I. Introduction: Spotted tentiform leafminer (STLM) has been an increasing problem in recent years because of resistance to organophosphate insecticides, notably azinphosmethyl (Guthion). A 160-fold increase in resistance was seen in Ontario, with cross resistance to phosmet (Imidan). There was no cross resistance with pyrethroids, but resistance to pyrethroids as well as methomyl (Lannate) has been found at some sites. Consequently, there is strong incentive for scouting for this pest. The degree of infestation is highly variable among orchards.

II. Hosts: A Connecticut study showed STLM to infest apple and wild crab apples. A broader host list is sometimes given, but this may arise from confusion between STLM and closely related species. A related species that is an economic problem in some northern areas, apple blotch leafminer [P. crataegella (Clemens)], develops on 17 host species in seven rosaceous genera, including apple, pear, sweet cherry, plum and quince.

III. Description: Adult moths are about 1/8 inch (3 mm) long when at rest, and have wings covered with golden-colored scales with silvery stripes (photo 1). Females are a little larger than males. The egg is translucent to white and less than 1/16 inch (1.5 mm) in diameter (photo 2). The larva passes through five instars. During the first three instars, the head capsule is flattened and wedge-shaped; the body is also flattened and legless. This is the "sap-feeder" stage, which reaches a length of 1/16 inch (1.5 mm) (photo 3). During the last two instars, the body is more cylindrical and the head is more oval, a typical caterpillar head capsule. Legs and prolegs are now more apparent. This is the "tissue-feeder" stage, which is slightly smaller than 3/16 inch (5 mm) long when mature (photo 4). For much of its development, the larva is white to pale green. During the final instar, color changes to a pale to deep yellow. The pupa is brown, tapering to the hind end, and is about 1/8 inch (3 mm) long. The shape of the antennae, legs, and wings can be discerned on the pupa.

IV. Biology: Pupae overwinter within fallen leaves in the orchard ground cover. Adults are relatively weak fliers and so first generation mines are usually concentrated on the lower tree limbs. Most adult activity and egg-laying is from mid-afternoon to sunset. At other times, adults may be found resting on leaves. Eggs are laid singly, usually on the lower leaf surface. Fecundity is highly variable, ranging from 1-152 eggs per female. Spring brood females (April-May) lay an average of 16, first brood (June-early July) 10.8, and second brood (late July-August) 12.8 eggs. The egg-laying threshold is about 48.2 F (9 C). Egg development requires about 172.5 degree days (DD) above a developmental threshold of 36.5 F (95.8 DD above 2.5 C).

The larva hatches from the underside of the egg, entering a slit directly into the leaf. The flattened head capsule of the sap-feeder slashes and shears cells; the larva then feeds on the released juices. The mine starts as a light green line visible only on the underside of the leaf. After the mine has progressed about 1/2 inch (13 mm), the larva stops to molt to the second instar. The larva now turns to complete a loop, and feeds in the enclosed tissue until the entire area is a light green (photo 5). Mines rarely cross leaf veins. The mine becomes visible on the top of the leaf during the tissue-feeder stage. The tissue-feeder removes patches of epidermis from the upper leaf surface, which gives the leaf a spotted appearance and causes the epidermis to form a tentlike ridge (photo 6). This gives the species its common name. Pupation occurs within the mine, which also contains a black fecal mass. After adults emerge, the empty pupal cases can be seen protruding from the lower leaf surface. Development requires 35 to 55 days per generation.

Spring brood moths begin to emerge from overwintering pupae at green tip to half-inch green. Peak emergence is from pink through bloom. Spring brood adults are present in April and May, first generation adults emerge in June to early July, and second generation adults are active late July to August. Most of the third generation pupae enter diapause for the winter; some third generation adults emerge from late August into the fall. These timings of activity are generalizations; there is actually overlap between generations, especially late in the season. There are three generations in most parts of its North American range; however, four and a partial fifth generation occur in the southern part of its range (North Carolina through Georgia).

A complex of parasitoids and predators exert biological control on STLM. Up to 16 parasitic species were found in one study in Connecticut. The three most common parasitoids (Hymenoptera) are Sympiesis marylandensis Girault (Eulophidae), Pholetesor (=Apanteles) ornigis (Weed) and P. pedias (Braconidae). Sympiesis can be seen as a white legless larva feeding on the leafminer larva. It emerges at about the same time as the host. Pholetesor is an internal feeder, but the cottony white, elongate oval cocoons (about 1/8 inch or 3 mm long) are commonly seen within the leaf mines.

V. Injury: Each mine reduces leaf area by about five percent. Loss of effective leaf area can reduce fruit quality (photo 7) and quantity, decrease size, cause premature fruit drop (especially in early varieties such as `Macintosh'), reduce fruit set the following year, and potentially reduce uptake of agricultural chemicals through the leaves.

Beginning at petal fall, monitor for sap-feeder larvae as evidenced by the appearance of U-shaped, incomplete mines (photo 5) when viewing the undersurface of leaves. Cornell University has developed a sequential sampling scheme to assess STLM populations. At petal fall, select three fruit clusters from around the canopy of each tree sampled. Count sap-feeder mines on the undersides of the second, third, and fourth leaves in each cluster, counting leaves in the order in which they unfolded. After two trees have been sampled, begin comparing the accumulated number of mines found with the limits in Table 1 for that number of trees. If the number of mines falls between the lower and upper limit, sampling is stopped and no treatment is required. If the total is greater than the upper limit, sampling is stopped and a treatment is recommended. If seven trees are sampled and the total number of mines is less than 63, no treatment is needed.

Use the sequential sampling scheme to assess second generation sap-feeder mines approximately five days after peak moth catch (late June to early July). Start at the orchard edge and, moving toward the center, sample every other tree until enough trees have been sampled. Begin by selecting five mature leaves from each of three trees, and count the sap-feeder mines on the undersides of the leaves (photo 5). After 15 leaves have been examined, begin comparing the accumulated total number of mines found with the limits given in Table 2 for that number of leaves. If the number of mines falls between the two values given, sample five more leaves from another tree, continuing to add the number of mines found to the running total while checking the chart again. Continue sampling in this fashion until the total number of mines falls below the lower limit or above the upper limit. If the total is less than the lower limit, sampling is stopped and no treatment is required. If the total is greater than the upper limit, sampling is stopped and an insecticide treatment is recommended. If ten trees (50 leaves) are sampled and the total number of mines is less than 98, no treatment is needed.

Table 2. Limits to determine status of second generation spotted tentiform leafminer infestations.

Monitor the underside of leaves in August for the appearance of third generation sap-feeder mines (photo 5). An average density of mines (old and new) in excess of two per leaf could increase fruit drop under some situations and, therefore, justify control.

VII. Management: Insecticides may be applied during the prebloom period to control adults or sap-feeder larvae as eggs hatch. For the remainder of the season, insecticide application should be based on the incidence of sap-feeder mines. Where monitoring reveals that control is justified, sprays should be applied while larvae are in the sap-feeder stage, because chemicals are much less effective against the tissue-feeder stage. Control of the third generation is generally discouraged unless absolutely necessary, because most insecticides are toxic to parasitoids which can cause a significant reduction in the overwintering population. It was discovered in Washington state that biological control can be expected if the level of parasitization is 30 to 35 percent. Raking and burning leaves in the home orchard in the fall will destroy the overwintering pupal stage.