Cornicle length measuremetns of Acyrthosiphon pisum (Harris) nymphs reared in the laboratory were instar-specific and unaffected by rearing temperature. A multimodal analysis of cornicle lengths of field-collected aphids clearly detected four distribution peaks (i.e., instars) in five different field populations, and there was generaaly little overlap between successive in star distributions. However, third and fourth instars in the spring field sample could not be separated accurately due to the shorter cornicle length of nymphs that developed from overwintered eggs. Cornicle length proved to be a useful criterion for separating virginoparous A. pisum instars in samples collected in southern Wisconsin.
Bibliographical noteFunding Information:
Appreciation is expressed to H. C. Coppel and J. L. Wedberg for reviewing an earlier draft of this manuscript. This reseaach was supported by the College of Agricultrual and Life Sciences, University of Wisconsin-Madison, and by Regional Research Project NC-149.