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: The wings of the adult moth are covered with golden-colored scales with silvery stripes (Plate 51). Adults are about 1/8 inch (3.5 mm) long when at rest. Females are a little larger than males. The egg is small and white, about 1/75 inch in diameter (about 0.3 mm). 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, reaching lengths of 1/16 inch (1-2 mm). During the last two instars, the body is more cylindrical and the head is more oval, a typical caterpillar head capsule. There are now more apparent legs and prolegs. This is the "tissue-feeder" stage, and is 1/7 inch (3.9 mm) long when mature. 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 (3.3 mm) long. The shape of the antennae, legs and wings can be discerned. The pupa is found in the mine, along with a black fecal mass.
IV. Biology: Pupae overwinter in the ground cover. Adults are relatively weak fliers and so first generation mines are usually concentrated on the lower limbs. Most adult activity and oviposition 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. The spring brood females (April-May) lay an average of 16, first brood (June-early July) 10.8, second brood (late July-August) 12.8 eggs. The ovipositional threshold is about 48.2 degrees F (9 degrees C). Egg development requires about 172.5 DD above a developmental threshold of 2.5 degrees 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 (1-1.3 cm), 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. Mines rarely cross leaf veins. The mine becomes visible on the top of the leaf during the tissue feeding stage. The tissue feeder removes patches of epidermis from the upper leaf surface, giving a spotted appearance, and causes the epidermis to form a tentlike ridge. This gives the species its common name. Development requires 35-55 days per generation. Spring brood moths emerge from the overwintering pupae starting in late delayed dormant. Peak emergence is from pink through bloom. The spring brood adults are present in April-May, first generation adults emerge in June-early July, and second generation adults are active late July-August. Most of the third generation pupae enter diapause for the winter; some third generation adults are active in August and September. 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 parasites and predators exert biological control on spotted tentiform leafminer. The two most common parasites are Sympiesis marylandensis Girault (Eulophidae) and Pholetesor (=Apanteles) ornigis (Weed) (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 1/10 inch or 2-3 mm long) are commonly seen within the leaf mines. Compare the parasitoid photos with the STLM pupa in this Cornell image). These parasites can build up to fairly high levels, and pesticide toxic to parasites should be avoided at this time.
V. Injury: Each mine reduces leaf area by about 5%. Loss of effective leaf area can reduce fruit quality and quantity, decreasing size, premature fruit drop (especially in early varieties such as `Macintosh'), reduced fruit set the following year, and potentially reduced uptake of agricultural chemicals through the leaves.
VI. Monitoring: A pheromone trap is commercially available for STLM. No such trap is available for apple blotch leafminer; however a red sticky trap may be used for this species, and may work for both species. At silver tip, place one trap per 3-5 acres (1.2-2 ha), at chest height.
Very high numbers of moths are often collected on pheromone traps; these cannot at present be correlated with economic damage. Catches are often influenced by leafminer numbers in the surrounding habitat. A tentative action threshold for visual traps is a cumulative moth capture of 12-13 in the prebloom period (silver-tip through late pink). The trapping data from either trap type can be used to help time other scouting and control measures.
Mines should be counted during the period of larval activity, beginning at petal fall. Ten fruit cluster leaves from each of several areas of the orchard should be examined. Scouting should emphasize appearance of sap-feeding larvae. At this time of the year, mines will most likely be U-shaped, incomplete mines. For the second generation, examine later unfolding 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-feeding 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 total of mines found with the limits in Table 4 for that number of trees. If the number of mines falls between the two values given, sample another tree. 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 a treatment is recommended. If seven trees are sampled and the total number of mines is less than 63, no treatment is needed.
Thresholds based on the Cornell sequential sampling plan are given in Table 4 for the first generation, and Table 5 for the second generation. Sprays for third generation should be avoided because of potentially heavy impact on the complex of parasites, complicating control in the following year. Control of first generation is preferable to control of second generation, because of reduced impact on natural enemies. A level of parasitization where biological control can be expected from Washington State is 30-35%.
Table 4. Limits to determine status of first-generation spotted tentiform leafminer infestations.
.........................Sap-feeding mines at
petal fall
No of
trees.................lower..........upper
sampled......................limit............limit
2...............................7...............30
3..............................13...............41
4..............................20...............52
5..............................27...............63
6..............................35...............73
7..............................63...............63
Table 5. Limits to determine status of second-generation spotted tentiform leafminer infestations.
..........................Second generation
sap-feeding mines
No. of
leaves.................lower..........upper
sampled.......................limit............limit
15..............................12...............46
20..............................22...............55
25..............................31...............65
30..............................41...............75
35..............................51...............85
40..............................70..............105
50..............................98...............98
