The Scale of the Problem

The Current State of Scale Management in Southeastern Peach Production— A Summary from 2017 Southeast Regional Fruit and Vegetable Conference Presentation

San Jose scale (SJS), Comstockaspis perniciosus (Comstock), is an herbivorous insect pest that infests an enormous variety of plants worldwide, and is currently one of the most distressing pests of peach production in the southeastern U.S.  San Jose scale is an indirect pest of peach, normally feeding on plant tissues, which can cause leaf chlorosis, twig or limb die-back, and even death of trees.  As the populations build, SJS can become a direct pest by feeding on fruit, which produces small, red lesions on the skin (See Figure 1).  Young trees can die within three years of heavy infestation and mature trees have drastically reduced vigor and yields, potentially reducing the productive lifespan of an orchard.

Combating such a severe and universal pest requires an effective integrated pest management program, and such a program starts with the knowledge of the targeted pest’s biology and life history.  SJS is a pest of peaches, nectarines, plums, and other tree fruits including apples, pears, and cherries.  Adult females of SJS are yellow, circular, sac-like insects.  They secrete and live beneath a protective covering that is round (1/16-inch in diameter), gray-brown, and made up of concentric rings surrounding a raised bump near the center.  Adult males are tiny, golden-brown, winged insects, about 1/25-inch-long with a narrow, dark band across the abdomen.  They mature under elongate oval scale coverings, about 1/24-inch-long, with the raised, dark bump near one end.  The immatures or “crawlers” are mobile and are yellow, somewhat oval, about 1/100-inch-long (See Figure 2).

We can use our understanding of the SJS life history to attempt to prevent initial infestations or outbreaks in the orchard.  Destruction or management of nearby alternative hosts, such as wild plum, abandoned peach blocks, and poplar may reduce immigration of scale into the orchard.  More importantly, burning and proper disposal of removed trees and limbs infested with scale will help prevent further spread of the pest within the orchard.

Even with preventative measures put into practice, trees and fruit should be inspected frequently for the presence of SJS.  Scales, particularly SJS adults, blend in well with peach bark and are generally hard to see and thus often go unnoticed until it is too late, but by exploiting their biology, we can make monitoring for SJS much more effective.  Female SJS produce a sex pheromone to attract males, thus we can bait sticky traps with synthetic SJS pheromone to monitor male SJS activity (See Figure 3).  In mid-March, place a SJS pheromone-baited trap in the upper half of a tree with previous history of scale infestation.  Check those traps twice weekly, and once males are captured, begin keeping track of accumulating degree-days (developmental base of 51° F).  Based on phenology models, crawlers begin to emerge 400 to 700 degree-days after adult males are caught.  To pinpoint peak crawler activity, wrap scale-infested branches with double-sided sticky tape and using a hand lens, check tape for captures of scale crawlers twice per week.  If crawlers are caught on the sticky tape, then it is time for intervention.

Currently, the primary means of scale management are two dormant applications of horticultural oils, which includes a delayed dormant application of oil plus chlorpyrifos or an insect growth regulator.  The oils kill primarily through suffocation, and are effective if applied properly: trees must be thoroughly covered, ensuring every scale is coated to smother the insects.  Since there is little, if any, residual value, and direct coverage of the pest must occur for mortality, oil applications must be performed correctly.  Growers are advised to apply 150-200 gallons per acre (gpa) of oil and/or insecticide solutions for proper coverage.  Such high volume is not always possible, and thus in-season monitoring of trees for crawlers is highly recommended.  Once crawlers are caught on the double-sided sticky tape, check the spray guide for insecticide recommendations, and apply sprays every seven to 10 days until no crawlers are captured.  The insect growth regulators (Pyriproxyfen, Buprofezin, and Spirotetramat) work well at this management timing.

When scale problems persist throughout the season, targeting the second generation of crawlers may be necessary.  This generally occurs 1,000 degree-days after peak first generation crawlers, which is around early July.  The insect growth regulators are also recommended for this timing, but it is very important to rotate chemicals in order to manage insecticide resistance.  Continue to monitor scale activity throughout the season to determine whether additional insecticide applications are necessary, making sure to end the season with a high-volume dormant oil application in the fall.                                                                         

In natural settings, scale insects are generally controlled by their natural enemies, which include predators and parasitoid wasps, but the frequent and necessary use of insecticides have negative impacts on these beneficial insects, which may be another factor in the continued pressure from scale insects.  Reduce pyrethroid use, especially during April and May when first generation SJS crawlers first emerge and are most vulnerable to attack by beneficial insects.  When it comes to SJS management, timing and coverage are key.  Providing the highest possible coverage per tree with high a gallonage rate, should ultimately lead to the highest SJS mortality, reducing the need for subsequent insecticide applications and thus increasing the beneficial insect abundance within the orchard.  With a combination of our understanding of scale biology, frequent monitoring, and targeted chemical intervention, we can hopefully keep SJS populations in peach orchards under control.

Figure 1: Scale damage to surface of peach.

Figure 2: San Jose scale adults (brown, circular shape) and “crawlers” (yellow, immature stage) on a limb of a peach tree.


Figure 3: Male San Jose scale caught on a sticky card. Courtesy of Steve Schoof, NCSU

CREDIT

by BRETT R. BLAAUW

Brett R. Blaauw is an assistant professor of Entomology at the University of Georgia Agriculture and Natural Resources program.