Crop rotation may improve production efficiency and reduce fertilizer N requirements for corn (Zea mays L.). Objectives were to determine effect of rotation and N on corn yield, efficiency of water use (WUE) and N use (NUE), and corn rootworm (Coleoptera: Chrysomelidae) beetle populations (CR). Rotations (started in 1990) were continuous corn (CC), corn–soybean [Glycine max (L.) Merr.] (CS), and a 4‐yr rotation of corn–soybean–spring wheat (Triticum aestivum L.) companion‐seeded with alfalfa (Medicago sativa L.)–alfalfa hay (CSWA). Nitrogen treatments for corn were corn fertilized for a grain yield of 8.5 Mg ha−1 (highN), 5.3 Mg ha−1 (midN), and no N fertilizer (noN). Average yield (1992–2003) was greatest (p = 0.003) under CS and highN (7.0 Mg ha−1). Yield differences (p = 0.001) among rotations increased with decreased fertilizer N. Average (1992–2003) yield with noN fertilizer was 5.8 Mg ha−1 under CSWA, 4.5 Mg ha−1 under CS, and 2.8 Mg ha−1 under CC. Nitrogen use efficiency differed (p = 0.096) only under midN with CSWA = CS > CC. Soil water (upper 1.8 m) for corn measured on 1 June (average of N treatments) was 55, 54, and 45 cm for CC, CS, and CSWA, respectively. For CSWA under highN, available water limited yield in 3 of 6 yr. At highN, CR adult populations were greater under CS compared with CC and greater at higher N fertilizer levels within CC. Rotations have potential to improve production efficiency; however, there is potential for reduced corn yield after alfalfa due to less available soil water.

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Abundance and head capsule width were 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 Þeld 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 m in the valleys between peaks. Multiple normal curves Þt to similar but partially overlapping peaks generated by northern corn rootworm suggested that division of Þrst to second and second to third instar can best be made for this species at 267 and 406 m, respectively (270 and 410 when measurements are made to the nearest 20 m). 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 ϫ 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 signiÞcantly 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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