FRUIT THINNING WITH CHEMICAL SPRAYS 39 



open. Less thinning was obtained if spray applications were made 

 earlier or later than this stage. Similar results have been obtained 

 with Beauty and Santa Iiosa plums. 



As compared with apples and pears, stone fruits have much less 

 foliage development at the blossoming period and for this reason less 

 indirect action of dinitro would be expected. It is quite possible, 

 therefore, that with stone fruits most of the thinning is a result of the 

 dinitro material functioning directly as a pollinicide. Thus, the 

 timing of the thinning spray with reference to pollination and fertili- 

 zation seems to be more critical than with apples in determining the 

 amount of thinning obtained. 



EFFECT OF THINNING SPRAYS ON POLLINATING INSECTS 



Cross-pollination is essential in the production of many apple 

 varieties and certain other deciduous fruits. Without adequate cross- 

 pollination good yields of those fruits would be impossible. The 

 honey bee is credited, by some authorities, with about 90 percent of 

 the pollen transfer in orchards. Although this may vary somewhat 

 from one area to another, it does emphasize the importance of the 

 honey bee in fruit growing. Furthermore, it is the only valuable 

 cross-pollinizing agent that can be managed by the orchardist. For 

 these reasons it is necessary to avoid the use of any sprays or dusts 

 that would prove harmful to hone} 7 bees while they are being used in 

 the orchard during the bloom period. States where fruit growing is 

 of commercial importance have laws to this effect for the protection 

 of these valuable insects. 



When dinitro bloom sprays were first suggested as a possibility for 

 chemical thinning the question of the toxicity of these chemicals to 

 honey bees was immediately raised. An investigation conducted on 

 this question by Goble and Patton (15) showed conclusively that 

 dinitro compounds were toxic to honey bees. Working under con- 

 trolled conditions the median lethal dose or concentration at which 

 50 percent mortality occurred was established by feeding bees dosages 

 of 0.004 ml. of a dinitro-sugar mixture of known concentration. It 

 was calculated that a bee need only consume 0.0028 ml. to get a lethal 

 dose of dinitro spray containing 1 ounce of the active ingredient per 

 100 gallons of water. 



This does not mean that dinitro bloom sprays for thinning cause 

 widespread destruction of bees. In fact, work and observations under 

 orchard conditions indicate quite the contrary. Dyce (9) observed 

 that when dinitro sprays were applied to trees, bees immediately 

 stopped visiting the blossoms but gradually returned as the spray 

 dried. Then the bees visited sprayed flowers of some varieties ■ in 

 greater numbers than unsprayed blossoms. It was suggested that this 

 phenomenon may have been due to the presence of more nectar in the 

 nectaries of blossoms burned by dinitro sprays than the nectaries of 

 normal flowers, thus resulting in sprayed bloom becoming more attrac- 

 tive to bees. 



