|Pikul jr, Joseph|
Submitted to: Journal of Economic Entomology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/28/2003
Publication Date: 8/1/2003
Citation: HAMMACK, L., ELLSBURY, M.M., ROEHRDANZ, R.L., PIKUL JR, J.L. LARVAL SAMPLING AND INSTAR DETERMINATION IN FIELD POPULATIONS OF NORTHERN AND WESTERN CORN ROOTWORM (COLEOPTERA: CHRYSOMELIDAE).. JOURNAL OF ECONOMIC ENTOMOLOGY. 2003. V. 96 p. 1153-1159. Interpretive Summary: Northern and western corn rootworms are important pests of corn that coexist throughout much of the U.S. Corn Belt and together account for significant insecticide use, often without knowledge of pest population levels. Most crop damage results from larval root feeding, but the larvae are difficult to monitor early in development before crop damage has occurred because of their small size, subterranean habit, morphological similarity between species, and the lack of validated indicators of developmental stage for field populations. Our study evaluated head capsule width as an indicator of rootworm stage using larvae readily retrieved live from maize root systems grown in the field under conditions that allowed most specimens to be assigned to species. For western corn rootworm early in the season, stage of larvae recovered from isolated root systems was further compared to that of larvae found in soil cores (7.5 inches in diameter and depth centered on a corn plant in leaf stage 5-7) from which the root systems had been removed. The results indicated that head capsule widths of 270 and 410 microns (0.0106 and 0.0161 inches) can be used to separate first from second and second from third stage larvae of both species with better than 95% accuracy; thus, developmental status of larvae from populations composed of both northern and western can be assigned without having to determine species. The data further showed that the relative abundance of the three western corn rootworm larval stages can be accurately assessed from populations recovered from maize root systems, a vastly easier process than recovery from large soil cores. These data contribute to the development of reliable and simple methods to monitor corn rootworms early enough in the season to apply rescue treatments should populations warrant.
Technical Abstract: Head capsule width was measured for northern (Diabrotica barberi Smith & Lawrence) and western corn rootworm (D. virgifera virgifera LeConte) larvae recovered primarily from maize root systems but also from large soil cores each centered 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 corn rootworm, presumably representing frequency of the three larval instars, with no larvae measuring 280 or 420 microns in the valleys between peaks. Multiple normal curves fit to similar but partially overlapping peaks generated by northern corn rootworm suggested that division of first to second and second to third instar can best be made for this species at 267 and 406 microns, respectively (270 and 410 when measurements are made to the nearest 20 microns). These results implied that instar of individuals from mixed northern and western corn rootworm populations can be accurately judged from head capsule width without having to determine species. The relative abundance of western corn rootworm instars was similar in root systems removed from the center of 19-cm diameter x 19-cm deep soil cores and in soil cores from which the root systems were removed. This result indicated that the former and much more convenient sampling unit can be used to assess stage of population development, at least early in the season when these tests were done and young larvae predominated.