549 



Even a few of these beetles suffice to assist the first summer generation 

 of P brassicae. This point must be reckoned with in plantmg summer 

 rape' if winter rape is grown. Another enemy of summer crops, and 

 one difficult to combat, is the cabbage aphis [Brevicoryne brassicae]. 



The relations between Meligethes and Ceuthorrhynchus and their 

 common parasite, the Hvmenopteron, Diospilus oleraceus, are of great 

 importance in the association of these pests with rape. Breeding 

 experiments seem to show that the great abundance of D. oleraceus in 

 Tulv and August (as compared with early spring) is due to its increase 

 at the cost of M. aeneus. The spring generation of D. oleraceus develops 

 in the autumn larvae of Ceufhorrhvnchus rilbsaameni and C. siilcicoiUs. 

 Infested autumn larvae carry their parasites underground and die 

 after building their earthern cocoons, and the parasites do not pupate 

 until spring " This explains the tardy appearance of D. oleraceus on 

 blossoming winter rape, as compared with the Ophionids, Isitrgus 

 nwrionellus, Holmgr., and 7. heterocerus, Thoms. The scantiness of 

 D oleraceus in spring is explained by the smaller numbers of 

 Ceuthorrhynchus as compared with Meligethes. The very important 

 check effected by Diospilus on Meligethes is thus closely connected 

 with the occurrence of Ceuthorrhynchus. Where the latter or allied 

 species are absent, Diospilus has no means of overwintering and 

 must be disregarded in the control of M. aeneus. 



Fortunately C. rilbsaameni is of little importance as a pest of rape 

 crops ; the slight damage done by it is amply compensated by its 

 usefulness, and no measures are required against it. 



M aeneus C rilbsaameni and C. sulcicollis, and D. oleraceus, therefore 

 form in conjunction with winter rape, a closely interrelated biological 

 unit and their numerical relation to one another will probably be 

 disturbed if winter crops are abandoned. It is remarkable that 

 M aeneus is able to reproduce itself in some numbers m early spring 

 before the appearance of Diospilus, and that part of the autumn 

 brood of C. riibsaameni and its eggs laid in the following spring remain 

 uninfested by Diospilus ; the latter would otherwise eradicate its 



hosts in a few years. ^y. a t \^c 



This adaptation of the parasite to the reproductive methods of its 

 hosts is much simpler in the case of Isurgus spp. They appear in 

 sprinij shortly after the oviposition of M. aeneus and occur through the 

 summer but although the larvae of Meligethes are thus constantly 

 exposed to attack, the parasite is not present m excess, so that it is 

 probable that the progeny of Meligethes is more numerous than that 



° Another pest meriting attention is Psylliodes chrysocephala. This 

 flea-beetle begins reproduction in autumn and continues it in spring 

 after an interruption during che cold weather. It has not been very 

 abundant around Naumburg during recent years, and m general may 

 be held to be one of the less common species. It is possible tnat 

 unfavourable hibernation conditions are responsible for this and it 

 would be useful to ascertain its abundance and mjury in districts 

 with lasting winter snow as compared with those m which snow is 

 occasional or slight. Any excess of this pest need then be combated 

 only in localities where the winter conditions favour it. Sometimes 

 there is a marked difference in the infestation of winter crops sown 

 at widely different dates. If one assumes that an observed freedom 

 from infestation of rape sown in September is due to the plants not 

 having reached by autumn a size suitable for oviposition, then it is 



