Citrus Peelminer

Citrus peelminer is a pest of susceptible citrus varieties in the Coachella and San Joaquin valleys. The San Joaquin Valley strain is genetically related to a strain from Mexico and probably arrived on infested fruit during 1998-99. Adults deposit eggs on stems and fruit of citrus and neighboring crops. A small larva hatches from the egg and begins feeding on the rind of the fruit just below the egg shell. As the larva grows, it molts four to seven times and mines the stem or fruit creating a winding tunnel that grows wider as the larva gets larger. Just before pupation, the larva changes to a pink form with spinning mouthparts. The pink stage larva leaves the mine and spins a flat cocoon on a twig, leaf, or fruit. It decorates the cocoon with silk balls and then pupates. There is no overwintering stage; the insect continues development throughout the year, but the length of a generation is shorter during warm temperatures. There are 6 to 8 generations a year occurring at about monthly intervals from May to November.

The citrus leafminer, a small moth that attacks citrus, differs from citrus peelminer because its larval stages leave a frass trail in the mine and it attacks only new flush leaves rather than stems and fruit. The leafminer pupa is found in the curled edge of a leaf and lacks the decorative balls that the peelminer creates. For more information on the differences between the two species, see Citrus Leafminer and Citrus Peelminer, UC ANR Publication 8321.

Larvae form mines on the surface of the fruit. Grapefruit, pummelo, and certain smooth-skinned navel varieties (Fukumoto, Atwood, TI) are most susceptible to damage, but the peelminer can attack any variety. Susceptible varieties can suffer 5 to 80% fruit damage, while nonsusceptible varieties rarely experience more than 3% damaged fruit. Under heavy infestations, peelminers will mine the leaves, but this is rare. Damage to fruit is cosmetic but one mine can render it unacceptable for fresh market. This pest can attack many different crops: grapes, cotton, beans, stone fruits, nuts, ornamentals (willows and oleander), various vegetables, and weeds. Susceptible varieties of citrus are at higher risk for damage if located next to crops where peelminer populations build up (cotton and beans). Because citrus peelminer is usually not an economic problem in the neighboring crops, it is rarely controlled in those crops. When those crops reach maturity and begin to dry, the moths emerge and move into neighboring citrus.

Pesticides are only somewhat effective against peelminer because of the difficulty in obtaining good spray coverage of the fruit and penetration of the pesticides into the mines. The moths prefer to lay eggs on fruit that is low and on the inside of the tree, which is the hardest fruit to reach with sprays.

There are many native parasites that attack peelminer to a limited extent. One native eulophid wasp, Cirrospilus coachellae, can control peelminers in the Coachella Valley. Parasites pupate in the mine and can be seen through the epidermis as small, black pupae surrounded by black dots (fecal material). Evidence of past parasite activity are exit holes near the remains of pupae. Long-term control of this pest will likely be biological. The arrival of citrus leafminer to California in 2000 improved biological control of peelminer because they share many of the same natural enemies.

Avoid planting susceptible citrus varieties such as Fukumoto oranges, grapefruit, and pummelos. Locate plantings of these susceptible varieties away from neighboring crops such as cotton and grapes that could be a source of peelminer. If cotton is grown next to a susceptible citrus variety, then Bt cotton should be used or an insecticide can be mixed with the defoliant to reduce the number of moths emerging from the cotton.

While citrus peelminer is sensitive to a number of different insecticides, it is difficult to control with insecticides because it attacks the inside low fruit (difficult to reach with sprays), the larval stage is protected by the mine, and it has 6 to 8 generations.

Studies have shown that the lower developmental threshold of citrus peelminer is 55°F. The biofix of first flight in noncitrus host crops usually occurs between the 3rd week of March and early April. From that point on, every 580 degree-days, the peelminer completes a generation. There are seven potential flights in the San Joaquin Valley and nine potential flights in the Coachella Valley.

Hot summer temperatures tend to stop development of the peelminer. The first two flights of citrus peelminer attack noncitrus hosts such as the stems of walnuts, willows, oleander, and various weeds. The third flight of moths finds the pummelo and grapefruit varieties suitable for attack and the 4th or 5th flight begins to attack susceptible navel orange varieties.

A treatment strategy is to use degree-days to estimate the timing of the flights that will attack a citrus variety and apply insecticides three times at monthly intervals to control the first three flights that could potentially deposit eggs on citrus fruit. For example, pummelo and grapefruit growers would spray flights 3, 4, and 5, while navel growers would spray flights 5, 6, and 7.

Diflubenzuron (Micromite) is effective primarily against the egg stage; time sprays of this insecticide to peaks in moth flights. Multiple applications of one-third or one-half rates of diflubenzuron are more effective than a single high rate. Addition of an adulticide such as a pyrethroid (fenpropathrin-Danitol or beta-cyfluthrin-Baythroid) to the diflubenzuron spray helps somewhat. However, these insecticides are fairly broad-spectrum and may cause flare ups of other citrus pests because they destroy natural enemies.

• Photos of citrus peelminer, citrus leafminer, and their damage