parasites, which were easily killed by orchard cover sprays. 



A broader view shows that this same scenario of 

 pesticide-resistant leafminers and pesticide-susceptible 

 parasites has led to significant pest problems from a 

 number of other Phyllonorycter species on apple or other 

 tree fruits in many areas, including P. blancardella (F.) in 

 New York, Michigan, and Ontario (Dutcher & Howitt, 

 1978; Free et al., 1980; Weires et al., 1980), P. corylifoliella 

 (Hubner) in Holland (Van Frankenhuyzen, 1975), P. el- 

 maella (Doganlar & Mutuura) in Utah (Barrett & Jor- 

 gensen, 1986), and P. ringoniella (Matsumura) in Japan 

 (Sekita & Yamada, 1979). This repeated pattern high- 

 lights the vulnerability of single-factor chemical controls 

 which, once breached by pest resistance, leave pests free to 

 multiply unrestrained by natural enemies which typically 

 are slower to develop pesticide resistance (if ever) and 

 hence cannot attack the pest in the sprayed environment. 



Because high density leafmincr populations can ad- 

 versely affect apple crops by increasing drop and reducing 

 flower bud formation (Reissig et al., 1982) and because 

 Phyllonorycter spp. have a high capacity to develop pesti- 

 cide resistance, it is important that orchardists in Massa- 

 chusetts broaden control systems immediately to protect 

 and encourage the specialist parasites that attack ABLM. 

 If this is not done, further resistance in the species likely 

 will occur to the specific carbamate and organochlorine 

 materials that are currently effective and used against high 

 density leafminer populations. 



Major Parasites of ABLM in Massachusetts 



Studies in Massachusetts (Van Driesche & Taub, 

 1983), Connecticut (Maier, 1984), New York (Weires et 

 al., 1980), and elsewhere have consistently shown that 

 Phyllonorycter species attacking apple are themselves ex- 

 tensively attacked by several species of parasites in the 

 absence of interference from pesticide applications. The 

 most important of these in Massachusetts at this time are 

 the braconid, Pholetesor omigis (Weed), and the eulophid, 

 Sympiesis marylandensis (Girault) (Van Driesche & Taub, 

 1983). 



P. omigis is a black species, 1/4 inch long, that attacks 

 tissue feeding host larvae, laying its egg inside the body of 

 the host larva. The mature parasite larvae exits from the 

 host and spins a white cocoon inside the leaf mine. Detec- 

 tion of parasitism by P. omigis is easiest in old mines since 

 parasite cocoons are readily visible to the naked eye and 

 remain present even after the adult parasite has left the 

 mine. Parasite cocoons are easily separated from both live 

 and emerged moth pupae. 



S. marylandensis, in contrast, is smaller (1/8 inch), 

 metallic blue-black in color, and attacks tissue stage larvae. 

 Its eggs are laid next to, but outside of, the host larvae and 

 are visible with a hand lens. Parasite larvae develop and 

 feed outside the host larvae and pupate in the mine, with no 



cocoon. Parasitism by this species can be detected at any 

 time because of its position external to the moth larva. In 

 addition to killing hosts by parasitization, 5. marylandensis 

 adults directly attack and kill other tissue feeding larvae 

 which they pierce with their ovipositor and then partially 

 consume as a source of protein. Larvae killed in this 

 manner appear dried out and shriveled 2md are often 

 attached to the mine "skin" at a single point, where the 

 parasite formerly fed. 



Other potentially important parasite species, not yet 

 found in Massachusetts, include the braconid, Pholetesor 

 pedias (Nixon), and the encyrtid, Holcothorax testaceipes, 

 both recently introduced to Ontario (Laing & Heraty, 

 1981) from New Zealand and Japan, respectively. 



Management Options Available to Growers to 

 Promote Biological Control of Apple Blotch 

 Leafminer Populations 



The major infiuence on parasite populations in apple 

 orchards is the pesticide regime, both in terms of its 

 duration and the particular chemicals selected for use. To 

 a lesser extent, proximity of wild or abandoned apple trees, 

 or wild cherry trees where parasite populations will be 

 found attacking various species of leafminers, can also 

 infiuence events within commercial apple orchards. 



Pesticide Management . Attempts have been made to 

 identify pesticides more toxic to the pest leafminer than to 

 their associated parasites (Weires et al., 1982; Van Dries- 

 che et al., 1985). While somedegree of selectivity has been 

 found (e.g., for oxamyl; Van Driesche et al., 1985), in 

 general all materials which provide effective control of 

 leafminer adults or larvae also seriously harm parasites. 

 Low rates of low-residual materials are the least damaging 

 to parasites. Nevertheless, complete elimination of chemi- 

 cal controls of leafminers in favor of reliance on parasites 

 should be the goal of orchardists trying to establish biologi- 

 cal leafminer control systems, as currently available pesti- 

 cides for leafminer control are relatively incompatible with 

 parasite survival. 



In addition to pesticide applications directly targeted 

 against leafminers, cover sprays against other insects also 

 represent a major obstacle to parasite effectiveness within 

 commercial orchards. While parasitism levels on non- 

 sprayed trees are high (often 60% or greater), levels inside 

 blocks sprayed for other insects are typically very low (less 

 than 5%), with some increase in the third leafminer gen- 

 eration in September after cover sprays have ended (Van 

 Driesche and Taub, 1983). It thus appears that the most 

 effective action growers can take to conserve leafminer 

 parasites in commercial orchards is to end regular cover 

 sprays as early in the season as possible, lengthening out 

 the insecticide-free period in which leafminer parasites 

 can increase in numbers. Currently, cover sprays are 



