A Threshold for Spraying Against Plum 

 Curculio Using Odor-baited Trap Trees 



Ronald Prokopy, Isabel Jacome, and Jaime Pinero 

 Department of Entomology, University of Massachusetts 



In the preceding article, we established several 

 characteristics that an odor-baited trap tree ought to 

 have in order to qualify as a site for monitoring fruit 

 injury by plum curculio (PC). 



Here, we present results of a 2003 experiment 

 aimed at determining a tentative threshold of PC injury 

 to fruit on a trap tree that would justify insecticide 

 application to rows 1 and 2 of an orchard block following 

 a whole-block spray at petal full. 



Materials & Methods 



We selected 12 blocks of frees in commercial 

 orchards in Massachusetts. Each block was comprised 

 of at least eight rows of trees and was bordered along 

 its entire 200 yard perimeter by continuous woods or 

 hedgerow. Each block was located in a different 

 orchard and was divided into three equal-size plots. A 

 trap tree baited with 1 dispenser ofgrandisoic acid plus 

 four dispensers of benzaldehyde (see preceding article) 

 was established at the center of the perimeter row of 

 each plot, 33 yards from either edge. Each of the three 

 plots per block was pre-assigned at random a threshold 

 of either 1, 2 or 4 freshly injured fruit out of 50 fruit 

 sampled on the trap tree. Each trap tree was sampled 

 for freshly injured fruit three times per week (Monday, 

 Wednesday, Friday), beginning 7 days after a petal fall 

 spray of insecticide. We presumed that residual activity 

 of insecticide extended at least 7 days after application. 

 Sampling involved examining 50 haphazardly chosen 

 fruit per tree at head height; 25 in the outer half of the 

 canopy, 25 in the inner half. Sampling was tenninated 

 on June 30, when no fresh injury was detected in 

 samples on any trap tree for two consecutive sampling 

 periods. 



All 36 plots received a grower-applied treatment 

 of Guthion or Imidan across the entire plot within 4 

 days after petal fall. Thereafter, only the first (= 

 perimeter) and second rows of a plot received 

 insecticide as applied by growers, who sprayed both 

 sides of first-row trees and the perimeter-facing side 

 of second-row trees. In all cases, such treatments were 



made within 24 hours of our sampling a trap tree and 

 our determination that the proportion of sampled fruit 

 showing injury had reached the pre-established 

 threshold of 1 , 2, or 4 freshly injured fruit. Once a plot 

 had received an insecticide treatment to rows 1 and 2, 

 we allowed 6-7 days before resuming examination of 

 fruit on trap trees for injury. Then, for each plot, we 

 waited until fresh injury to sampled fruit on a trap tree 

 again reached the pre-established threshold for that plot 

 before calling for the next insecticide application to rows 

 I and 2. To guard against invasion of PCs into plots 

 from an exposed lateral side or from rows deeper than 

 the seventh row, growers applied insecticide at 7-to- 

 1 0-day intervals to orchard trees abutting trees in test 

 plots. 



To evaluate the plot-wide outcome of insecticide 

 application against PC as driven by varying thresholds 

 of allowable injury on trap trees, during the first week 

 of July we examined 20 fruit at head height in the outer 

 half of the canopy on each of five trees in each of 

 rows I, 2, 3, 5, and 7 of each plot for evidence of any 

 injury caused by PC (total of 100 fruit per row per 

 plot). Fruit on the trap tree were excluded from 

 consideration, because such fruit would normally 

 comprise a very low percentage of all fruit in an orchard 

 block. For example, for a 2.5 acre square block of 

 medium-size trees on M.26 rootstock, fruit from a trap 

 tree at the center of a 110 yard perimeter row would 

 constitute less than 0.2% of the total amount of fruit in 

 the block. 



Results 



The mean number of insecticide applications made by 

 glowers to trees in rows 1 and 2 declined successively 

 (though not significantly) from 1.56 to 1.44 and 0.89 

 sprays as the pre-assigned threshold calling for spray 

 application increased successively from I to 2 and 4 

 freshly injured fruit out of 50 fruit sampled on trap trees 

 (Table 1). Conversely, the mean proportion of fruit 

 injured by PC in samples taken during the first week of 

 July (i.e., at the conclusion of the injury season) on 



14 



Fruit Notes, Volume 69, Winter, 2004 



