Not too many years ago, most growers and pest control advisors were unaware that earwigs were a potential pest problem in citrus.
Earwigs simply were not often found in large numbers in citrus orchards. Earwigs' increasing pest status is probably related to advances in integrated pest management techniques and attendant reductions in use of broad-spectrum organophosphate and carbamate insecticides for control of common citrus pests.
On the plus side, fewer toxic, broad-spectrum pesticides treatments reduced the safety hazard for pesticide applicators, field workers and the environment and biologically integrated pest management has been effective for controlling most pests.
However, once general broad-spectrum pest suppression was removed by significant reductions in these insecticides, some secondary pests, or insects that were not known to be pests, began to do serious economic damage to citrus under some conditions.
In April 2000, samples of earwigs collected in the act of chewing on citrus fruit by Robert Walther, private pest control advisor in Kern and Tulare counties, were sent from the University of California Cooperative Extension Office in Bakersfield to the California Department of Food and Agriculture for identification.
These earwigs were identified as the European earwig (Forficula auricularia). The adult European earwig is about three-fourth inches long, with a reddish brown head and darker body. A distinctive feature of the adult earwig is a pair of long appendages that resemble forceps at the tail end of its body. These forceps are straighter in the female and more curved in the male.
The European earwig has wings hidden under short, hard wing covers. Earwigs are capable of flight, but when disturbed during daylight hours, usually scurry and hide under the nearest available cover. Immature insects look like adults except are smaller and lack wings.
Females lay eggs in the soil and produce a single, if somewhat extended, generation per year. Earwigs are active and feed mostly at night, especially during hot days in spring, summer and fall. They prefer to inhabit cool, moist and dark places.
Generally, earwigs will return to the ground before daylight after feeding in citrus at night. During the day they are often found in tree wraps commonly placed on the trunks of young citrus for frost protection and under heavy leaf litter adjacent to irrigation emitters in mature orchards.
For earwigs to be an economic problem in citrus, they usually have to be present in large numbers. High populations of earwigs do not normally develop in citrus unless protective, shaded habitat is present. Fifty earwigs in a tree wrap is not an unusual find in infested young orchards.
Earwigs damage citrus leaves and small-diameter developing fruit. Buds, newly expanded leaves and soft, fully expanded leaves are all susceptible. Earwigs gouge leaves, and chew irregular holes through and around the edges of leaves. Recently expanded spring flush leaves can be chewed down to the midrib. Heavy infestations of earwigs in newly planted trees may require treatment, in that severe defoliation may result from their feeding activities.
In mature orchards the principal damage results from the earwigs chewing newly developing fruit in April and May. This damage is typified by holes gouged at the base of the fruit near the attachment to the stem or shallow crescent or star shaped slashing marks across the fruit.
Badly damaged fruitlets will fall from the tree, but the scars on fruit that remain on the tree continue to expand as the fruit grows, and the fruit will not be marketable. Earwigs usually stop feeding on fruit larger than about an inch in diameter.
Pruning citrus so that branches do not contact the ground and blowing or raking leaf litter from under the tree into the row middles away from the wetted irrigation pattern can reduce earwig populations in mature orchards. In young orchards, simply removing trunk wraps can remove the earwig problem.
Finding pesticides labeled for control of earwigs in citrus may be difficult. Some growers say after treating an ant infestation with an appropriately labeled chlorpyrifos formulation, that earwigs were controlled as well.