Stable carbon isotopes were used to examine colonization of an ephemeral crop habitat by Hippodamia convergens Guérin-Méneville. A laboratory study indicated that H. convergens carbon isotope ratios, d13C, shifted rapidly after its food supply was changed from a C4 to a C3-based diet of aphids produced on either grain sorghum or cotton, respectively. Final isotope ratios of adult H. convergens were closer to that of the new diet than the C4-based diet on which the beetles completed adult development, but most change in d13C occurred within three days. Field collections of eggs and adults also were used to assess colonization, feeding and reproductive behavior of H. convergens. Isotope ratios of beetles collected in cotton suggested nearby grain sorghum was a continuous source of H. convergens until many area sorghum fields senesced. When cotton aphid (Aphis gossypii Glover) prey were absent, carbon isotope ratios of adult beetle populations did not change over time and virtually no egg production by H. convergens adults was detected. This indicates beetles were feeding very little on other resources originating in cotton, but were still retained in the crop. When cotton aphids were present, beetle isotope ratios decreased towards the d13C of cotton, indicating feeding by adult beetles in their new habitat. As a result, egg masses produced had carbon isotope ratios in the C3 range of values. Results suggest that this predator may be retained in habitats without large prey populations present, a quality that may be essential in controlling crop pests in agricultural systems.
Species 1: Coleoptera Coccinellidae Hippodamia convergens (convergent lady beetle)
Species 2: Homoptera Aphididae Aphis gossypii (cotton aphid, melon aphid)
Keywords: cotton, sorghum
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