The Plum Curculio, Conotrachelus nenuphar (Herbst), is one of the most important insect pest attacking fruit trees. It is particularly destructive to peaches and plums where it can cause extensive damage to fruit. The larvae tunnel in the developing fruit and render the fruit inedible. Also, adults chew holes in fruit as they feed and for depositing eggs. These holes provide entry for the brown rot fungus and produce cat-facing on peaches similar to that caused by stinkbugs.
The hosts of the plum curculio are apple, nectarine, plum, cherry, peach, apricot, pear and quince. This insect can also survive on some wild hosts including wild plum hawthorn and native crabapple.
Biology and Description
The adults are snout beetles, about 1/4 inch long, gray to black with lighter gray and brown mottling. Four pairs of ridges occur on the wing covers, but because the middle humps on each wing cover are larger, it appears to have only two humps. The mouthparts are at the end of a curved snout that is about 1/4 the size of length of the body.
Adult plum curculios overwinter in protected places in woodlands, along fencerows and under ground debris. They become active in the spring when mean temperatures reach 50° to 60°F for three to four days, and begin migrating toward trees (orchards) when the maximum temperature reaches 70°F for two or more days. They feed on the foliage and flowers until the small fruits form.
Shortly after the shuck-split stage on plums, plum curculios begin to lay eggs, but the peak of egg laying is not reached until up to a month later. Egg laying may continue for a period of six to eight weeks following shuck split. Egg laying begins as soon as the young fruit forms. Eggs are deposited in shallow cavities chewed into the fruit surface by the female curculio. A single female lays between 60 to 150 eggs. Sometimes during the egg laying process, the weevil may introduce spores of brown rot fungus to the fruit, which usually leads to entire fruit rotting from the inside out.
Eggs are about 3/16 in diameter, pearly white and elliptical. Eggs hatch in two to twelve days, with the average being 5 days to hatch. The newly –hatched larvae tunnel into the fruit and feed.
The larvae are slightly curved, white to yellowish- white, legless grubs with brown heads, and a light brown shield behind the head.
Larvae develop through four larval instars and the full grown larvae are 3/8 inch long. Larvae feed in the fruit for 8 to 22 days, depending upon temperature.
The full grown larva then tunnels out of the fruit, falls to the ground, enters the soil and constructs a small earthen cell, usually one to three inches below the surface. After about two weeks in the soil, the larva transforms into a pupa. The pupa is whitish or cream-colored and measures about 1/4 inch. After the pupal stage is complete, the adult emerges from the pupal case and digs its way out of the soil. First generation adults usually emerge about four to five weeks after larvae enter the soil. A generation, from egg to adult, is completed in about five to eight weeks. Most plum varieties are harvested by the time the first generation is completed and many of the adult curculios then migrate to other nearby host fruit trees. Here, they deposit their eggs in the developing fruit. Therefore, second generation larvae may be found in other later maturing fruit at harvest time.
After the second generation larvae have completed development, they pupate in the soil and emerge as adults between June and August. They join the first generation adults in feeding on foliage until the onset of cold weather, at which time individuals from both generations seek hibernation sites.
Annual variations in this life cycle depend on climatic conditions. During hot, dry seasons, fewer individuals of the first generation reach maturity and the second generation rarely becomes large enough to inflict economic injury to peaches. The overwintering population, following a hot, dry season, is composed almost entirely of first generation adults.
A related species is the plum gouger, Coccotorus scutellaris LeConte. Its primary host is native plums but is often found feeding on and causing damage to cultivated plums and peaches. Damaged fruit becomes gnarled and distorted from adult feeding and egg-laying punctures, and the larvae feed on the kernels of the pits. Adult plum gouger weevils have, smooth, dark brown wing covers and yellow-brown heads and legs (Drees and Jackman, 1998).
Plum Curculio Injury and Damage
Injury caused by the plum curculio can be grouped into two categories:
- Wounds resulting from feeding and egg-laying by the overwintering beetles early in the spring appear as crescent-shaped scars (oviposition injury) on plums and apples. The oviposition wounds on peaches results in shiny areas or disturbed fuzz on peaches. on the fruit, or as bumps (feeding injury) that protrude from the fruit at harvest. Badly attacked fruit may be knobby, gnarled and scarred at harvest
- The larvae’s burrowing in the fruit causes internal injury. Most of the larvae-infested fruits drop to the ground during April and May or results in unusable fruit that matures prematurely and is grub infested.
Suggested Management Guidelines for Plum Curculio in Peaches and Plums
It is important to use cultural control practices such as clearing of fallen leaves and dropped fruit from under the fruit trees on a regular basis and discard properly. This is to reduce potential sheltering sites for the adults and to keep larvae from reaching the soil where they pupate. Close moving of the orchard after harvest and weed control also discourages curculio adults from overwintering in the orchard. In addition, natural control of the curculio can result from winter mortality, predation by birds and other predators and from parasites.
One tactic that can be implemented into a plum curculio integrated pest management program is referred to as jarring. Jarring is not a widely performed pest management tactic for plum curculio and its efficacy may not be very efficient, however, for those who want to reduce potential insecticide applications, this tactic may have some value for plum curculio management. Jarring is a mechanical control tactic that is performed by suddenly beating the tree with a padded mallet. This in turn causes the adult beetles to loosen their grip on the tree, contract their legs and fall to the ground. It is recommended that this be done early morning when the temperatures are still cool. Place sheets on the ground to collect beetles and then destroy them after they fall on the sheets. This method should be avoided on young trees as severe damage can result if beaten too much. This needs to be done when the fruit is very small, because later jarring of trees when the fruit is larger can cause the fruit to be knocked-off the tree.
Satisfactory control of the plum curculio generally depends on properly timed application of effective insecticides. Insecticide sprays provide a protective barrier to prevent overwintering adults from laying first generation eggs. No economic threshold levels have been established for the plum curculio. One method to assist with proper timing of insecticide applications is monitoring for adult plum curculios. Monitoring can be when to performed by vigorously tapping the foliage or small branches in the early morning during petal fall. Hold a beat cloth (white sheet) under the branches that are being tapped. Look for the adult weevils that fall onto the cloth. Check the developing fruit for feeding and/or egg-laying injury. The activity of this pest is closely tied to temperature. The ideal temperature for this pest is around 70-75° F. However, activity will take place at temperatures much cooler and warmer than this. Once adult curculios are found, then management tactics should be initiated immediately.
Normally, several applications of an insecticide are generally needed for early curculio control. Apply the first spray at 75 % petal fall and make 2 additional applications at 7-14 day intervals. Repeat applications as long as new damage appears or adults continue to be found through monitoring. Sprays targeting the overwintered plum curculio generation also provide control of oriental fruit moth and suppress stinkbugs moving into orchards. This is the time in the curculios life cycle when the adult is actively moving around the branches and beginning to lay eggs. Spraying to kill the adults at this point will provide the best control. Remember, once the eggs and larvae are in the fruit, spraying an insecticide will not reverse previous damage or kill larvae already inside the fruit.
Second generation plum curculio may infest mid- and late season peach cultivars in infested orchards. To protect peaches from this generation, it may be necessary to apply an insecticide at six, four, and two weeks before harvest.
Be sure to read and follow label instructions when using pesticides, including the pre-harvest interval between the last allowed application and harvest date.
For additional information on suggested pesticides for managing these other insect and disease pests of peaches, plums and pecans for homeowners, obtain a copy of “E-145, Homeowner’s Guide to Pests of Peaches, Plums and Pecans” from the Texas Cooperative Extension On-line Bookstore at “http://tcebookstore.tamu.edu” or by contacting your local Texas Cooperative Extension Office. In addition, more information can be obtained from the 2006 Southeastern Peach, Nectarine and Plum Pest Management Guide, which can be found at the following link “http://www.ent.uga.edu/peach/peach_guide/title.htm”.
Drees, B. M. And J. A. Jackman. 1998. A Field Guide To Common Texas Insects. Pp.191-192. Houston, TX.
Horton, P. Brannen, B. Bellinger and D. Ritchie. 2006. Southeastern Peach Growers’ Handbook-Plum Curculio.
Howitt, A. H. 2005. Common Tree Fruit Pests – Plum Curculio. Michagan State University Extension, Fruit IPM Fact Sheet.
Fig 1. Adult plum curculio (Photo courtesy of Clemson Cooperative Extension).
Figure 2. Plum curculio egg laying scar with egg (Photo courtesy of Cornell Cooperative Extension).
Figure 3. Plum curculio larva and damaged fruit (Photo courtesy of University of Georgia).