INSECTS ATTACKING WEEDS IN MINNESOTA 



S. MARCONITCH 



The growing- of crops has been aptly called a tight against 

 weeds. Weeds are so closely associated with the farm economy 

 that a knowledge of the weed-infesting insects is highly desirable. 

 When seen from its various phases, the subject is extremely com- 

 plex, so that it is sometimes difficult to tell wh.ere tlie usefulness 

 of an insect Ijegins and where it ends. 



Certain insects are so closely adapted to particular weeds that 

 they will always remain with them, and are undoubtedly useful. 

 For example, the midge, Dasyneiira gibsoni and the fly, Trypcta 

 nificanda, so completely destroy the seeds of Canada thistle that 

 it is sometimes difficult to obtain good seed. Such insects are just 

 as beneficial as our seed-eating birds ; and considering the great 

 number of insects, their value cannot be overestimated. As is well 

 known, weeds are Avidely distributed through the agency of man. 

 Their seeds are transported with the crop seeds, thus becoming 

 established in new localities. Such weeds may become even more 

 troublesome in their new situations, because their insect enemies 

 were left behind, ^[oreo^■er. if we had a complete knowledge of 

 the weed insects, it might be possible to establish certain insects 

 which are known to attack weeds, in parts of the world where those 

 weeds or their relatives occur, but the insects do not. 



As this paper is about to go to press, an interesting note appears 

 in Science N. S. XoX. XLV:380, indicating the practicability of 

 importing insects for such purposes. Mr. Jarvis, Assistant Ento- 

 mologist of Queensland, made a trip to Hawaii for the purpose of 

 gathering the lantana Agromyzid which very successfully prevents 

 the seeding of this most troublesome weed in Hawaii. This agro- 

 myzid was introduced into Hawaii by Br. Koebele, who it will be 

 remembered was responsible for the successful introduction of the 

 Australian ladybird (Nov'uis cardinalis), the imported enemy of the 

 fluted scale. As many of our worst insect pests have been intro- 

 duced accidentally, it is also probable that some beneficial importa- 

 tions were likewise introduced. According to Hyslop, Jour. Econ. 

 Ent. Vol. X:278, Ceutorhyuchus marginafus Schonh appears thus 

 to have been introduced accidentally. Its host plant is the noxious 



*This study was undertaken at the suggestion of Professors William Moore and W. L. 

 Oswald. I am greatly indebted to Professor Oswald for the determination of the plants and co- 

 operation during the progress of the work. I also wish to thank Messrs. Busck, Aldrich and 

 Pierce of the P>ureau of Entomology for the identification of many of the insects. 



