Spiders in Second-level and First-level 

 Apple IPM Blocks 



Joanna Wlsniewska, Yanghe Yang, and Ronald Prokopy 

 Department of Entomology^ University of Massachusetts 



One of the principal practices of full second- 

 level IPM is to control key summer fruit pests by 

 a combination of behavioral and ecological tech- 

 niques, thus allowing beneficial predators and 

 parasitoids to increase enough to control sum- 

 mer foUar pests. Spiders may be an important 

 group of such predators. Insecticides can reduce 

 numbers and diversity of spiders in apple or- 

 chards. Therefore, full second-level IPM, which 

 eliminates insecticide use Eifter early June, may 

 allow spiders to prohferate in apple orchards. 



In 1992, we assessed spider populations in 

 blocks of apple trees under full second- level IPM 

 compared with first-level IPM practices. Addi- 

 tionally, we conducted a laboratory test of the 

 effects of Guthion™, Thiodan™, and Omite™ on 

 the most common spider species found in second- 

 level IPM blocks. 



Besides these evaluations, we also were in- 

 terested in the effects of herbicides on spider 

 populations in these orchards. Frequently, veg- 

 etation growing under apple trees is controlled 

 in commercial orchards with herbicides applied 

 early in the growing season, v/hile vegetation 

 between the tree rows is mowed throughout the 

 season. These practices reduce competition for 

 nutrients, lower humidity (which may contrib- 

 ute to higher disease pressure), and eliminate 

 alternative sources of food and shelter for many 

 orchard pests. Herbicides can decrease spider 

 numbers in vegetable production systems 

 (Riechert and Bishop, 1990), but their effects 

 have not been examined in an orchard system. 

 Hence, in 1992, we examined the effects of 

 herbicide treatments on the number of spiders 

 on apple trees in second-level IPM blocks. 



Spider Numbers in Full Second-level 

 and First-level IPM Blocks 



Spiders were sampled in six apple orchards. 



Each orchard contained a six- to nine-acre block 

 imder full second-level IPM and a nearby six- to 

 nine-acre block under grower-supervised first- 

 level IPM. Guthion and Thiodan were used in 

 both types of blocks through early or mid-June 

 to control early-season insect pests. After mid- 

 June, second-level blocks received no insecti- 

 cides while first-level blocks received an average 

 of 2 sprays of Guthion or Imidan (once in July 

 and once in August). All blocks were sprayed 

 with carbaryl in early June to thin finiit. 



Beginning in early July, spiders were 

 sampled on twenty randomly chosen trees every 

 two to three weeks in both tjT)es of blocks in each 

 orchard by tapping tree branches with a rubber 

 mallet over a two-by-two-foot tray. Spiders were 

 preserved in 70% alcohol and returned to the 

 laboratory for identification, a process not yet 

 complete. 



Figure 1 summarizes the results obtained 

 over the entire season. The mean number of 

 spiders collected per tree at the beginning of the 

 sampUng period (July 2) was very low and W£is 

 the same in both second-level and first-level 

 IPM blocks. As the season progressed, however, 

 these numbers increased to 1.5 spiders per tree 

 by late September in the second-level blocks 

 compared with 1.0 spiders per tree in the first- 

 level blocks. Data trends were similar for each 

 orchard considered, except for one orchard 

 where there seemed to be no difference between 

 the two types of blocks. 



The low numbers of spiders per tree in early 

 July in all blocks may have been due to spraying 

 for plum curculio, which extended through early 

 to mid-June in all blocks. Beyond mid-June, 

 growers continued to use insecticides in the first- 

 level blocks but not in the second-level blocks. 

 This difference probably accounted for the 

 greater abundance of spiders in the second-level 



20 



Fruit Notes, Winter, 1993 



