explanation is that the weather during peak 

 female activity was warm and dry in 1991, 

 highly conducive to a maximum amount of 

 egglaying per female. 



It is difficult to turn these data into a firm set 

 of recommendations. On the one hand, it is not 

 clear that a situation of explosive leafminer 

 growth from one generation to the next, such as 

 we had in 1991, will happen again soon. On the 

 other hand, the results from this year suggest 

 the need to take a more conservative approach 

 to early-season leafminer management. Some 

 growers may give up on the traps altogether; 

 however, it should be remembered that the 

 traps have given reliable results in most years 

 for most growers for nearly a decade. Also, the 

 effectiveness and non-disruptiveness of most of 

 our pesticides for leafminer control is greatest 

 when the pesticides are applied as early as 

 possible. This fact forces us to make a control 

 decision at a time when we are not sure of what 

 will happen with egglaying rate per leafminer 

 female, which is driven in large part by weather 

 conditions during the critical period. The ideal 

 situation would be to have a non-disruptive 

 material that could be used effectively against 

 first- or second-generation mines, allowing a 



more confident reliance on thresholds. Unfortu- 

 nately, this is not likely to happen in the near 

 future; even Dimilin™, the most non-disruptive 

 material under testing for leafminer control, is 

 somewhat less effective when used against later 

 generations. And, we have no assurance that 

 Dimilin will be labelled for use against 

 leafminer on apples. 



The approach taken in the 1992 March Mes- 

 sage is to reduce further the threshold for taking 

 action against leafminers to two per trap on the 

 trunk traps by tight cluster, four per trap on the 

 trunk traps by pink, and two per trap on the 

 canopy traps by pink. These are extremely 

 conservative thresholds but they should provide 

 a high degree of certainty that if the threshold is 

 not reached, leafminers will not reach damaging 

 levels. 



Because those growers in our second-level 

 IPM program who had even moderate numbers 

 of leaf mines eventually did apply control mea- 

 sures, we were not able to follow leafminer 

 populations through the season and make any 

 correlation between leafminer pressure and 

 early fruit drop. Thus, we will leave our summer 

 action thresholds at the same level of 13 mines 

 per 100 leaves in the first generation, and 100 



Adult Leafminers on Trunk Traps 

 and Resulting First-Generation Uinea 



10 20 30 40 50 60 70 80 90 

 Adults per trap, cumulative through tight cluster 



1.0 

 0.9 

 0.8 



0.7 

 0.6 

 0.5 

 0.4 

 0.3 

 0.2 

 0.1 

 0.0 



10 20 30 40 50 60 70 80 90 

 Adults per trap, cumulative through pink 



Figure 2. The relationship between the capture of leafminer adults on rectangular red traps 

 and first-generation leaf mines. 



Fruit Notes, Spring, 1992 



19 



