sprayed foliage and fruit along with two spheres 

 were brought to the laboratory for determination of 

 surface dislodgeable residues of dimethoate (i.e. 

 residues that would be available to individuals that 

 came into contact with foliage, fruit or sphere). Resi- 

 dues were removed from samples of apples and from 

 spheres by thoroughly wiping twice the fruit or 

 sphere surface with a piece of cheese cloth moist- 

 ened with water. This is standard toxicological 

 methodology for removing surface residues. A 

 slightly different (but also standard) approach was 

 used for removing surface residues from samples of 

 sprayed foliage (owing to potential tearing of leaf 

 surfaces by wiping with cheesecloth). Precise 

 amounts of dimethoate residues in each sample were 

 determined by standard pesticide anal3d;ical meth- 

 odology. 



Results 



All four types of sphere caused mortality to 100% 

 of alighting apple maggot flies when tested on the 

 first day of deplo)Tnent, before any rain fell. After 

 five weeks and 3.45 inches of cumulative rainfall, 

 mortality fell to 0% for Type A spheres (1994 ver- 



sion single-layer spheres) but remained at a sub- 

 stantial level for the other types of spheres: 43% for 

 Type B (two layers), 50% for Type C (three layers, 

 linseed oil as third layer), and 70% for Type D (three 

 layers, shellac as third layer) (Figure 1). After ten 

 weeks and 6.4 inches of cumulative rainfall, mor- 

 tality fell to 0% for Type B spheres, 10% for Type C 

 spheres, and 30% for Type D spheres. When a 20% 

 sucrose, 80% water mixture was applied to each 

 sphere at ten weeks, mortality was restored to a 

 level of 70-75% for each sphere type. This result 

 suggested that the principal reason for decline in 

 effectiveness of each sphere type over time was loss 

 of feeding stimulant and, to a much lesser extent 

 loss, of toxicant. 



This suggestion was confirmed in assays of 

 spheres exposed to artificial rainfall. As shown in 

 Figure 2, Type A spheres lost 100% of sucrose after 

 one inch of rainfall, whereas 100% loss of sucrose 

 did not occur in Type B and Type C spheres until 

 four and six inches of rainfall, respectively. Even 

 after seven inches of rainfall, Type D spheres re- 

 tained 30% of original sucrose. Unfortunately, the 

 third layer of mixture (shellac) that conferred this 

 greater retention of sugar in Type D spheres some- 



0) 



c 



O) 



I*- 

 o 



o 



E 



o 

 re 



V 



> 

 < 



0(0) 1(0 2) 3(0.75) 5(3 45) 7(6.2) 10(6 4) 10 retreatment 



Weeks of sphere exposure in orchard (cumulative inches of rainfall) 



Figure 1. Residual activity of four types of dimethoate-treated spheres for controlling apple 

 maggot flies. 



Fruit Notes, Spring, 1996 



