no. lo tropisms of lepidoptera — mcindoo 5i 



Summary and Discussion 



In order to throw light on the biology of the codling moth, a 

 thorough investigation of the tropisms of this insect was begun in the 

 spring of 1927. Definite results were obtained only by using the larvae. 

 In all 154 larvae, belonging to the two broods at Silver Spring, Md., 

 were tested individually in the laboratory under various conditions. 

 In bright light, although not direct sunshine, larvae of the first instar 

 were weakly photopositive. Certain tests indicated that objects are 

 perceived and located by the senses of smell and sight, and by mere 

 ' chance. Chance alone seemed to be only 30 per cent efficient ; sight 

 and chance combined, 40 per cent efficient ; whereas smell, sight, and 

 chance combined were 65 per cent effective. Therefore, since larvae of 

 the first instar have photopositive eyes, they remain in the open on 

 apple-tree foliage and search freely for food, apparently not being 

 aided by their senses until within a few millimeters of the food. The 

 larvae were found to be easily repelled by odorous substances, but 

 attracted with difficulty. 



Larvae of the second, third, and fourth instars were weakly photo- 

 positive to weak light, but indifferent to strong light. Larvae of the 

 fifth instar sometimes acted indifferently to light but generally were 

 weakly photonegative. Larvae of the sixth instar were either weakly 

 or strongly photonegative, the degree depending on their age ; and 

 those with blackened ocelli did not respond to light. At cocooning time 

 the larvae were strongly photonegative, geopositive, and thigmoposi- 

 tive, whereas during their earlier instars they either behaved indiffer- 

 ently to light, gravity, and touch, or were photopositive, geonegative, 

 and thigmonegative. Consequently, when the larvae are ready to spin 

 cocoons they avoid bright light as much as possible, usually move 

 toward the ground and hunt for dark and tight places in which to 

 pupate. When bands are placed around the trunks of apple trees to 

 serve as a supplementary control method, we are merely taking ad- 

 vantage of nature's laws. It therefore seems that so far as tropic 

 responses are concerned the vulnerable period in the life history of 

 codling-moth larvae is brought about by a change in tropisms. 



It is well known that certain varieties of apples are more susceptible 

 to codling moth injury than are other varieties ; why, no one knows, 

 but several factors, including thickness, toughness, and waxiness of 

 apple peel, and the odorousness and acidity of apples, might be con- 

 sidered. Owing to one or more of these factors the larvae probably 

 gain entrance to the more susceptible varieties with less difficulty ; or 

 the female moths perhaps distinguish differences between apple trees. 



