Pattern of egg distribution by Adzuki bean beetle,Callosobruchus chinensis (L.) (Coleoptera: Chysomelidae) in stored chickpea under natural infestation

Under laboratory experimental condition female Adzuki bean beetles, Callosobruchus chinensis (L.) are known to evenly distribute their eggs over grains available for egg laying. However, currently there are no data on pattern of egg distribution by Adzuki bean beetle in large quantity of chickpea grain stored for extended period of time. The objective of this study was to determine the number and distribution of eggs among grains of chickpea stored in large quantity for 8 months and subjected to natural infestations. The experiment was conducted under room temperature and relative humidity at Debre Zeit Agricultural Research Center. The chickpea cultivars used were Natoli (desi type, 22 g per 100 seeds with Tuberculated seed coat texture) and Arerti (kabuli type, 26 g per 100 seeds with smooth seed coat texture) and for each cultivar there were 21 replicas of bags (40 cm wide and 70 cm high). From each bag, which contains 10 kg each, 25 chickpea grain samples were randomly selected at monthly intervals and examined for the presence or absence of Adzuki bean beetle eggs. Grains with eggs were categorized into frequency classes (i.e. 0, 1, 2, …, n) based on the number of eggs present on them. The variance to mean ratio was used to decide the type of egg dispersion pattern. In all the sampling dates and on both cultivars, the variance to mean ratios were highly significantly greater than one indicating that egg distribution by female Adzuki bean beetles on grains of chickpea was aggregated type. Moreover, further analysis of data had shown that the egg distribution pattern perfectly fits the negative binomial distribution, which confirms that the pattern of egg distribution on chickpea grains was aggregated/clumped type.