Postharvest losses and population dynamics of bruchids in resistant common bean (Phaseolus vulgaris L.) genotypes

Dissertation

Tanzania is a leading country in common bean production in Africa, and it ranks 5th among top producers in the world. Dry beans are essential sources of human dietary protein, calories, vitamins, micronutrients and income. Postharvest losses are relatively high due to infestation by storage insect pests (40-80%). The bean bruchids namely Acanthoscelides obtectus and Zabrotes subfasciatus inflicts economic losses in stored dry common beans. This study aimed at reducing postharvest losses by evaluation of resistant bean genotypes against major bean bruchids Acanthoscelides obtectus and Zabrotes subfasciatus. Specifically, the study sought to: (i) determine the effect conferred by resistant bean lines on the life stages and population dynamics of major bean bruchids and (ii) evaluate selected common bean lines for resistance to major bean bruchids under the farmer’s storage conditions. Methods Determination of the effect conferred by resistant bean lines on the life stages and population dynamics of major bean bruchids, the study was laid out in a 2x7 factorial experiment arranged in a completely randomized design (CRD), with two (2) factors and four (4) replications. Factor A entailed the backcrossed common bean genotypes with seven (7) levels i.e., five (5) resistant lines; AO-1012-29-3-3A, 65/44-30-2-3A-1, ROBC (8-54) AA, 30/59-96-2-3A-1, ROBC (14-34) AA CIAT and two (2) susceptible control lines: 44-NJANO and 59-SOYA. While Factor B included the major bruchids species of common bean with two (2) levels i.e., A. obtectus and Z. subfasciatus. The quantitative data were subjected to analysis of variance. Mean separation test was conducted using Duncan’s Multiple Range Test (α = 0.05). The promising bean lines AO-1012-29-3-3A and ROBC (8-54) AA which had the lowest number of holes per grain and emerged adults were on farm evaluated for the period of 90 days as per specific objective 2. Evaluation of selected common bean lines for resistance to major bean bruchids under the farmer’s storage conditions, the study was laid out in a 2x4 factorial experiment arranged in randomized complete block design (RCBD) with two (2) factors and five (5) replications (randomly selected farmers per agroecological zone). Factor A entailed the common bean genotypes at four (4) levels i.e., two (2) resistant lines; AO-1012-29-3-3A, ROBC (8-54) AA, and two (2) susceptible lines; 44-NJANO and 59-SOYA as control. While Factor B consisted the Agroecological zones at two (2) levels namely; Southern Highlands Zone (Nambala village in Mbozi district, Songwe region) and Northern Zone (Mungushi village in Hai district, Kilimanjaro region). Each bean genotype weighing 0.75 kg was placed in a 1 kg brown paper bag and stored in farmers’ warehouse under natural infestation of bruchids. The quantitative data for number of holes per grain, number of emerged adults, percentage grain damage and percentage weight loss were subjected to analysis of variance. Mean separation test was conducted using Duncan’s Multiple Range Test (α = 0.05). Findings The results showed that, bruchids infesting AO-1012-29-3-3A had significantly (p<0.001) highest number of days from larva to pupa (31), and pupa to adult emergence (17) at 90 days after infestation (DAI) than other genotypes. AO-1012-29-3-3A and ROBC (8-54) AA had significantly (p<0.001) lowest number of holes per grain (0.3; 0.4), number of emerged adults (0.3; 0.4), emerging adults’ percentage (0.3%; 0.4%), surviving adults (0.3; 0.4), and percentage adult survival (0.3%; 0.4%) at 90 DAI than other genotypes. The highest number of emerged A. obtectus adults occurred the earliest (at the 6 th week ≈ 42 days) in bean genotypes 44-NJANO (64 adults) and 59-SOYA (48 adults). While the lowest number of emerged A. obtectus adults were observed (at the 8 th week ≈ 56 days) in the bean genotype AO-1012-29-3-3A (2 adults) followed by ROBC (8-54) AA (4 adults) and 65/44-30-2x3A-1 (7 adults). The highest number of emerged Z. subfasciatus adults occurred the earliest (at the 5 th week ≈ 35 days) in bean genotypes 44-NJANO (133 adults) and 59-SOYA (108 adults). While the lowest number of emerged Z. subfasciatus adults were observed (at the 8 th week ≈ 56 days) in the bean genotype ROBC (8-54) AA (4 adults). Regardless of the agroecological zones, the bean genotypes AO-1012-29-3-3A and ROBC (8-54) AA had significantly lowest weight loss (0.23%; 0.65%), seed damage (4.1%; 4.25%), number of emerged adult bruchids (2.4; 2.9) and number of holes per grain (0.42; 0.3), respectively than 44-NJANO and 59-SOYA. While the bean genotype 59- SOYA had significantly highest weight loss (11.5%), seed damage (45.0%), number of emerged adult bruchids (111.9) and number of holes per grain (8.9).