LIFE HISTORY OF HYDROUS ( HYDROPHILUS) TRIANGULARIS. 



35 



impressed with the magnitude of this agent of destruction. It must cause the death 

 of almost unbelievable numbers of these beetles every year. 



Under certain conditions, particularly in early summer, large numbers of 

 insects are sometimes found along the shores of the Great Lakes. Needham has 

 published a paper on this subject in The Canadian Entomologist for 1904, but 

 he failed to mention any of the large water beetles as being destroyed in this 

 way; but Carl L. Hubbs, of the Field Museum at Chicago, noted May 20, 1917, 

 on the beach near Winnetka, 111., amongst a similar lot of insects a hundred or more 

 Hydrous triangularis (unpublished records). These insects had been washed ashore 

 after a storm, but it was not ascertained definitely whether they were all dead, 

 although it is probable that most of them were. Evidently this is another way in 

 which numbers of these beetles may be destroyed yearly. Cinnamon teal, pintail, 

 and wood ducks were recorded by D. C. Mabbott (1920, p. 62) as eating species of 

 adult Hydrous beetles, but not in sufficient numbers to make them serious enemies. 



GENERAL SUMMARY. 



Both sexes of Hydrous triangularis, in the larval as well as the adult stage, 

 feed upon fish fry whenever conditions are favorable, and thus may become a 

 serious menace to pondfish culture. If adult fish are kept in the same pond with 

 the fry, it is not probable that the beetle larvae will more than hold their own. 

 Many adult fish, notably the sunfishes, eat Hydrous larvae and thus keep them 

 within due bounds; but if the fry are placed in a separate pond, and especially 

 if any effort is made to increase the supply of natural food, precautions against 

 the beetles may become necessary. 



There are several ways in which the beetles may be checked. If the pond is 

 thickly carpeted with such water plants as Naias or Potamogeton, or is plentifully 

 supplied with blanket algae, duckweed, or Elodea canadensis, the danger from these 

 water beetles will be greatly diminished; but this kind of vegetation is not alto- 

 gether desirable in such quantity. 



The mating season of the adults comes about the last of May, and at that 

 time both sexes fly about at night in considerable numbers. If a trap lantern be 

 constructed according to any of the standard patterns close to the shore of the 

 pond and be furnished with a light for two or three weeks at that season, it is 

 probable that many of the adult females could be caught before they began to 

 lay eggs, and in this way the larvae could be reduced in numbers. 



Again, if the pond be raised 15 or 20 inches for two or three hours once a 

 week during July and August when the larvae are pupating, the pupal chambers 

 wiU be flooded and the pupae wiU aU be drowned. This reduces the number of 

 adults, and there are not as many to produce larvae; but it is a precaution against 

 the future rather than a remedy for the present. 



When fuU-grown larvae actually appear in sufficient numbers to menace the 

 fish fry, they must be removed at once, if the remedy is to be effective. The best 

 and practically the only way to do this is to drain the pond, removing the fish fry 

 temporarily to another pond or to large aquaria. After the water is out the large 

 beetle larvae become very conspicuous and may be quickly and entirely removed. 



