New Trap-capture Thresholds for Tarnished 

 Plant Bug and European Apple Sawfly In 

 Massachusetts Apple Orchards 



Kathleen Leahy, Ronald J. Prokopy, and William M. Coli 

 Department of Entomology, University of Massachusetts 



IPM trap-capture thresholds for tarnished plant 

 bug(TPB) and European apple sawfly (EAS) originally 

 were developed in 1980. Changes in input costs and 

 produce prices have reduced the reliability of these 

 thresholds. In this article we are presenting new 

 threshold values as well as a general format for estab- 

 lishing a trap-capture threshold. With this format 

 growers could use their own price and production 

 figures to calculate appropriate thresholds and update 

 them as their data change. 



The actual mechanics of how the new thresholds 

 were determined can be seen in the Table 1. Clearly, a 

 number of assumptions needed to be made about per- 

 acre fruit production, types and costs of chemicals 

 used, etc. Some of the assumptions used were: 



- 15 minutes to spray one acre 



- labor cost = $7/hr 



- equipment cost = $8.68/hr, developed from 

 publications by New York agricultural 

 economists, updated by W. R. Autio 



- use of an organophosphate insecticide 



- per-acre production = 500 bushels 



- average 80% Extra-Fancy packout 



- fruit values = $15/bu U. S. Extra Fancy, $9/bu 

 U. S. Number 1, $7.50/bu Utility, $2/bu 

 Processing 



In all cases, where a choice of assumptions was pos- 

 sible, we used the more consei-vative figure. 



One obvious fact that emerged in calculating these 

 new thresholds was that the intended market for the 

 fruit made a difference in how much injury a giower 

 could tolerate. A gi-ower selling fruit through a farm- 

 stand, or who could otherwise market fruit graded less 

 than U. S. Fancy, can afford more injuiy than a gi-ower 

 for whom any fruit that does not meet the U.S. Fancy 

 gi-ade will be priced as processing fruit. Thus, we found 

 that wholesale growers need to be considerably more 

 conservative in their pest management decision-mak- 

 ing than retail gi-owers, a fact that probably will not 

 surprise most growers. In response to this situation, 

 we developed separate thresholds for the two situ- 

 ations. 



Thus, we considered two possibilities for down- 

 graded fruit: first, U. S. Extra Fancy downgraded to a 

 mix of U. S. Number 1, Utility, and processing (20, 70, 

 and 10 %, respectively, of those fruit downgraded); and 

 second, U. S. Extra Fancy downgraded to processing 

 only (100 % of those fi-uit downgraded). We also used 

 data from a number of packout studies (e.g., Morin and 

 Bahn, Fruit Notes 1981) showing that only about 10% 

 of plant bug injury actually is downgraded at all, since 

 it tends to occur in the calyx of the fruit, hidden from 

 view. 



Mostly what these new thresholds demonstrate is 

 that some gi-owers can tolerate more, and even a good 

 deal more, tarnished plant bug injuiy than had previ- 

 ously been assumed. In the case of sawfly, the tolerable 

 threshold likewise is slightly higher. But we do not 

 expect that growers will be able to save many sprays for 

 sav^y, since a petal fall insecticide would usually be 

 necessary for plum curculio or other insects even if 

 sawfly were not a problem. In ceilain years when 

 curculio adults begin attacking apples later than nor- 

 mal (e.g., a week after petal fall), sawfly trap captures 

 can indicate whether sawflies are sufficiently abun- 

 dant to spray at petal fall or not. 



Fruit Notes, Spring, 1989 



