Effects of different bean varieties on oviposition behaviour and fecundity of acanthoscelides i obtectus say (coleoptera: bruchidae).

Abstract

The interactions between the bean bruchid Acanthoscelides obtectus and six bean varieties (Red haricot, Mwitemania, Rose coco, Nyayo, Canadian wonder and Mwezi moja) under cultivation in Kenya were studied under laboratory conditions. Experiments were conducted to investigate: (1) The effects of bean varieties on oviposition site preference by the beetles. (2) The effects of bean varieties on fecundity (3) The effects of delayed bean provision on oviposition (4) The " susceptibility of bean varieties to attack by the beetles using a susceptibility index. Similarly, bean seed sizes, testae thicknesses and hardness of seeds of the various bean varieties were determined. Later on they were used to find out whether their variations could be used to explain the differences in preference, fecundity and susceptibilities. The experimental room was maintained at temperatures of 27 ± 1°c and 70 ± 5 % relative humidity. The results indicated that Mwezi moja was the most preferred variety as oviposition site, whereas Mwitemania was the least preferred. Number of eggs laid by the beetles were significantly different between varieties (X2 = 32.1, d.f = 5, P < 0.05). Beetles laid the highest number (54.6 ± 4.7) of eggs in Mwezi moja and the least (35.4 ± 5.2) in Mwitemania. Egg laying -xiduration (days) was not significantly different between varieties (F = 0.62, d.f = (5, 54), P > 0.05). Beans induced oviposition in the majority of females. When provision of bean varieties was delayed for 0 - 16 days, beetles laid the highest number (15) of eggs on Mwezi moja and the least (7) on Mwitemania. Delayed bean provision reduced fecundity and prolonged the egg laying duration. Number of progeny of A. obtectus emerging were significantly different between bean varieties (X2 = 77.0, d.f = 5, P < 0.05). The highest number' (490.4 ± 40.1) of progeny emerged from Mwezi moja and the least (299.7 ± 39.9) from Mwitemania. Duration taken by 50 % of progeny to emerge was significantly different between varieties (F = 6.55, d.f = (5, 54), P < 0.05). Beetles took the shortest period (36.6 ± 0.1 days) to emerge from Mwezi moja and the longest period (39.2 ± 0.5 days) from Mwitemania. Susceptibility index was significantly different between bean varieties (F = 3.33, d.f = (5, 54), P < 0.05). Mwezi moja was the most susceptible variety to attack by A. obtectus with a susceptibility index of 5.81 ± 0.4 and Mwitemania was the least susceptible with a susceptibility index of 3.36 ± 0.6. Seed size and thickness of testa affected fecundity of the beetles. The differences in preference of bean varieties by the beetles was attributed to seed size. Physical characteristics of seeds of the bean varieties played a subordinate role on varieties. susceptibility in most of the bean