The Diamond-hack Moth. By George Bolam. 381 



effect that Mr Paynter, of Alnwick, had, on a visit to the Fame Islands on 

 10th July, found the insects swarming upon the rocks, and, upon question- 

 ing the lighthouse keepers regarding them, was informed that " such a 

 great cloud of moths was driven over by the north-east wind, that they 

 were obliged to keep sweeping them off the lantern throughout the whole 

 night in order to allow the light to be seen at sea." The inference 

 deduced was, of course, that the moths had come "from abroad," and the 

 statement of the lighthouse keepers, taken in conjunction with the fact 

 that the outbreak of the caterpillars occurred soon after this date, was 

 held up as " conclusive proof " of the correctness of the theory. Other 

 persons also recorded their experience in meeting with great numbers of 

 the moths near the coast : and Mr John E. Robson has remarked upon their 

 sudden appearance in the neighbourhood of Hartlepool, on 24.th June, in 

 such profusion that he felt convinced that they could not have been bred 

 in the locality without having attracted previous observation. 



Plutella cruciferarum, however, is not the only insect which has sporadic 

 years of increase. There are periodically recurring " Clouded Yellow 

 years,' ' Camberwell Beauty years,' ' Gamma Moth years,' and the like, 

 while every now and then we have the familiar Tnrnip Beetle (Haltica 

 nemorum and its allies)* numerous enough to do injury to our early crops ; 

 but it seems scarce!}' necessary to fly to the hypothesis of a Continental 

 invasion to account for their excessive abundance. The season may be 

 particularly favourable to the increase of some particular insect, or it may 

 have been exceptionally destructive to some enemy which usually holds it 

 in check. A very slight mortality amongst the ichneumons, which are 

 parasitic upon it, would present to a species like P. cruciferarum, which has 

 several generations in the course of a season, an opportunity to increase, 

 which would readily account for its abundance last year. Climatic in- 

 fluences have also an important bearing upon the domestic economy of the 

 insect world. Damp, in any form, is known to be detrimental to many 

 species, and it is not unlikely that the long continuance of dry weather in 

 the spring of 1891, may have had a beneficial effect upon the development 

 of the early broods of the Diamond-back Moth. Man also, to some extent, 

 supplies a means of increase, to many creatures, by providing them 

 artificially with a supply of food ; and it has been remarked as a curious 

 incident in the life-history of Plutella cruciferarum, that the later cater- 

 pillars, which feed upon our cultivated plants, have a more prolonged life, 

 ill that state, than those which, earlier in the season, find a means of 

 sustenance upon wild mustard and other weeds.f 



I am not sufficiently conversant with the order of moths, to which it 

 belongs, to be able to say whether it is possible that P. cruciferarum may 

 sometimes remain, for more than one year, in the pupal state, but some of 

 the larger Lepidoptera are well-known to do so. An instance of this, which 

 came within my own observation, in 1891, may not be entirely mal-a-propos 



* Phyllotreta undulata and P. nemorum. 



t See the Reports on Injurious Insects by Charles Whitehead, F.L.S., 

 etc., issued by the Agrionltural Department, No. C. 4986, page 57. 



