548 



were ascertained. In the Naumburg region Phyllotreta atra is the most 

 common and destructive species. In 1920 its infestation reached the 

 maximum in August, but this mass occurrence was of short duration ; 

 from 2,000 individuals per catch-unit on 4th August the number fell 

 to 8 on 21st August in rainy weather, though a catch of 130 was taken 

 towards the end of September. A summer crop, planted in drills on 

 10th August, soon recovered from attack, and the winter crops, 

 similarly planted later, were not affected. P. nigripcs was practically 

 contemporaneous with P. atra, but of much less importance. It is 

 clear that in dealing with these two species the winter crops must be 

 sown from August onwards. 



The conditions are different if P. iindidata is concerned, the 

 maximum catch-unit of 83 individuals occurring on 6th October. 

 Whether this delay occurs each year is not yet certain. P. iindulata 

 was not ver}'' abundant in Naumburg in 1919 and 1920, and P. nemonim 

 was not represented at all in the 1920 catches. The latter appears 

 to reach its maximum infestation somewhat earher than P. atra and 

 P. nigripes, whereas P. itndnlata seems to reach its maximum after them. 

 If P. undidata continues to be as rare as in 1919, it may be disregarded, 

 and sowing may be regulated by P. atra. In conclusion it may be 

 said that winter crops may be protected against flea-beetles as follows : 

 In warm districts, and in the reduced abundance of P. undulata, sowing 

 must be delayed until the mass occurrence is over ; in cool districts, 

 and with a predominance of P. imdidata, sowing must be advanced 

 in order to strengthen the plants prior to infestation. The trap-crop 

 method is useless, as the beetles oviposit only after hibernation. 



It was not possible to obtain data in the spring of 1920 regarding 

 the mass occurrence of flea-beetles in relation to the sowing time of 

 summer crops. 



As regards Meligethes aeneits, this beetle appears so early that there 

 is no hope of growing a winter rape crop capable of avoiding attack 

 on its buds by blossoming and fructifying before the appearance of 

 the pest. In growing a winter crop other desiderata obtain, foremost 

 being the capacity of the plant to ramify abundantly, as this permits 

 it to outgrow the injury better than is the case with a weak plant. 

 Trap-crops are useless for the same reason as in the case of flea-beetles. 

 One point that seems important is the contemporaneous blossoming 

 of the plants in various fields, as late-blossoming fields seem to attract 

 all beetles busy with reproduction in the neighbourhood. From 

 the catch figures of 1920 it is perhaps possible to find a way of avoiding 

 injury by M. aeneus, the mass occurrence of which is in April, May and 

 June, by replacing winter crops with summer ones. Where it is 

 inadvisable that cultivation be restricted to summer mustard, the 

 growing of rape crops might be confined to late summer in districts 

 where they are annually badly injured by M. aeneus. It is a question 

 of finding whether the varieties suited to sowing in late summer can 

 be selected so as to give a larger yield without prolonging their present 

 short time of development. The growing of such summer crops, 

 however, involves giving up winter rape crops, for experimental 

 sowings of summer rape, made with the object of providing food and 

 breeding facilities for the rape pests under investigation, led to an 

 enormous increase of a very injurious midge, Perrisia {Dasyneura) 

 brassicae. 'J'his midge has several generations in the warm season, 

 and its numbers rise rapidly when Ceuthorrhyncliiis assimilis and 

 allied weevils facilitate its oviposition by boring into the shoots. 



