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The Oriental Fruit Moth (OFM), Grapholita molesta, (Lepidoptera: Tortricidae) is an introduced pest of orchards native to China. It was accidentally introduced in Washington, D.C. from Japan on nursery stock in 1913. By 1945 the OFM had spread nationwide on infested fruit and nursery stock. It is now found on all continents where stone-fruit is grown.
OFM is a small (7-10 mm) moth, dull grayish brown in color with a row of black dots near the end of the forewing. Larvae are thin, 10-13 mm in length, appear pinkish to creamy-white, and have a reddish brown, apparently bisected head capsule.
In the spring, adult OFM females lay their eggs on the growing terminals of plum, cherry, quince, pear, apricot, apple, and especially nectarine and peach trees (stone-fruits). Each larva attacks at least three shoots to develop into adulthood. As much as 90% of shoots per tree can become attacked resulting in a bushy tree of lesser value. The hatching larvae mine down the new shoots causing the shoots and leaves to wither and die. This damage is often called “flagging” or “shoot strike”. Summer generations of OFM larvae attack the ripening fruit and are especially severe on peaches. Larval feeding causes blemishing on the fruit exterior or brown rot on the interior. Early developing fruit drops off the tree if attacked. When OFM became established in the Grand Valley of Colorado, it caused heavy damage and severely threatened the peach industry. It is only a problem in the Upper Grand Valley area near Palisade and Clifton including Orchard Mesa and East Orchard Mesa.
During the first several decades of OFM infestation, chemical controls were ineffective against the moth larvae because larvae bore through the fruit/branch skin and thereby do not consume the sprayed material. OFM also have the potential to develop insecticide resistance. A more economical, sustainable, and safe method of control is biological. Biological control of pests is the use of natural enemies such as insects, mites, pathogens or other animals to reduce the spread, reproductive ability or reduce density of the target pest. The state of Colorado is fortunate to have a large and active program that works with biological control agents on many problematic pest species in the state. The Colorado Department of Agriculture has raised a parasitic wasp of the OFM in its insectary in Palisade, CO since 1946. The adults of this small wasp, Macrocentrus ancylivorus (Hymenoptera: Braconidae) called “Mac” for short, lay their eggs in the larvae of the OFM. After Mac eggs hatch, the wasp larvae feed on the OFM larvae until the host is consumed. Releasing Mac in late spring or early summer can control the OFM before they attack the ripening fruit. However, Mac does not overwinter on OFM so millions of parasitoids are produced indoors at the Palisade Insectary for orchard release every new growing season.
Female moths lay eggs after 2-5 days and each can lay up to 200 eggs during the week long oviposition period (A). Larvae grow to 12 mm, complete 4-5 instars, and leave the fruit (B) to construct cocoons and pupate on the lower tree trunk or in the substrate below (C). Pupae can also overwinter in the substrate (D). OFM completes 3-7 generations per year based on length of growing season.
Last year, OFM traps were placed in 13 orchards to monitor the adult flights. The OFM traps were examined weekly and the locations and counts recorded. Once OFM are found, Mac releases begin in earnest. In an average year, the Palisade Insectary releases over 1.5 million Mac into nearby peach orchards. Early research (1948) indicated a 50% reduction in fruit damage after Mac wasp releases and the wasps were just as effective as chemical spray at the time.
The insectary in Palisade distributes the OFM parasitoid “Mac” each year without charge. The Mac are released in the pupal (cocoon) stage at a rate of 1,000 per acre of peach orchard. The releases come in small paper bags with some packing material and a string to tie the bags to the trees. Over the coming days, the adult wasps will emerge from their pupal casings and search the orchard for OFM larvae to attack. Parasitoids insert the ovipositor through the OFM feeding tunnel entrance. It then lays eggs on the OFM larvae which hatch and consume the host from the inside. First and second instar OFM and those moving from shoot to shoot are the most heavily parasitized.
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