New sources of cowpea genotype resistance to cowpea bruchid Callosobruchus maculatus (F.) in Uganda

Cowpea bruchid Callosobruchus maculatus (F.) is a major constraint to cowpea production throughout subsaharan Africa. The identification of sources of C. maculatus resistance and their incorporation into breeding programs would be a beneficial strategy to combat the devastation caused by the bruchid in stored cowpea. We evaluated 145 cowpea genotypes from Uganda and introductions from Kenya and Nigeria for resistance to bruchids. The mean number of eggs and number of holes, percentage pest tolerance, percentage weight loss, bruchid developmental period, bruchid growth and Dobie susceptibility index were significantly different among the 145 genotypes. Based on Dobie susceptibility index value, there were 18 resistant, 114 moderately resistant and 13 susceptible genotypes. Dobie’s susceptibility index correlated negatively with insect development period and percentage pest tolerance, and positively with number of eggs, growth index, number of holes and weight loss. The study identified new sources of cowpea from the studied genotypes that could be used by cowpea breeders to develop cultivars with relatively high resistance to cowpea bruchid. However, further investigations and identifcation of biochemicals that are responsible for cowpea seed resistance to bruchid are recommended. * Corresponding Author: Weldekidan Belay Miesho  belaymiesho@yahoo.com International Journal of Agronomy and Agricultural Research (IJAAR) ISSN: 2223-7054 (Print) 2225-3610 (Online) http://www.innspub.net Vol. 12, No. 4, p. 39-52, 2018 Int. J. Agron. Agri. R. Miesho et al. Page 40 Introduction Cowpea (Vigna unguiculata (L.) Walp.) is an important indigenous legume providing dietary protein, minerals, carbohydrates, fats, vitamins and income to many poor people in Africa, Asia, and central and South America (Enwere et al., 1998; Popelka et al., 2004; Langyintuo et al., 2005; Agbogidi, 2010). Its protein content ranges from 24.7–33.1% with low anti-nutritional factors (Nielsen et al., 1994; Rangel et al., 2003). Globally, more than 12.32 million hectares of cowpea are harvested, 98.1% being from Africa (FAO, 2016). However, cowpea production in these producing countries is limited by insect pest attacks (Beck and Blumer, 2007). In storage, cowpea weevil Callosobruchus maculatus (Coleoptera: Chrysomelidae) is the most destructive pest (Deshpande et al., 2011). The insect females deposit their eggs on seed coat, and embryogenesis is completed after 3 to 5 days (Beck and Blumer, 2007). After eclosion, the larvae penetrate the cotyledons where they develop by consuming the energy reserves of cotyledons, reducing both the quantity and quality of seeds, making them unfit for planting, marketing and human consumption (Ali et al., 2004). Adult emergence occurs after 25-30 days (Oliveria et al., 2009). The loss in quality is due to contamination with insect exudate, eggs, dead insects and holes, conversion of seed contents (Ali et al., 2004). The loss in quantity is attributed to seed weight loss (Maina et al., 2012). In Sub-Saharan Africa, chemical control using insecticdes is a common practice used by the majority of farmers to minimize losses due to bruchid infestations (Olakojo et al., 2007). However, the method is expensive, pose health hazards to farmers and consumers and their continuous use can lead to development of insecticide resistant bruchids (Boyer et al., 2012). The use of resistant genotypes offers a promising alternative control method to the hazardous pesticides for the management of C. maculatus, especially where huge quantities of grains are involved (Cruz et al., 2015). Several studies have assessed the performance of C. maculatus infesting different genotypes (Singh et al., 1985; Shade et al., 1999). In Nigeria, for example, out of the 8000 germplasm lines screened, only three C. maculatus resistant lines (TVu-2027, TVu 11952 and TVu 11953) were identified by the International Institute for Tropical Agriculture (IITA), and C. maculatus showed decreased survival and increased developmental times during infestation of those seeds. However, the use of resistant genotypes is affected by the durability of resistance (Appleby and Credland, 2004), which is rapidly being overcome by changes in pest populations (Keneni et al., 2011) and by lack of highresistance sources (Leach et al., 2001). A study in Nigeria, for example, showed that the already identified bruchid resistance genotype, TVu-2027 has been overcomes by the pest population (Shade et al., 1999). Such breakdown of genetic resistance of improved cowpea genotypes to bruchids highlight the need to search for new sources of resistance from different cultivated varieties and wild species. In Uganda, information on sources of local and improved cowpea bruchid resistant genotypes is scarce. Therefore, in this study, we investigated the susceptibility and resistance of 145 V. unguiculata genotypes to infestation and damage by C. maculatus. The aim was to identify new sources of cowpea genotypes resistant to bruchid in Uganda for the improvement of the breeding programme. Materials and methods Sources of cowpea genotypes Seeds of 145 cowpea genotypes (130 Ugandan, one Kenyan and 14 genotypes from IITA Nigeria) were used for the study (Table 1). To generate sufficient seeds for laboratory testing, each of the genotypes were grown at the Makerere University Agricultural Research Institute Kabanyolo (MUARIK) (0°28’N and 32°37’E, approximately 1200 m asl), between May and December 2015. Bruchid laboratory culture Adult C. maculatus (F.) were obtained from the National Agricultural Research Laboratory, Kawanda. A permanent laboratory culture of the insect was established at MUARIK by allowing the insects to lay eggs on a susceptible inbred line IT71. Insects were reared on 12 kg seeds kept in four transparent plastic buckets of five liter capacity whose tops were covered Int. J. Agron. Agri. R. Miesho et al. Page 41 with muslin cloth to provide aeration and prevent the insects from escaping. The insects were allowed to oviposit and their progeny maintained by regularly replacing the infested seeds with fresh seeds. Table 1. Cowpea genotypes evaluated for bruchid resistance. Genotype Cultivar type source Genotype Cultivar type source Genotype Cultivar type source 182 Landrace Uganda MU9 Landrace Uganda 5T 3B Inbred line Uganda 2282 Landrace Uganda NE13 Landrace Uganda 5T × Acc12 Inbred line Uganda 2309 Landrace Uganda NE15 Landrace Uganda 5T×4W Inbred line Uganda 2392 Landrace Uganda NE19 Landrace Uganda ACC12 × 3B Inbred line Uganda 2419 Landrace Uganda NE23 Landrace Uganda ACC12 × 2W Inbred line Uganda 2434 Landrace Uganda NE30 Landrace Uganda ACC2× ACC12 Inbred line Uganda 3306 Landrace Uganda NE37 Landrace Uganda ACC2 × IT Inbred line Uganda IT109 Improved IITA NE39 Landrace Uganda ACC23 × 4W Inbred line Uganda IT97 Landrace IITA NE39 × SEC2 Inbred line Uganda ACC25 Landrace Uganda KVU-27-1 Improved Kenya NE39 × SEC4 Inbred line Uganda ACC26 x ACC2 Inbred line Uganda NE20 Landrace Uganda NE4 Landrace Uganda ALEGI x 4W Inbred line Uganda NE51 Landrace Uganda NE40 Landrace Uganda ALEGI Local Uganda 3B x 2W Inbred line Uganda NE44 Landrace Uganda ALEGI×3B Inbred line Uganda ACC12 x 5T Inbred line Uganda NE48 Landrace Uganda ALEGI×5T Inbred line Uganda ACC23 x 3B Inbred line Uganda NE5 Landrace Uganda ALEGI × ACC2 Inbred line Uganda ACC26 * IT Inbred line Uganda NE51 × SEC3 Inbred line Uganda CIG Inbred line Uganda EX-1Seke Landrace Uganda NE51 × SEC4 Inbred line Uganda EBELAT×NE39 Inbred line Uganda IT × ACC23 Inbred line Uganda NE55 Landrace Uganda EBELAT×NE51 Inbred line Uganda IT ×ALEGI Inbred line Uganda NE67 Landrace Uganda WC32 × SEC5 Inbred line Uganda IT2841 x BROWN Inbred line Uganda NE70 Landrace Uganda IT71 Inbred line IITA MU17 Landrace Uganda NYBOLA Landrace Uganda IT84 Improved IITA MU20B Landrace Uganda OBONQ1 Landrace Uganda IT889 Improved IITA MU24C Landrace Uganda SEC1 × SEC4 Inbred line Uganda MU15 Landrace Uganda NE21 Landrace Uganda SEC5 × SEC2 Inbred line Uganda WC5 Landrace Uganda NE31 Landrace Uganda SEC5 × NE39 Inbred line Uganda WC55 Landrace Uganda NE32 Landrace Uganda SECOW2W Improved Uganda WC60 Landrace Uganda NE36 Landrace Uganda SECOW5T Improved Uganda WC44 Landrace Uganda NE41 Landrace Uganda UW × 5T Inbred line Uganda WC46 Landrace Uganda NE45 Landrace Uganda 2W×Acc2 Inbred line Uganda WC62 Landrace Uganda NE46 Landrace Uganda 4W × 5T Inbred line Uganda WC63 Landrace Uganda NE49 Landrace Uganda W10 Landrace Uganda WC64 Landrace Uganda NE50 Landrace Uganda W32 Landrace Uganda WC67 Landrace Uganda NE53 Landrace Uganda WC10 Landrace Uganda WC674 Landrace Uganda NE6 Landrace Uganda WC13 Landrace Uganda WC67B Landrace Uganda NE71 Landrace Uganda WC15 Landrace Uganda WC68 Landrace Uganda SEC1×SEC3 Inbred line Uganda WC16 Landrace Uganda WC684 Landrace Uganda SEC5× SEC1 Inbred line Uganda WC17 Landrace Uganda IT82D 716 Improved IITA WC2 Landrace Uganda WC18 Landrace Uganda IT84s-2246 Improved IITA WC29 Landrace Uganda WC19 Landrace Uganda IT97K-499-35 Improved IITA WC35C Landrace Uganda WC21 Landrace Uganda TVu-2027 Improved IITA WC42 Landrace Uganda WC26 Landrace Uganda IT90K-277-2 Improved IITA WC52 Landrace Uganda WC27 Landrace Uganda IT90K-76 Improved IITA WC58 Landrace Uganda WC30 Landrace Uganda IT95K-207-15 Improved IITA WC69 Landrace Uganda WC32A Landrace Uganda IT98K-205-8 Improved IITA WC7 Landrace Uganda WC35A Landrace Uganda IT99K-1399 Improved IITA WC8 Landrace Uganda WC35D Landrace Uganda WC41 Landrace Uganda WC36 Landrace Uganda 2W x IT Inbred line Uganda WC37 Landrace Uganda SEC5 x SEC2 Inbred line Uganda WC48 Landrace Uganda SEC5 x NE39 Inbred line Uganda WC48A Landrace Uganda Infestation and data collection Seeds of each of the 145 cowpea genotypes were dried in an oven at 40oC for 24 hours to eliminate any bruchid infestation coming from the field and to keep moisture level of the seeds uniform (Amusa et al., 2014). Ten randomly selected