Larval sampling and instar determination in field populations of northern and western corn rootworm (Coleoptera: Chrysomelidae)
2003
Hammack, L. | Ellsbury, M.M. | Roehrdanz, R.L. | Pikul, J.L. Jr
Abundance and head capsule width were determined in 1999 to 2001 in South Dakota, USA, for northern (Diabrotica barberi) and western maize rootworm (D. virgifera virgifera) larvae recovered primarily from maize root systems but also from large soil cores each centred around a root system. Larvae for measurement derived from field populations under infestation and rotation regimes that allowed most specimens to be assigned to species. A frequency distribution of head capsule widths indicated three separate peaks for western maize rootworm, presumably representing frequency of the three larval instars, with no larvae measuring 280 or 420 µm in the valleys between peaks. Multiple normal curves fit to similar but partially overlapping peaks generated by northern maize rootworm suggested that division of first to second and second to third instar can best be made for this species at 267 and 406 µm, respectively (270 and 410 when measurements are made to the nearest 20 µm). These results implied that instar of individuals from mixed northern and western maize rootworm populations can be accurately judged from head capsule width without having to determine species. The relative abundance of western maize rootworm instars was similar in root systems removed from the centre of 19-cm diameter × 19-cm deep soil cores and in soil cores from which the root systems were removed. Furthermore, the number of larvae from root systems correlated significantly with that from the surrounding soil. These results indicated that the former and much more convenient sampling unit can be used to estimate population developmental stage and possibly density, at least early in the season when these tests were done and young larvae predominated.
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