2 BULLETIN 902, U. S. DEPARTMENT OF AGKICULTUKE. 



the majority of flea-beetles, tliis species does most harm to young 



plants, and, as an instance of its destructiveness, it has been reported 



to come in swarms, like black clouds, completely covering the plants. 



This species is not a periodical pest, like army worms and others, 



but it is more or less injurious year after year in the regions which 



it inhabits. It is, however, like most other flea-beetles, subject to 



considerable fluctuation in numbers for reasons which have not yet 



been entirely explained, but which are doubtless due to atmospheric 



conditions either at the time that the insect is breeding or when it 



is in hibernation. 



DESCRIPTION. 



BEETLE. 



The adult of the western cabbage flea-beetle (fig. 1) is shining 

 metallic copper in color, measuring one-sixteenth of an inch or a little 

 more in length. The body is elongate oval, 

 much flattened. The antenna? are slender and 

 the same in both sexes. The thighs of the 

 hind legs are strongly developed, fitting the 

 insect for jumping, whence its common name 

 of "flea" or flea-beetle. 



Following is the original description of 

 PTiyllotreta pusilla (3, 1 p. 302): 



Form narrow, elongate, depressed, piceous, surface with 

 distinct eeneous lustre. Antennae slender, half as long as 

 the body, piceous, joints 2-3 paler. Head scarcely visibly 

 punctate. Thorax less than twice as wide as long, widest 

 Fig. 1.— The western cabbage at jnfddle, sides arcuate, apex slightly narrower than base, 

 flea-beatle (Phyllotreta pus- n . ', . r . 7, , 



ilia) : Adult, highly magnified. dlsc convex > surface shining, the punctures moderate, 

 closely placed, but not convex. Elytra wider than the 

 thorax, humeri obtuse, punctation coarser than that of the thorax, closely placed, 

 very little finer near the apex, but less dense, surface shining. Body beneath and 

 legs piceous, abdomen sparsely punctate. Length .06 — .OS inch; 1.5-2 mm. 

 Male. — Last ventral [segment] with a feeble triangular impression in the apex. 

 Female. — Last ventral simple. 



The antennae are alike in both sexes and the joints 3 to 10 vary 

 little in length, although slightly broader externally. 



This species is very easily confounded with related forms of similar 

 habits. Prominent among these is PTiyllotreta albionica Lee, which 

 it resembles so nearly hi form, size, and color that the females can 

 scarcely be separated. It is, however, more sliming, the head is 

 nearly smooth, and the thorax and elytra are less densely punctate. 

 Moreover, Ph. albionica may easily be separated by the male antennae 

 which have the fifth joint dilated. The female antennas of the two 

 species are almost identical. The color of pusilla is sometimes olive 

 brown and inclined to black, but examination of a large series of 

 properly preserved specimens does not show any material variation, 



1 Reference is made by number (italic) to " Literature cited," p. 21. 



