Sampling for Early-season Leafminer 

 Populations in Massachusetts in 1991 



Kathleen P. Leahy and Ronald J. Prokopy 

 Department of Entomology, University of Massachusetts 



Leafminers are among the most enigmatic of 

 all problem pests encountered in Northeastern 

 apple orchards. Leafminers did not become a 

 problem in Massachusetts until 1980. Since 

 then, however, they have appeared in many 

 orchards throughout the State, sometimes 

 briefly and sometimes for several years in a row. 

 The population dynamics and movement of 

 leafminers are not well understood, and there is 

 still some question as to the level and timing of 

 infestation that will cause harm, especially 

 when other pests or environmental stresses are 

 present. Additionally, in New England, we 

 experience the further complication of dealing 

 with not one, but two closely related leafminer 

 species: the spotted tentiform leafminer, which 

 is present in most North American apple grow- 

 ing regions, and the apple blotch leafminer, 

 which is confined mainly to eastern New York 

 and New England, although it does appear else- 

 where. 



Although we did not have the resources to 

 investigate in a major way the myriad of factors 

 involved with leafminer management, we were 

 hopeful that the large number of blocks (12 

 second-level IPM test blocks and 12 paired 

 blocks managed by the growers) that were 

 sampled as part of the IPM Program in 1991 

 would give us an opportunity to look more 

 closely at seasonal patterns of leafminers. Since 

 all 12 growers refrained from using pre-bloom 

 leafminer controls, we were able to look at the 

 relationship between pre-bloom visual trap cap- 

 tures and first-generation mine counts. The 

 visual traps that we used were rectangular red 

 traps, which were either stapled at knee height 

 to the trunk on the south side of the tree ("trunk 

 traps") or hung at about head height in the 

 canopy as close to the trunk as possible ("canopy 



traps"). We also compared the number of mines 

 on leaves of different ages and types (small and 

 large fruit cluster leaves and small and large 

 non-fruit-cluster leaves) to determine which 

 gave the best indication of the extent of 

 leafminer infestation. 



Because of the unusually mild winter and 

 warm spring of 1990-91, leafminers apparently 

 had a very high survival rate and emerged in 

 large numbers anywhere that there had been 

 appreciable third-generation populations in 

 1990. The majority of growers in the second- 

 level IPM project needed to apply pesticide 

 against leafminer. All of these growers opted to 

 wait and treat the second-generation adults or 

 mines. The explosive growth of leafminer popu- 

 lations in the early part of the season may have 

 played some role in the difficulty we had in 

 interpreting the data. 



We consistently found more mines on the 

 smaller (older) fruit cluster leaves than on any 

 other type of leaf (Figure 1), confirming that 

 these are the best leaves to check for determin- 

 ing whether or not a control is needed for 

 leafminer. (IPM recommendations since 1980 

 have been to examine the older fruit cluster 

 leaves in order to make control decisions.) 

 Leafminer adults generally begin to emerge at 

 half-inch green and continue flying through 

 pink, so that the leaves that are out at that time 

 are available to the females for laying eggs. 



The New York State Simplified IPM (SIMP) 

 program uses a leafminer population assess- 

 ment of number of first-generation leafminer 

 eggs to determine whether or not a pre-bloom 

 treatment is needed. We attempted to make this 

 assessment, but were late in beginning our 

 monitoring for eggs, as we found virtually no 

 eggs at all. 



Fruit Notes, Spring, 1992 



17 



