537 



higher the temperature the sooner the insect succumbs. Experiments 

 showed that cutworms died within about ten days if kept ^^^thout 

 food during the summer months. The existence of the larvae is 

 considerably prolonged if dry maize stalks are left on the land. It 

 is obvious that the cutworms hatching from eggs laid about May 

 cannot be destructive to the next season's crop ; they must either 

 pupate during the dry season or die of starvation. Pupation takes 

 place an inch or two below ground level, emergence of the moth 

 occurring in two or three weeks in the summer. The generations 

 of cutworms are quite irregular throughout the year. Adults have 

 been reared during every month, but they are most numerous in 

 January and early February. In summer the life-cycle may occupy 

 a minimum of 50 days ; in the field in Rhodesia there are probably 

 four complete generations of E. segetuni during the year, as compared 

 with one only in the British Isles and similar climates. Cutworms 

 will eat almost any growing plant that is not too hard or woody, 

 but their preference is for potato, tobacco, maize, lettuce and grasses 

 of a particularly succulent nature. 



There are various factors in Rhodesia that tend to check the 

 increase of cutworms, though they are not sufficient to keep the 

 pest in constant subjection. The climate undoubtedly encourages a 

 number of generations durmg the year, but the prolonged dry season 

 has an adverse effect. The warmth of Rhodesian winters favours' 

 the development of the insect, at a time when food-plants are exceed- 

 ingly scarce. Epizootics of disease are common among cutworms, 

 particularly during the latter half of the wet season (February and 

 March), and it may be found possible to cause the spread of these 

 artificially. Insectivorous birds and mammals undoubtedly exercise 

 a considerable measure of control over cutworms, although the habit 

 of burrowing into the soil during daylight affords the larvae a certain 

 measure of protection. Predaceous insects include the ant, Dorylus 

 helvolus, and its allies. An Asilid fly has also been observed to attack 

 the adult moth. Parasites bred from E. segefum include at least 

 five species of Tachinids, four Ichneumonids and one Braconid. 

 The most persistent cutworm parasite appears to be the Tachinid, 

 Gonia bimaculata, Wied., which has been reared from cutworms in 

 several localities, but has not been bred by the author from any 

 other host. Another common species is Wagneria nigricans, Meig. 

 There are unfortunately many practical difficulties in the way of 

 utilising natural parasites for control of cutworms. Their action 

 is too slow to control the sudden outbreaks that occur, though they 

 may reduce subsequent generations. The Tachinids that are the most 

 efficient parasites have not as yet been successfully reared in confine- 

 ment, and even if this method proved possible, it would require a 

 large amount of labour to attain very uncertain results. 



The influence of various classes of cultivation and of different kinds 

 of soil in relation to outbreaks of cutworms is discussed. Green 

 vegetation is very scarce in Rhodesia during the dry season ; wherever 

 this is found, therefore, cutworms are likely to concentrate. Rich, 

 low-lying land that quickly develops a crop of weeds after a few 

 early showers is very liable to attack. The use of kraal manm^e, by 

 increasing the humus content of the soil, encourages cutworms. In 

 small areas, such as kitchen gardens, cutworms may continue to be 

 (C523) B 



