WALNUTSCALE- DEVELOPMENTOFANEFFECTIVEMONITORINGMETHODAND

PHENOLOGYMODEL

Richard L. Coviello, Mark Freeman, Richard E. Rice, William H. Krueger

ABSTRACT

Double-sided sticky tape was wrapped around branches of walnut trees withheavy populations of walnut scale in order to monitor emergence of adultmales and crawlers. The tape successfully trapped both males and crawlers,even in inclement weather. Trap counts gave good biofix points from onegeneration to the next and allowed calculation of temperature thresholdsand degree-days per generation. A threshold of 50°F and an upper limitof gO°F gives the lowest variation between biofix points and results in ageneration time of approximately 2060 degree-days per generation.

The tapes were also used to time treatments and evaluate results of applyingmethidation at the dormant period, delayed dormant when male adults areemerging, and when crawlers emerge. The dormant treatment was not as effec-tive as the others in the latter part of the season. The crawler emergenceapplication reduced the scale to the lowest numbers but gave indicationsof disrupting biological control of other pests.

OBJECTIVES

Current walnut scale control recommendations are to apply insecticidesduring first generation adult male flight. No effective and convenientmethod for monitoring male emergence has been available to time this treat-ment. Recent data evaluating alternate timings is also lacking. Ourobjectives were as follows:

1. To evaluate and demonstrate the effectiveness of double-sided stickytape for monitoring the emergence of males.

2. To evaluate the effectiveness of a dormant treatment for scale control.

3. To develop a phenology model of walnut scale for alternate treatmenttiming and for future linkage with a walnut plant model to eventuallyassess the economic impact of the scale.

PROCEDURE

Twenty trapping sites on ten trees in an untreated orchard were selected forthe phenology study. Two branches, one to two inches in diameter and havingheavy scale populations, were chosen in each tree. A small area of bark wasscraped clean around the branch and encircled with the tape. The tapes werereplaced twice a week and all adult males and crawlers stuck to them counted.Numbers were plotted as scale per tape per night to exclude variation due tothe observation interval.

The timing trial was conducted by applying methidathion to five single treereplications, either during the dormant period, during first generation maleemergence, or during first generation crawler emergence. The treatmentswere applied by handgun sprayer at 400 gallons per acre with 2 lbs. a.i. peracre. Five trees were left untreated for a control. Effects of the treat-ments were evaluated by the sticky tape method.

56.

RESULTS

The results of the phenology study are shown in Fig. 1. Both adult malesand crawlers were easily trapped by the sticky tape, even during periods ofcold wet weather. The data provided very clear biofix points in eachgeneration, from which degree-day calculations were made. Temperature datawere obtained from a NOAAthermograph station approximately 1/2 mile fromthe trapping site. Degree-day accumulations were calculated between variousbiofix points from one generation to the next, using several combinations oflower and upper thresholds. The coefficient of variation (CV) betweenbiofix points for each threshold was calculated and the smallest CVwasassumed to indicate the closest approximation of the correct threshold withthe concomitant correct number of degree-days per generation (Table 1). Thecurrent year's data indicate approximately 2060 degree-days per generation ata 50°F threshold and gO°F upper limit. This model must be validated byobservation from several locations in the next few years.

The results of the timing trial are shown in Figs. 2 and 3 and Table 2.The dormant treatment reduced the first generation male flight and crawleremergence significantly but failed to hold downthe second. The delayeddormant application reduced the scale population through both generationsbut towards the end of the second crawler stage appeared to lose someeffectiveness. The treatment at first generation crawler emergence heldthe scale levels very low throughout the remainder of the year. These sameplots will be observed next year to see howlong the treatments will hold.

The crawler stage treatment appears to offer the best control of scale;however, observations of European red mites showed significantly higherlevels in the June treatment than in the dormant application, (Table 3).Predator mite populations were just the reverse, indicating that a crawlerstage application may lead to serious secondary pest problems if the orchardis under heavy mite pressure. Walnut aphid populations were lower in thecrawler treatment plots, indicating some control of that pest. However,the ratio of live to parasitized aphids was higher showing a disruption ofthe biological control of the aphid. This orchard has not been treated withmethidathion before, so there was no selection for resistance to this material.Aphid control by the scale treatment could not be counted upon in a routinelysprayed orchard. Evaluations of other materials will be carried on in thenext few years to find a less disruptive one.

57.

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Table 1. Degree-Days and Coefficient of Variation (CV) at SeveralThresholds

Threshold/UDDerLimit 1/ Mean Dearee-Davs CV

40/40/9040/8545/-45/9045/8550/-50/90

.50/8555/-55/9055/8560/-60/9060/85

310930512967260325452469211520561972166116011518126412051121

6.867.217.725.675.905.835.485.445.556.746.416.169.038.487.98

1/Means followed by the same letter are not significantly different(DMRT= 0.05).

58.

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Table 2. Mean No. Scale Per Night at Population Peaks

1st Generation 2nd GenerationTreatment and Date Males Crawlers Males Crawlers

Dormant 4.03a 1/ 64.39a 2.19ab 58.07 b2/11/83

Male Flight 0.87a 14.80a O.77a 8.lla3/23/83

Crawler Emergence 13.73 b 3.96a 0.26a 0.36a6/1/83

Control 15.16b 455.29 b 3.07 b 57.67 b

Table 3. European Red Mite (ERM),Mite Predators and Walnut

Aphid Populations !/ Sampled July 19, 1983

lIMean no. per leaflet.

~Means followed by the same letter are not significantly different(DMRT- 0.05).

59.

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Treatment and Predatory Walnut Live AphidsDate ERM Mites Aphid Mummies

Dormant 0.95a Y 1.28ab 2.45ab 0.03a2/11 /83

Male Flight 1.72ab 1.80ab 2.99 b 0.08ab3/23/83

Crawler Emergence 2.85 b 0.20 b 1.17a 0.11 b6/1/83

Control 1. 08ab 1.91 a 2.84 b 0.06ab

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