Do aphids maintain differential densities on plant parts? A case study with Aphis craccivora Koch (Hemiptera: Aphididae)

Differential aggregation of aphids on host plant parts may indicate site-specific optimal densities for efficient utilization of plant parts, besides the usual inference of preferential colonization. Differential densities of aphids within the plant  were studied using the legume aphid Aphis craccivora Koch in cowpea Vigna unguiculata (L.) Walp. ssp. unguiculata. In field-collected infested sample stems and pods (n=60), colonies were demarcated, aphid colony size, length and circumference measured, and colony area and density calculated. The results indicated that colony dimensions and colony size were significantly higher in pod than in stem whereas colony density did not differ significantly between the two plant parts. Colony density was significantly higher in leaflets of top most leaf than in leaflets of top 2nd or 3rd leaf. Overall, the four plant parts could be graded in the descending order as stem>pod>leaflets of top most leaf> leaflets of top 2nd or 3rd leaf for colony density. Significant positive curvilinear and linear relationship between colony size and colony density in both stem and pod indicated that A. craccivora showed a propensity to spread out colonies at low populations but tended to compact them with a rise in population levels. Identical colony density in stem and pod suggested that the aphid may not require differential densities to overcome host defenses or utilize food from these two plant parts. In top most leaf and top 2nd or 3rd leaf, finite leaflet size apparently limits proliferation of the aphid. Higher density on top immature leaves could be more an outcome of nutritional suitability than the need to overcome host defenses. Variable colony densities on the four parts of V. unguiculata indicated differential optimal densities.