices are applying a delayed dormant oil spray and avoiding insecticides toxic to these predators.
Monitoring Mite Populations: Use a regular monitoring program to follow the buildup of mite populations and to determine if and when supplemental applications of a miticide are necessary to avoid economic damage. Monitor each contiguous block of apples weekly beginning when adult mites first appear (which may vary from mid May to early July). Within each block, examine 5 leaves from each of 10 trees with a visor lens or hand lens. Rather than counting the total number of mites on each leaf, record the number of leaves infested with one or more mites, and estimate the mite density on a per-leaf basis from the table below.
Determining the Need for Miticides: When mite populations reach a density of 5 to 10 mites per leaf (80 to 90 percent infested leaves) decide whether to use biological control or a miticide to prevent mites from increasing to higher densities. Count the actual number of A. fallacis on sample leaves with a visor lens. If the ratio of A. fallacis to TSSM is between 1 to 5 and 1 to 15, biological control is possible. For biological control with S. punctum to occur, the ratio should be 2.5 S. punctum to 1 TSSM. S. punctum should be sampled by counting the number of adults and larvae observed during a timed 3-minute search around the periphery of mite-infested trees. S. punctum larvae must almost always be present if this predator is to control mites. If neither predator is present at sufficient levels for biological control to occur, and mite populations are between 5 to 10 mites per leaf, apply a miticide.
In areas where Alternaria blotch is a problem on Delicious apples, biological control is usually not an option. In the presence of Alternaria blotch, mite populations must be maintained at very low levels to avoid high levels of Alternaria and premature defoliation. If preventive control measures are not used, a modified threshold level of 1-2 mites per leaf should dictate the need for miticides.
| % Mite-infested leaves (1+ mite/leaf) |
Expected number of mites per leaf |
|---|---|
| 40 | 0.7 |
| 45 | 0.9 |
| 50 | 1.1 |
| 55 | 1.3 |
| 60 | 1.6 |
| 65 | 2.0 |
| 70 | 2.6 |
| 75 | 3.4 |
| 80 | 4.7 |
| 85 | 6.8 |
| 90 | 11.4 |
| 95 | 26.4 |
Insect and Mite Management Overview
Insect and Mite Index
Southeastern Apple Production Mountain Horticultural Crops Research & Extension Center 455 Research Drive Mills River, NC 28759 Phone: 828.684.3562 ~ Fax: 828.684.8715 Email: jim_walgenbach@ncsu.edu |
Web Crafters: Anne S. Napier and
Steve Schoof |
Updated March 12, 2007