maintained until mid-August or even into September to 

 protect fruit against apple maggot flies. A "Second-stage 

 IPM" strategy now being tested in Massachusetts may 

 offer a means to extend the length of the insecticide-free 

 period by terminating regular cover sprays at the end of 

 May after the period of plum curculio attack has ended. 

 Control of apple maggot fly in July, August, and Septem- 

 ber then would be based on intensive trapping using red 

 sticky traps developed by Prokopy and removal of wild or 

 abandoned apple trees adjacent to the orchards. While 

 initial data (1987) show that this system increases numbers 

 of predator mites, data are not yet available as to its effect 

 on leafminer parasites. Such data, however, will be col- 

 lected in future tests beginning in 1988. 



Habitat Management . Abandoned or wild apple 

 trees, or wild cherries in forested areas near orchards 

 typically have Phyllonorycter spp. Icafmincrs (including 

 ABLM) that serve as hosts for P. omigis and 5. marylan- 

 dcnsis. Increased parasitism in commercial orchards in 

 late summer and fall results from both within-orchard 

 build up of the parasites that survived early season pesti- 

 cide applications and perhaps from immigration of para- 

 sites into orchards from hosts living on unlcndcd trees. 

 The relative importance of these two sources is unknown. 

 Wild hawthornc, wild apple trees, and abandoned apple 

 trees near orchards all serve as sources of apple maggot 

 flies that can enter orchards and hence such trees should 

 be removed. Various species of native cherries also serve 

 as possible breeding sites for ABLM parasites, but are 

 likely to be too few in number to do more than "re-seed" 

 orchards that have lost their leafminer parasites due to 

 extensive pesticide use. The size of mid- to late-season 

 leafminer parasite populations in commercial orchards is 

 therefore less likely to be determined by the habitat out- 

 side the orchard than by the pesticide application history in 

 the orchard itself. 



State Actions of Potential Value in Promoting 

 Biological Control of the Apple Blotch Leafminer 



Existing native parasites of ABLM, while fairly effec- 

 tive, are not necessarily the species with the highest poten- 

 tial to suppress leafminer populations. Because of the 

 problems that have arisen in various regions with other 

 Phyllonorycter species, much is known of the parasites 

 attacking a variety of leafminer species. Some species 

 seem to have potential to increase the level of control over 

 that provided by our native parasites. For example in 

 Ontario, the introduced parasite, Pholetesor pedias, has 

 produced levels of parasitism 2 1/2 times greater than the 

 native Pholetesor omigis (Laing & Heraty, 1987). This 

 parasite has been established in Ontario and more recent- 

 ly in New York (Weires, personal communication). An- 

 other parasite of potential value is the encyrtid, Holcotho- 

 rax testaceipes, which is the major parasite attacking P. 



ringoniella in Japan (Sekita & Yamada, 1979). It has 

 recently been established in Ontario and is becoming the 

 dominant parasite there as well. A recently approved 

 Northeast Regional Apple Project has provided funds to 

 Dr. Chris Maier of the Connecticut Agricultural Experi- 

 ment Station to introduce both of these parasites to New 

 England. 



How to Monitor Parasitism Levels in Your 

 Orchard 



Leafminer populations can be monitored either by 

 counts of adult moths caught on red sticky traps, or by 

 counts of mines per leaf for each leafminer generation. 

 Thresholds currently in use in Massachusetts for the sec- 

 ond system are 0.13 mine/leaf for the first generation and 

 1 .00 mine/leaf for the second generation. These levels are 

 lower than thresholds currently used in other states to 

 account for compounding effects of mites or drought 

 stress. The use of 0.13 mine/leaf as a treatment threshold 

 is based on the concept that, given the 7 to 8 fold increase 

 typical from the first to the second leafminer generations, 

 more than 0.13 healthy mines in the first generation will 

 result in damaging populations (more than 1.0 mine/leaf) 

 in the second generation. This first generation threshold, 

 however, should be raised if levels of parasitism are high 

 (Figure 1). Parasitism levels can be determined by select- 

 ing one hundred old mines at the end of a generation and 

 opening them with needle-nose tweezers. Mines then can 

 easily be classified into ones in which moths have devel- 

 oped, ones in which parasitoids have developed or ones in 

 which the larvae were killed by feeding of adult parasites. 

 Percentage parasitism can then be calculated as number of 

 mines with dead larvae plus the number with parasitized 

 larvae or parasite pupae or cast parasite pupal skins 

 divided by the total number of leaf mines sampled. Levels 

 of parasitism can be taken into consideration only if 

 leafminer levels are assessed in the first generation and 

 required treatments are then made in the next generation 

 against leafminer adults. This strategy currently has the 

 drawback that available materials for killing leafminers 

 tend to make mite control problems worse. This effect 

 becomes increasingly more severe as the season pro- 

 gresses, and thus, given existing pesticides, treatments 

 made early against first generation leafminer larvae are 

 less disruptive to mite population dynamics than treat- 

 ments made for second generation leafminer adults. 

 Unfortunately, treatments targeted against first genera- 

 tion leafminer larvae cannot use thresholds modified by 

 levels of parasitism because parasitism occurs late in the 

 leafminer larval stage. If the insect growth regulator 

 difiubenzuron (Dimilin™) is registered for leafminer 

 control, it will be easier to utilize a strategy based on 

 assessing first generation mine numbers and levels of 

 parasitism at that time and then treating second genera- 



