Effects of Tree Size and Planting Density 

 on Control of Apple Maggot Flies with 

 Odor-baited Red Spheres 



Juan Riill, Ronald Prokopy, Starker Wright, and Jonathan Black 

 Department of Entomology, University of Massachusetts 



There is an increasing tendency among New En- 

 gland apple growers to replace trees on semidwarf 

 rootstocks with dwarf trees. Although this has clear 

 advantages from the orchard-management perspective, 

 little is known about the impact of this horticultural 

 practice on pest control. Behavioral control of apple 

 maggot fly (AMF), a key pest of apples in Massachu- 

 setts, relies on interception of females immigrating into 

 orchards using sticky red spheres. Female AMF are 

 intercepted by traps placed on perimeter trees before 

 they can penetrate and cause damage to fruit within 

 the orchard. 



It is a widely known fact that some insects modify 

 their behavior on plants of different sizes. It is con- 

 ceivable then, that changes in AMF behavior on apple 

 trees of different sizes could affect their response to 

 interception traps and result in more or less fruit dam- 

 age. 



As part of a study encompassing the effect of tree 

 size on all IPM practices in apple orchards, we studied 

 the effect of tree size and planting density on control 

 of AMF using odor-baited red spheres. 



Materials & Methods 



We conducted experiments during the growing 

 seasons of 1997, 1998, and 1999 in eight commercial 

 orchards in Massachusetts. In each of the orchards, we 

 selected six square blocks of apple trees, two each of 

 small, medium, and large trees (M.9, M26, and M.7 

 rootstock, respectively). All blocks consisted of seven 

 rows of Mcintosh and/or Cortland trees perj^endicular 

 to the hedgerow or woods at the orchard margin (Fig- 

 ure 1). All blocks in every orchard were sprayed until 

 early June to control insects and diseases. Thereafter, 

 one block of each tree size in each orchard received 

 odor-baited traps hung on perimeter trees every 6 yards 



to intercept immigrating flies (IPM blocks). The other 

 three blocks received insecticide to control AMF (con- 

 trol blocks). To compare populations of tlies inside EPM 

 and control blocks, we placed four unbaited spheres 

 near the center of each block and counted the number 

 of flies captured by those spheres every 2 weeks. Fruit 

 injury was compared by sampling 20 fruit on 10 trees 

 at the interior of each block every 2 weeks. 



Additionally, we released flies marked with dif- 

 ferent colors at the interior and exterior of IPM blocks 

 of different tree sizes. Marked flies released inside 

 blocks allowed us to determine the fate of flies that are 

 able to penetrate IPM blocks, whereas flies released 

 outside blocks permitted us to assess to what extent 

 immigrating flies are intercepted by perimeter traps 

 before entering EPM blocks of different tree sizes. 



Results 



To compare results in IPM and control blocks, 

 we calculated the ratio of wild AMF captures by inte- 

 rior monitoring spheres in IPM vs. control blocks. 

 Ratios were greater than one for all block types in 1 997 

 and 1998, indicating slightly greater captures of wild 

 AMF by monitoring traps in IPM blocks (Figure 2). 

 Ratios were highest for large trees, although this pat- 

 tern did not hold during 1999. Injury to fruit was less 

 in IPM blocks than in control blocks of small trees 

 whereas the reverse was true tor blocks of large trees 

 (Figure 3). 



Marked AMF released inside blocks were recov- 

 ered in larger percentages by perimeter traps in IPM 

 blocks of small and medium sized trees than by those 

 in blocks of large trees in 1997 (Figure 4). In 1998, 

 there was no detectable pattern in recovery of released 

 AMF. For marked AMF released outside of IPM 

 blocks, more AMF were intercepted by perimeter traps 



18 



Fruit Notes, Volume 6A (Number 4), Fall, 1999 



