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Keywords:

  • Dispersal;
  • diet-switching;
  • inter-crop movement;
  • stable isotopes

1. Dispersal plays a key role in structuring the local population densities of many insect species, yet the movement patterns across the landscape of most species are poorly understood. By measuring the stable isotope of carbon (δ13C) from multiple tissues, a novel approach applied to field-collected insects, we were able to infer differences in movement patterns of two species of mobile generalist insect predators.

2. Coccinella septempunctata L (7-spot ladybeetle) and Harmonia axyridis Pallas (multicoloured Asian ladybeetle) were collected in agricultural habitats in 2003 and 2004, and were assayed for δ13C in the elytra (slow turnover) and fat/reproductive tissues (fast turnover). δ13C values were used to infer diet use of C3 versus C4 crops.

3. Coccinella septempunctata was relatively more faithful to a particular habitat and tended to stay in alfalfa and soybean (C3-based photosynthetic crops) over long periods during the summer. This contrasts with H. axyridis which showed isotopic evidence consistent with frequent late-season movement between C3 and C4 crops such as corn in the landscape.

4. These differing patterns suggest that in the late summer season H. axyridis individuals traverse the environment more extensively and utilise broadly dispersed aphid resources, whereas C. septempunctata adults are more specialised on alfalfa and soybean crops.