Q. I planted two apple trees about six years ago. They have grown well, but every year, most of the apples fall off long before they mature. What causes this to happen, and how can I get more fruit to stay on the tree until it is ripe?

A. Fruit drop occurs for a number of reasons. All types of fruit trees -- apples, pears, peaches, plums and cherries -- typically shed some fruit early because they cannot support every piece of fruit that grows. Known as June drop, it is perfectly normal and there is nothing you can do to stop it.

However, plenty of fruit should be left after June drop. In fact, more thinning is usually needed so that the remaining fruit develops to a good size. Apples are usually thinned to one per cluster, spread 6 to 8 inches apart on each branch.

Incomplete pollination can also cause small fruit to drop. This occurs when there are not enough bees and other pollinators active in your yard. Avoid spraying insecticides on fruit trees when they are in full bloom. Always spray first thing in the morning or last thing in the evening, when bees are safe in their hives for the night. Excessive rain and/or cold temperatures can also inhibit pollinators.

Apples require cross-pollination from another variety of apple for good fruit set. You can usually find pollination compatibility charts in fruit tree catalogs.

The use of the common insecticide Sevin (carbaryl) also causes some fruit to drop before it matures. Sevin is often a component of home orchard spray, which combines insecticides and fungicides to protect fruit trees from a wide range of insect and disease problems.

A number of insect pests that commonly feed on apple and pear trees cause premature fruit drop. One of the following insects could be the culprit if you do not follow a regular spray program:

Tarnished plant bugs overwinter as adults under leaf litter, in bark crevices or in other protected areas. They migrate to fruit trees to feed as buds swell in the spring. Severely injured buds never develop, and small fruit that have been attacked often abort.

Tarnished plant bugs cause the most damage from green tip (when leaves are 1/4 inch long) through petal fall, when 90 percent of the tree's petals have fallen. Adults are brown with a white or yellow "Y" on the thorax. They are oval-shaped, and about 1/4 inch long. Nymphs or immatures are pale yellow to green with black spots. Since broadleaf weeds and legume crops can attract these pests, keep them away from your apple trees. Also, mowing near your trees can drive the bugs into them.

Plum curculio overwinter as adults in wooded or overgrown areas adjacent to orchards. They are most active during the warm weather just after petal fall, when temperatures are above 70 degrees. Adult feeding damages flowers and leaves. The most severe damage is caused when adults lay their eggs in the fruit, leaving a crescent-shaped scar. Severely infested fruit drops prematurely.

The 1/4-inch long adults are dark brown with white patches and have long snouts that account for one-third of their body length. The larvae are legless, white with a brown head.

Mature larvae drop from the fruit and pupate in the soil for roughly two weeks. New adults hatch in August and feed briefly before migrating to protected sites for the winter. Control is most important for about six weeks after petal fall. An effective but labor-intensive control method is to spread tarps or sheets under the trees and shake the trees. The beetles will fall onto them and can be crushed.

Codling moths overwinter as mature larvae in cocoons under tree bark, in leaf litter and in other sheltered sites. They pupate in spring when apple buds show pink and are about to open. Adult moths become active around petal fall and continue to emerge for about six weeks. Newly hatched larvae tunnel into the fruit, usually at the blossom end. The larvae feed on the core and developing seeds, often causing the fruit to abort. After three or four weeks, mature larvae leave the fruit and spin a cocoon in a sheltered site.

Codling moths can attack fruit from its earliest development until harvest. Adult females are about 3/8 inch long and grayish-brown with coppery patches at the tip of each wing. Larvae are cream to palest pink with brown heads. There are no effective controls for this pest.

Apple maggots overwinter in the soil, emerging as adults in July and August, when females lay eggs just under the skin of the apples. Larvae hatch in a few days and tunnel through fruit as they feed. Infested apples drop prematurely.

Adults are black flies about the size of a housefly. They are marked with three or four white stripes across the body and a prominent white dot in the middle of the back. They have clear wings with four black bands somewhat in the shape of the letter "F." The white, legless larvae are known as maggots.

In small plantings such as yours, you may be able to control apple maggots by using sticky red spheres. These plastic "apples" are coated with a sticky substance such as Tanglefoot that captures them. Spheres and Tanglefoot are available from mail-order orchard supply companies or local agricultural supply companies such as Agway. Hang one sphere per 100-150 real apples.

The easiest way for home gardeners to control insect and disease problems on fruit is to use a home orchard spray that combines malathion and/or Sevin (carbaryl) with a fungicide such as captan. There are organic versions that contain rotenone, pyrethrins and sulfur.

Timing is everything, so it is important that you follow the spray schedule that comes with the product. Or follow Penn State University's recommendations, available on the Web at ssfruit.cas.psu.edu/; click on Pests and Pesticides.