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Entomology

14. PLANT PROTECTION

During the_season the Entomologist carried out a survey of insect pests of crops in the Central, Western and Southern Province areas in Tanganyika. The following is the text of his report, the conclusions having already been presented on page 122.

Central Province

(a) Groundnuts. The present position is satisfactory. Several species of insects. have been observed feeding on haulms, roots and nuts, but none has reached epidemic population. Curl grubs were responsible for a loss of about 2 per cent. of the plants on certain areas of Unit 2, and various soil insects damaged about 1 per cent. of the kernels on both Units. Caterpillars and young grasshoppers were observed on the hauim. A few small colonies of aphides were observed during January, but these were exterminated by the larvæ of Syrphids and Coccinellids.

Only a few isolated plants showing rosette disease were found towards the end of the growing season. Neither aphides nor traces of a previous infestation were found on any of these plants, but white flies and leaf-hoppers were present on some. (In the Belgian Congo, white flies are still suspected as vectors of rosette.) Rosette has thus been practically absent from both African crops and unit production of groundnuts at Kongwa for two seasons, since very little rosette- (and that late in the season) was seen in the very wet season of 1945-47.

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Hilda patruelis Stal., a leaf-hopper, feeds on the roots and underground por-- tions of the pegs, and has caused serious damage in parts of Tanganyika. has been found on late-planted groundnuts and on. leguminous and composite. weeds at the Kongwa Experiment Station, on nearby flattened areas and on Unit 2. Although numerous eggs, both unhatched and hatched, have been found, nymphs were rarely present. It is possible that the dry condition of the soil during late May-June was unfavourable.

(b) Sunflowers. Several species of plant bugs have been observed feeding on the stem and back of the heads, but, no evidence of damage was seen.

(c) Sorghums. Aphides were found in considerable numbers on the young plants, but were later almost completely exterminated by the larvæ of Syrphid flies. The later-sown imported dwarf combine varieties, and the local tall type, Buhura Mahemba, were severely damaged by the larvæ of a fly, Atherigona indica infuscata, which bored into and killed the growing points of main shoots and tillers. This not only reduced the stand but also lengthened the growing period, since the plant had to produce new tillers, some of which did not in the time available develop sufficiently to produce heads.

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The early-sown dwarf combine varieties at Kongwa Experiment Station, which in 1947-8 had standing bush down two opposite sides, were heavily damaged as the grain was filling out by so-called "blue bugs" (Calidea dregei) which fed on the maturing grain and stopped or held back its development. Damage was not so severe on plots in an area of former open grassland (on Unit 2), perhaps on account of the greater distance of the crop from standing bush.

The maturing heads of two later sowings of Feterita Managil and Martin's Milo, sown on 9th March, were not attacked by adult Calidea, which evidently preferred the adjacent sunflowers on which they were found in large numbers. Calidea bred in large numbers on a wild Hibiscus sp. which grew abundantly on cleared areas which had not been sufficiently prepared for planting. The close proximity of sorghum or sunflower to Hibiscus is evidently very favourable to Calidea. The adults first appeared on sorghumn and sunflowers in March and fed on the developing heads of the two crops. When the ovaries matured in April, they flew to the Hibiscus, on which the eggs were laid and the immature- stages fed. The adults of this generation returned to sunflowers chiefly, but were observed topfeed of the developing pods of legumes, pastor oil cotton, safflower.

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and various weeds.

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