THE ALFALFA WEEVIL. 7 



Besides the two modes of diffusion previously mentioned (that is, 

 by rail and by water), there is a third, which, while perhaps not 

 disseminating the insect so far in an}^ given time, is certainly a method 

 of dispersion of the greatest importance. This is the migration of 

 the adult insects, wdiich occurs from about the middle to the 20th of 

 June in the vicinity of Salt Lake City, after nearly all of the larvae 

 or worms have developed to adults. About this time great num- 

 bers of the insect seem to take wing and scatter from the fields 

 where they have originated, being aided and their course shaped by 

 every strong wind or gale that happens to occur at this time. As 

 illustrating the importance of this habit of the beetles, on June 18, 

 1910, with a strong wdnd from the south, both the Government and 

 station entomologists observed the air to be full of adults moving 

 with the wind. Just how high these insects fly w^ould of course 

 indicate whether they can be carried by air currents over the moun- 

 tain ranges or whether they must follow up and down the canyons. 

 This, as well as the question as to how" far they can fly with the aid of 

 the wind, is a matter that has not yet been definitely determined. 

 As a matter of course, the dispersion of the insect occurs almost en- 

 tirely in the adult stage. Larvae can only be carried in numbers from 

 one field to another by the flowing water in the irrigation ditches that 

 runs through or beside fields badly infested with the pest. They 

 have been found floating along with the current in this way. 



It must not be inferred from what has been said of this migration 

 that all of the beetles leave the alfalfa fields in summer. It seems 

 that these voyagers constitute only what might be termed the over- 

 flow. Vast numbers remain in the alfalfa fields, and while they do 

 not eat the crown of the plant, as do the larvae or young, they gnaw 

 off and devour the outer surface of the stem, thus skeletonizing and 

 killing the plants, giving a field thus attacked much the appearance 

 of having been scorched by fire. If at this time a field is mown off, 

 or when the plants have all been destroj'ed. leaving no more food for 

 them, the remainder move out of the field, evidently seeking new 

 food supply. 



FUTURE INVESTIGATIONS. 



As will be observed from a study of the map (fig. 10), the pest 

 has become established in the midst of the alfalfa region of the 

 West, where this is by far the most important crop grown. From 

 what is known of the insect in Europe, and in the light of its be- 

 havior since its discover}^ in this country, there seems little hope for 

 anything from it but dispersion and destruction. It has no natural 

 enemies except frogs and toads, both of which are by far too few in 

 numbers to restrict its ravages to the slightest degree. 



