Larva: Five larval instars and two distinct morphological forms develop during the larval stage. Instars 1 through 3 are dorsoventrally compressed, apodous (without legs), deeply segmented and plasmophagous (sap-feeding). Instars 4 and 5 are eruciform (caterpillarlike) and histophagous (tissue feeding). Body length ranges from about 1 mm for the first instar to about 5 mm for the fifth instar.

Pupa: The pupa is cylindrical and elongated and tapered at the posterior end. Pupae are yellow to light brown during early development and gradually darken to a deep brown as development continues. Overall length is 3 to 4 mm.

This insect attacks primarily apples but has also been found on various crabapples. It is found in apple-growing areas of the northern United States and Canada.

The larvae mine the leaves of the apple. About 4 percent of the leaf area is disrupted by each larva. When populations densities are high, severe defoliation occurs. A heavy infestation can cause stunting of the fruit growth, reduced terminal growth, early leaf drop, premature ripening, drop of fruit and reduced fruitset the following season. Extensive mining combined with drought conditions can worsen plant damage.

Parasitism under natural conditions is nearly 100 percent. Birds and predator insects reduce populations. Mortality in winter can be high, especially under conditions of alternate thawing and freezing.

In the north central states, this leafminer has three generations per year.

Spring-generation adults emerge in late April and continue flying until the middle of June. The life span of the adults in the laboratory is five to seven days. The eggs are laid soon after emergence and require less than one week to develop. The larva exits the egg through the flat surface adjacent to the leaf and enters the leaf directly. The first instar larva begins feeding around the entry point by shearing open the spongy mesophyll cells with its specialized mouthparts. This instar and the two subsequent instars feed wholly on the protoplasm of these cells and are called sap-feeding larvae.

Instar 2 larvae form a linear mine between the lower epidermis and the spongy mesophyll tissue of the leaf. Instar 3 larvae extend the boundaries of the mine and from a blotch-shaped mine. When the third instar molts, the caterpillarlike fourth instar emerges and feeding habit changes. Instars 4 and 5 larvae feed on whole cells of the leaf's parenchyma tissue and are called tissue-feeding larvae. The fifth instar feeds through the columnar parenchyma tissue up to the upper epidermis of the cell, causing irregular, translucent feeding sites over the entire mined area. This gives the mine a spotted appearance, providing the insect with its common name.

The larvae pupate within the mine in a loosely knit cocoon at one end of the mine. After a three-week development period, the pupa pushes its way through the lower epidermis of the leaf and the adult emerges. The adult rests on the leaf surface for a few hours until it is ready for flight. At this time, it is very vulnerable to various predators that inhabit the leaf surface.

The time span between spring generation adult emergence and first-generation adult emergence is about 60 to 70 days. The second generation develops like the first but, because of the warm summer temperatures, requires only 30 to 40 days to develop.

The third generation begins in mid-August and continues until the advent of cold weather, when it then pupates in the leaves. It spends the winter in pupal stage in leaves on the ground. Adults begin to emerge the following spring in late April.

Depending on the need, there are a number of methods to monitor the tentiform leafminer.

Growers should note that, during the prebloom periods, there is a correlation between numbers of adults and mines per leaf. To monitor adults, use a pheromone in a trap hung about 5 feet from the ground in the interior of the apple tree. Use one trap for every 3 to 5 acres. If the total prebloom capture from silver tip to pink stags is more than 12 adults pre trap, apply a pesticide against the adults.

Sampling for first-generation eggs at the pink stage will provide information on the need for the first brood control. Control at this time can help prevent damaging populations of the second generation. To sample, select four clusters from each of five trees in a block. Using a lens, count the total number of eggs on the undersides of the second, third and fourth leaves in each cluster. If the total for all clusters is more than 60, apply a treatment either after the sampling analysis or at petal fall. If the total is less than 60, no treatment against first-generation adults is required.

Another way to determine the need for spraying is to count the mines. At petal fall, randomly select five trees of the same variety and pick 10 terminal leaves per tree. Count the number of mines on the undersides of the leaves. No treatment is needed for a find of zero to one mine per leaf. For second-generation monitoring, pick younger leaves rather than terminal leaves. Count incipient mines made by the second generation. If you find two or fewer mines per tree, treatment is not needed.

Pheromones are unreliable in the mid and late season for indicating whether sprays are needed.

In abandoned orchards, biological control of this pest is nearly 100 percent. At present, there is no apparent biological window when chemical control could be applied without serious effects on the parasites, though some pesticides applied at the prepink stage for control of tentiform leafminer eggs and adults have minimal effects on parasites. Low numbers of leafminers in the first generation do not guarantee that heavy infestations will not occur in the second and third generation.

The need for control of first-generation leafminers can be determined at petal fall by counting the number of incipient mines on the undersurfaces of the leaves. The spotted tentiform leaf miner has demonstrated great proclivity for developing resistance to chemicals. If effective, chemicals can be applied at the prepink stage as ovicides and adulticides.