BUTTERFLIES OF THE DISTRICT OF COLUMBIA 51 



meadows west of Cabin John on October 15 disclosed only about a 

 dozen G olios eurytheine^ about the same number of Phyciodes tharos, 

 and a single Pieris protodice. 



EXTIRPATION OF ONE BUTTERFLY BY ANOTHER 



During the past four years the orange clover butterfly {C olios 

 euin/theme) has enormously increased in numbers in the District 

 area, and within the past two years the numbers of the yellow 

 species {G. jfMlodice) have not only relatively but actually decreased. 



The orange species is a more active butterfly than the yellow and 

 is stronger and swifter on the wing. When the two species exist to- 

 gether the orange males are frequently observed to annoy both sexes 

 of the weaker yellow species. 



If we may judge by the frequent appearance of the males in num- 

 bers as puddle butterflies, the yellow species is sensitive to overcrowd- 

 ing. The same is true of the small pale form (ariadne) of the orange 

 species, which, although rare in this region, appears with the yellow 

 species as a puddle butterfly. The larger and darker forms of the 

 orange species {keewaydin and eurytheme) seem to be unaffected by 

 overcrowding and never appear as puddle butterflies. 



Wherever the two species occur together the yellow species would 

 be affected by the males of the active and powerful orange species 

 more than it would by its own males, whereas the orange species 

 would not in any way be affected by the yellow. Thus overcrowd- 

 ing in the case of the yellow species would occur in proportion to 

 the numbers of the males of the two species combined and not in 

 proportion to the numbers of the males of the yellow form alone. 

 With the increase in the numbers of the orange species the yellow 

 would gradually decrease in numbers through the driving out from 

 the breeding areas of the yellow males, and through the decrease 

 in the number of successful matings as a result of the constant 

 annoyance of both sexes. 



This seems to be what is actually taking place. The strong and 

 active orange species {G olios eurytheme) is causing a progressive 

 diminution in the numbers of the weaker and less active yellow 

 species {C. pMlodice). It will be interesting to see whether the 

 yellow species will be able to maintain itself in the face of the con- 

 tinued annoyance by the orange, or whether it will be reduced to 

 insignificant numbers or even perhaps die out. 



The yellow species has never been a pest, possibly because of its 

 sensitiveness to overcrowding. The orange species, on the other 

 hand, which seems not to be affected by overcrowding, from time 

 to time becomes a more or less serious pest in certain regions in 

 the West, and in some sections its caterpillars — known locally as 



