Petal Fall is the Most Attractive 

 Development Stage of Mcintosh 

 Apple Trees to Plum Curculio Adults 



Tracy LeskeyS Michele Bakis^ Holly Gagne^ Larry Phelan^, 



and Ronald Prokopy^ 



^Department of Entomology^ University of Massachusetts 



^Ohio Agricultural Research & Development Center, Ohio State 



University 



In the 1996 winter issue oi Fruit Notes, we 

 reported the results of laboratory assays 

 conducted in 1995 aimed in part at examining 

 plum curculio responses to odors emitted from 

 Mcintosh apple trees. We obtained some 

 evidence indicating that attractive volatiles 

 were emitted from all parts of Mcintosh trees 

 (not just fruit) and were emitted in the most 

 attractive form at petal fall. Here, we report on 

 1996 laboratory bioassays of curculio responses 

 to extracts of Mcintosh twigs, leaves, and fruit 

 at eight different tree- developmental stages, 

 using different solvents. 



Materials & Methods 



Hexane and water extracts were made from 

 twigs, leaves, or fruit of Mcintosh at each of the 

 following stages of development: pink, bloom, 

 petal fall, and 2, 3, 4, 5, and 6 weeks after 

 bloom. 



Curculios used in bioassays were collected 

 from unsprayed wild plum and apple trees and 

 sexed in the laboratory. For all tests, curculios 

 were starved for 24 hours prior to testing. Tests 

 were conducted at the beginning of darkness. 

 One curculio was placed into each petri dish 

 bioassay chamber and allowed to move toward 

 odors emitted from a hexane or water extract of 

 Mcintosh tissue (the treatment) or toward 

 hexane or water alone (used as a control). 

 Hexane was allowed to evaporate before testing 

 and curculios were given 2 hours to respond. 



To measure the power of a Mcintosh odor 

 extract to stimulate curculio response, we used 

 a response index. The response index was 

 calculated by subtracting the number of 

 curculios responding to the control from the 

 number responding to the treatment, dividing 

 this amount by the total number of curculios 

 tested, and multiplying by 100. The greater the 

 value of the index, the more attractive was the 

 Mcintosh odor extract. We consider an index of 

 25 to be the minimum for suggesting 

 attractiveness. 



Results 



Hexane extracts of Mcintosh twigs, leaves, 

 or fruit at petal fall were more attractive than 

 hexane extracts made at any other develop- 

 mental stage (Figure 1). Hexane extracts of 

 fruit at petal fall (index = 43) were similarly 

 attractive to extracts of twigs or leaves at petal 

 fall (index = 40 for each). These response indices 

 are nearly identical to those recorded in 1995 

 for curculio responses to hexane extracts of 

 Mcintosh fruit, twigs, and leaves at petal fall. 

 Finally, both males and females responded 

 equally well to extracts made with hexane. 



Water extracts of Mcintosh twigs, leaves, or 

 fruit at pink, petal fall, and two weeks after 

 bloom were the most attractive developmental 

 stages (Figure 2). At petal fall, extracts of 

 Mcintosh fruit (index = 71) were slightly more 

 attractive than extracts of Mcintosh twigs 



Fruit Notes, Volume 62 (Number 1), Fall, 1997 



13 



