Every fruit crop has its insect nemesis. For peaches and nectarines, it is the Oriental fruit moth, the biggest insect pest headache for growers since it first appeared in the U.S. in early part of the 20th century. OFM thrives in both humid and dry peach growing areas and has many traits that have allowed it to adapt, survive, thrive, and cause mischief.
OFM is an Omnivore
Part of the reason that Oriental fruit moth (OFM) is successful is that it likes many different fruit crops. Although the primary hosts of the oriental fruit moth are peach and nectarine, it will also attack quince, apricot, apple, plum, cherry, pear, rose, and flowering cherry.
For Michigan, oriental fruit moth (OFM) was considered to be a minor pest on apples, usually worse when peaches were grown in the vicinity. Over the past 20 years, OFM has adapted to become a major pest in apples, at times the larvae of this pest is more common in fruit at harvest than codling moth.
A Cloud of OFM
Oriental fruit moth overwinter as larvae in silken cocoons on the tree or in the duff on the orchard floor. In the spring the larvae become pupa, and begin to emerge as adults during April, shortly before peach trees bloom. Time from egg laying to hatch can take anywhere from 5 to 21 days depending on the temperature.
The first-generation targets foliage and the rest seek out fruit. The first-generation flight is relatively well defined, but later generation flights two, three, and four blur together in a “cloud.” It is thought these later generations overlap because of different rates of larval development, depending on the different hosts and food sources.
As a result, it difficult to time summer sprays based on development models because there are no distinct peaks. Although pheromone trap catches can’t pinpoint the start of the summer and fall generations, they can indicate the relative size of the population and to some extent the need to apply insecticide.
Typically, a catch over a threshold of 10 to 20 or more moths per trap in a week can be taken as an indication that ongoing insecticide treatment is needed.
A Sneaky Pest
OFM can be a real problem in young peach orchards where early season larvae invade and distort tender young shoot tips, frustrating growers trying to grow a well-structured tree, and perhaps causing future canker problems. OFM are also capable of infesting young apple shoots, but the damage to these are generally minor and often overlooked. Infested shoot tips, particularly in peach orchards, become a major source of new moths for the second and third generations. As female moths can lay up to 200 eggs, the OFM population can build quickly, plus it has the reputation of being a sneaky pest.
The sneakiness is due to the summer and fall generation larvae often burrowing into fruit close to the stem. If it is a small larvae the entry hole is tiny, almost invisible, and the infested fruit are really difficult to sort out in a packing line. Packers have had to abandon complete fruit loads that have even a small amount of infestations of this type.
OFM, like many other moth pests of fruit, is notorious for developing resistance to insecticides. With building resistance problems, a grower will see that insecticides do not last as long in the orchard, and, trap catches can skyrocket, reaching over 100 moths per trap in a single night.
Since the early 1990s, OFM has developed resistance to a wide array of insecticide types including organophosphate-types like Guthion and Imidan, carbaryl (Sevin), and the various pyrethroid insecticides, as has been well documented in Ontario, Illinois, and New Jersey. Fortunately, there are several classes of insecticides available for OFM management that can be used in rotation to help keep resistance to one class from building.
The good and also bad news is that resistance of OFM to an insecticide class can decrease over several years if use of that class is cut back. Although this seems good, it does leave the crop manager uncertain as to what is the resistance level to a particular insecticide in an orchard. Testing for insecticide resistance in an orchard is a slow and expensive undertaking, and currently is not practical for management decisions.
It takes real discipline to follow a good insecticide rotation program and there are always temptations to deviate. For example, pyrethroid insecticides are relatively cheap and they are a valued mix partner due to their broad spectrum activity against other pests such as tarnished plant bug, brown marmorated stink bug, and San Jose scale. Even though another insecticide may be in the mix to control OFM, the presence of the pyrethroid partner will keep nudging the insect population to maintain high pyrethroid resistance.
And don’t forget that pheromone disruption is a straightforward method to help control OFM. Disruption has proven to be particularly useful for reducing OFM populations that have gotten out of control. Mating disruption has its nuances as described in a recent article by Michigan State University researchers Larry Gut and Mike Haas. It is well worth reading.