some weeks that the diagnostic "hopper burn," or yellowing of leaf margins, 

 began to appear. In a few severe cases, particularly where trees were also 

 stressed by drought, leaves later became curled and somewhat browned about the 

 margins. 



Since PLH has only been a sporadic pest on apple, the effects of this 

 injury on affected trees are not well known. Because PLH inject a toxin into 

 the plant, in many crops (such as potatoes) the threshold for these insects is 

 very low. It is likely, however, that fruit trees, because of their size and 

 the enormous amount of vegetative growth of which they are capable, can 

 withstand considerably higher numbers of PLH. Until we know better, we have 

 adopted a provisional threshold for PLH that is the same as that for white 

 apple leafhopper, 25 leafhoppers or signs of feeding per 100 terminals 

 observed. 



PLH are extremely mobile, rendering monitoring and management decision- 

 making difficult. For one thing, PLH will take flight at a fairly slight 

 disturbance, so that moving terminals for examination may cause the leafhoppers 

 to fly off. (According to researchers working with PLH, ovipositing females 

 are much less easily disturbed than males. This fact may make it possible at 

 least to determine whether the insects are still present, but difficult to 

 assess actual population levels.) 



It is also difficult to assess the susceptibility of potato leafhopper to 

 insecticides. In 1986 in New York, where PLH infestation was more severe than 

 it was in Massachusetts, PLH were observed on terminals in blocks which had 

 been treated with organophosphate insecticides. This observation led tree 

 fruit entomologists there to speculate that resistance had developed, but 

 vegetable specialists raised the possibility that reinfestation had occurred in 

 those blocks, and that no resistance was present. Vegetable specialists in 

 both New York and Massachusetts note that PLH are highly dispersed over a wide 

 geographic range and a variety of plant hosts, many of which are never exposed 

 to pesticides. These factors would tend to inhibit the development of 

 resistance in most cases. 



Pesticide control results in Massachusetts in 1987 have been inconclusive, 

 but we can state with confidence that we have not seen any sign of continued 

 infestation of trees treated with organophosphate insecticides (OP's). In 

 fact, in the one monitored orchard where an OP was used against PLH, 

 infestation declined from ^7% of terminals the week before treatment to 0% the 

 week after. The reason that it is not possible to state definitely that this 

 decline was due to the OP treatment is that similar declines also occurred in 

 untreated orchards, over the same period of time, possibly because terminal 

 growth was beginning to harden off and PLH were moving onto other hosts. 



Based on the recommendations of vegetable specialists, and the fact that 

 no obvious OP resistance has been noted on any crop in Massachusetts in 1987, 

 we are proceeding with the assumption that PLH are indeed susceptible to OP's, 

 including Imidan and Guthion. 



Since it appears that potato leafhopper migration is determined primarily 

 by prevailing weather conditions in the southern U.S., it is impossible to 

 predict whether this insect will continue to be a problem on apples in 

 Massachusetts. However, since such outbreaks have happened in the past, we 



