356 MAINE AGRICULTURAL EXPERIMENT STATION. I916. 



addition migrates for a part of the year to a secondary host. 

 But it would be a rash person who felt safe in the conviction 

 that such a cycle could not be. 



Many migratory aphids, to be sure, alternate their primary 

 and secondary host plants at regular intervals, each time entirely 

 deserting the one for the other, thus existing for a part of the 

 year only upon each. Rhopalosiphum nympheae Linn is an 

 example of such a cycle with its winter and spring habitation 

 on the plum and its summer residence upon various water 

 plants. 



For many reasons it becomes evident that a failure with a 

 migration test gives no data. 



If an investigator fails in one hundred attempts to colonize 

 thistle with migrants from plum that will not be a safe reason 

 for him to conclude that he is not working with Aphis cardui, 

 or that this thistle aphid has nothing to do with the leaf 

 deformations of the plum in the spring. It has been my own 

 experience that negative data with aphids under such conditions 

 are just no data at all. If the structural characters are such 

 as warrant the migration test in the first place, they warrant 

 a patient continuation even in the face of repeated failures. 



On the other hand (and this is the most encouraging and 

 stimulating circumstance in connection with aphid migration 

 tests), a single success goes a long way to prove the case. Bar- 

 ring complications, a single success is enough, and repetitions 

 and verifications are needed only as safeguards in that respect. 

 For these insects are remarkably stable as to their exclusive 

 tastes in vegetable juices and a given species will die before it 

 will submit to the sap of any plant not. on its approved dietary. 

 So if the progeny of the migrants accept the food plants given 

 them in the laboratory to the extent of developing upon it from 

 the first instar to maturity, it is safe to conclude that that food 

 plant is one which they would accept in the field under favor- 

 able conditions, even though, with the wider choice of the open, 

 a different one might be given preference in certain localities. 

 Such proof should rest with the behavior of the progeny of the 

 migrants and not with the migrants . themselves, for. the 

 migrants, as has been suggested, have many ways of tantaliz- 

 ing the hopeful investigator. 



