Cool Temps Delay PC Migration

Plum curculio stings in early sweet cherry

Synopsis: Sweet cherry and other stone fruit develop a week or so before most apple varieties, and are often hit first by PC. An untreated sentinel cherry tree along the orchard border can assist apple growers in determining the early migration of the adult plum curculio (PC) beetle. Begin scouting for fruit with feeding and first ovipositional stings by the adult plum curculio this week along the orchard wooded edge. As the temperatures begin to climb a week or so from now keep a close eye on the border fruit. Tree fruit growers can postpone the petal fall spray during this cold spell and buy some time befor PC becomes active. Since the migration of plum curculio lasts for 308 Degree Days (base 50) a delay in the PC application may help save an application during this cool period of delayed and possibly prolonged adult PC migration, while a timely spray can pick up the first wave of PC migration.

Introduction: The plum curculio (PC), a snout beetle native to North America, overwinter as adults in ground litter or the soil. Cornell professor Dr. S.E. Link summarized plum curculio spring migration in a fact sheet focusing on temperature dependent activity, following several days of either a mean temperature above 15.5C (60F) or maximum temperatures above 24C (75F). In Highland this season we have not had a day in May above 70F, with only a single day in which the average temperature was above 60F. (Table 1.) Temperature forecasting by NOAA predicts the average daily temperature over the next 5 days (to May 13th) to range between 48F and 55F with winds mostly out of the northwest to northeast. Cool temperatures and heavy rains (2.02″ since May 1st) have generally reduced insect presence (European red mite and pear psylla), feeding activity and egg laying.

Generally, commercial orchards are free of resident PC populations while organic orchards may have endemic overwintering PC populations. Infestations by adults occur during periods of PC migration moving in from adjoining hedgerows and woodlands. Therefore, injury in most commercial orchards is normally heaviest close to these sites.

Beginning May 15th forecasts predict dramatic increase in temperatures and the onset of PC activity. Postponing PC applications until the 14th of May will provide highest levels of residual as PC adults begin migration into orchards. The use of Carbaryl (Sevin) at PF will also provide some additional insecticidal insurance against PC during the thinning window over the next week (Carbohydrate model HVRL).

Scouting: It goes without say that frequent tree fruit scouting in historically problem blocks is your best source of real orchard information on insect presence and onset of fruit injury. As of Monday the 8th of May, we are seeing relatively low levels of leafroller feeding, rosy apple aphid within curled leaves, with continuing bloom in mid-late varieties. Ginger Gold and Red Delicious along with the early varieties have set with fruitlets at 5-7mm. Border trees and preferably an unsprayed ‘Sentinel’ tree, provides an early warning of insect presence, especially true for the onset of PC injury.

Emergence: As PC move into orchards the adults fly or crawl (if cooler temperatures prevail) to the trees. There they feed on the buds, flowers, and newly set fruit. cutting a hole in the skin of the fruit to hollow out a cavity about 3mm deep. After a preoviposition period that varies from 6-17 days the female cuts a cavity under the fruit’s skin with her snout. She then turns around and deposits an egg in the hole. Turning around again, she pushes the egg into the cavity and cuts a crescent-shaped slit, protecting the egg from being crushed by the rapidly developing fruit.

Upon hatching, the young larvae bore into the fruit. On stone fruits, the larvae will feed to but not on the seed. Larvae will feed on the seeds of pome fruits. Larvae are killed in apples by the pressure of the growing fruit cells and can only complete their development in dropped fruit.

Table 1. NEWA Temps. May 1st – 8th 2017

Management: Although the emergence period for PC lasts for several weeks, 40-60%of the total emergence occurs on a single day. Signs of first oviposition should signal the need for the first application of effective insecticide to manage the insect. The most effective materials with the longest residual should be selected, many of which are in the table below. The initial application should concentrate along the wooded edge to reduce migration.

Remember, that although Actara is effective against the PC at the highest labeled rate, used alone it is ineffective against the codling moth, which will become a pest target during the 1st and 2nd cover applications.

When PC Management is No Longer Needed: To access the plum curculio forecast model link to the NEWA site. Choose a station near your orchard, select “Pest Forecasts”, then “Apple Insects”, selecting plum curculio from the list. The site will estimate your degree-day (DD) accumulation after petal-fall once you enter the PF date of McIntosh. When 308 DD base 50F have accumulated after petal-fall, plum curculio migration ceases and insecticide applications are no longer necessary.

Insecticides to Manage PC at PF