A COMPARISON OF INSECTICIDAL SOAP AND AMITRAZ 

 AS SUMMER SPRAYS AGAINST PEAR PSYLLA 



William M. Coli, Anthony Rossi, and Kathleen Leahy 



Departments of Entomology and Plant & Soil Sciences, University of Massachusetts 



The pear psylla (Psylla pyricola) is well known as 

 an important pest of pears, causing damage in the form of 

 reduced tree vigor and through the accumulation of excre- 

 ment (honeydew) and resultant sooty mold fungus on fruit, 

 foliage, and wood. In commercial pear orchards, psylla are 

 often difficult to control due to pesticide-induced resis- 

 tance to many registered pesticides. We frequently have 

 seen excessive psylla injury, giving trees a blackened ap- 

 pearance, even in blocks which received a regular spray 

 program, and a number of growers have reported increas- 

 ing difficulty with psylla control in recent years. 



To some extent, psylla are a problem because 

 growers often do not recognize that a particular material 

 that had been effective in the past is no longer providing 

 control. For example, some growers may .still be able to use 

 azinphosmcthyl or phosalone against psylla, but in many 

 instances these materials will not adequately control the 

 pest. Synthetic pyrethroids generally still are effective, but 

 continued use, especially of multiple applications in a 

 season, will almost certainly result in the development of 

 resistance. For most pear growers, dormant oils will 

 suppress psylla in the early part of the season, but fre- 

 quently one or more applications of amitraz, a highly toxic 

 material, are required during the July through August 

 period. 



Because of the potential for psylla to develop 

 resistance to amitraz and the negative effects of this and 

 other registered pesticides on beneficial arthropods, it is 

 imperative that alternative approaches to managing pear 

 psylla be developed and tested. For example, researchers 

 in Washington achieved limited success by washing honey- 

 dew from fruit with water sprays. In this article, we describe 

 a trial conducted in 1987 at a small commercial pear 

 orchard to determine if insecticidal soaps have any poten- 

 tial in commercial psylla control programs by "cleaning" 

 fruit, by direct psylla control, or by both. For more informa- 

 tion on pear psylla management strategies, see the 1988 

 March Message (University of Massachusetts Cooperative 

 Extension). 



The trial was conducted in a 1 acre orchard of well- 

 pruned, 8-year-old Bartlett pear trees in Belchertown, MA. 

 Treatments were laid out in a randomized complete block 

 design, using 6 replications of 3-tree plots for each treat- 



ment. Weekly sampling for all psylla life stages (eggs, 

 softshcll nymphs, hardshell nymphs, and adults), honey- 

 dew, and beneficial arthropods was conducted by observ- 

 ing the last four leaves on 10 succulent terminals from 

 throughout the canopy of the center tree of each plot. 



Treatments were: (1) untreated check, (2) ami- 

 traz @ 2 pints per 100 gal. water, and (3) SaferTM soap @ 

 2 gal. per 100 gal. water. All treatments were applied until 

 runoff with a motorized hydraulic handgun sprayer at 200 

 psi, after 30% of terminals were infested with active psylla 

 stages (July 28). Because trees were not cropping heavily, 

 at harvest fruit from all trees in each 3-tree plot were 

 combined and examined closely for signs of injury. Data 

 were analyzed using analysis of variance, and means were 

 separated using Duncan's New Multiple Range Test. 



Data in Figure la indicate that both the soap and 

 amitraz treatments caused a significant reduction in num- 

 bers of all active psylla stages (softshell, hardshell, and 

 adult) compared to the check on August 7 and August 12. 

 By August 19, the quantity of active psylla stages on treated 

 trees did not differ significantly from those on the check; 

 however, treatment-related pest reduction allowed fruit to 

 be harvested with no further treatment and with no down- 

 grading of fruit from honeydew or sooty mold when soap 

 or amitraz was used. 



When life-stage data were analyzed separately, 

 results indicated that neither treatment caused a significant 

 reduction in numbers ofsoftshell nymphs (Figure lb). This 

 result may be due to the protection afforded this stage by 

 drops of honeydew which they secrete and hide within. 

 Softshcll nymphs typically also are protected by their 

 tendency to feed in leaf axils where sprays may not ade- 

 quately reach. Also, since neither treatment was expected 

 to have an effect on eggs, undoubtedly a certain portion 

 hatched after the treatment date, resulting in higher 

 softshell nymph numbers. This finding suggests that there 

 is a need for back-to-back soap applications to prevent 

 nymph numbers from continuing to expand, which is 

 important because it is nymphal feeding and excrement 

 which sap the tree of its fluids and soil the fruit. Hardshell 

 nymphs were reduced by both treatments as of the August 

 7 sample date (Figure Ic). Although significant differences 

 disappeared by subsequent sample dates (as surviving soft 



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