99 



FLAX CATERPILLARS IN KENYA COLONY, WITH SPECIAL 



REFERENCE TO THE LIMITATIONS OF THE ROPING 



METHOD OF COMBATING THEM. 



By F. W. Dry, M.Sc, 

 Rcccntlv Assistant Government Entomologist, Kenya Colony. 



Caterpillars on flax in Kenya Colony are farm pests of the first rank. They belong 

 to more than one species, but the one most commonly reared from material from the 

 field is Phvtometra (Plnsia) orichalcea, F. ; Heliothis obsoleta, F., is also met with. 



Caterpillars have been reported from all the chief flax-growing districts of the 

 country : the Kikuyu district, including Kabete, Kyambu, Limuru and Thika, and 

 the districts of Nakuru, Lumbwa and the Uasin Gishu. Near Kericho, which is 

 wetter and colder than Lumbwa, from farms up to within a distance of six miles 

 from those of the Lumbwa area, which have suffered badly, caterpillars have been 

 reported, but no serious damage has been done. Mr. T. J. Anderson, the Government 

 Entomologist, has taken the moths of Phytometra orichalcea in the Trans-Nzoia 

 district far from European cultivation. 



The caterpillars have a wide range of food-plants. In East Africa they have been 

 found to attack, in addition to flax, Canadian Wonder beans, potatoes, rape and 

 other crucifers, as well as weeds. 



The attack on the flax crop may take place at any stage, from the time the crop 

 is only just above the ground up to the time of pulling. In its early stages the crop 

 may be eaten up completely. In the later stages the seed-bolls are especially attacked. 

 In a bad attack on the partly grown crop all the leaves and the tops of the stems 

 are consumed, all that remains being the stripped stalks. When the caterpillars are 

 less numerous a frequent form of damage is that the growing-point at the apex of 

 the plant is killed and several weak secondary stems are thrown out. This results 

 in shortening of the fibre and irregularity in the ripening of the seed. 



The damage caused by flax caterpillars has been very serious, and many fields have 

 been destroyed. There have been heavy losses in each of the three years 1918, 

 1919 and 1920, the worst reports having been received during the last year. Most 

 of the damage is done from May to August, following the long rains, in the main 

 growing season, but outbreaks have also occurred in the season following the short 

 rains, which are due from October to December. 



The life-history of Phytometra orichalcea was worked out in the laboratory at Kabete 

 during the months of June to August, 1918, comparatively cool months. Eggs were 

 dsposited on the leaves or stems of the plants provided. The cocoons were spun on 

 the host-plants, several leaves or stems being bound together. The caterpiflars were 

 fed on the weed Galinosoga par vi flora. Little variation was found in the length of 

 the life-cycle of different individuals. The average times for the difterent stages 

 were : — 



Egg ... ... ... ... ... ... ... •■• 10 days. 



Caterpillar (hatching to spinning cocoon) 41 days. 



Cocoon (spinning to emergence of moth) ... ... ... 30 days. 



Eggs were laid a day or two after the emergence of the moths, making the length of 

 the life-cycle, from egg to egg, about twelve weeks. 



The caterpillar stage lasting, as it does, about six weeks, there is some little time 

 for the detection of the pest while the caterpillars are still small. The large caterpillars 

 are quite conspicuous, but the younger ones harmonise so well with their surroundings 

 that unless a careful search be made they may easily be overlooked. 



