Humidity Preference of the Alfalfa Weevil, Hypera postica,1 as Affected by Microenvironmental and Physiological Conditions2

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Humidity preference of adult Hypera postica (Gyllenhal) (Coleoptera: Curculionidae) was studied in the laboratory using modified petri dish and modified T-tube humidity chambers. Both chamber types offered 2 choices, with humidities being controlled by solutions of sulfuric acid and distilled water. Test results showed that sulfuric acid had no inherent effect upon alfalfa weevil behavior. The source of weevil, whether field collected or laboratory reared, had little effect on humidity behavior except under extreme conditions. Pretest starvation accentuated but did not change the direction of response to humidity gradients by the weevil. Both increasing age and increasing temperature resulted in increased selection of the higher of the 2 humidities. The higher of the 2 humidities was selected more frequently in the light than in the dark. Weevil response to moving and still-air gradients was similar. Weevil adults could distinguish humidity differences as small as 5% at 35°C. Discussion is presented to correlate results obtained and field behavior. It was felt that weevil presence on plants at night might be due in part to the low humidity preferred in decreasing light and temperature conditions. This lower humidity would be found near plant tips rather than in the duff. Decreasing weevil numbers on plants with increasing weevil age might similarly be correlated with a tendency to select higher humidities (as found in the duff) with increasing age. Correlation of experimental results and field conditions indicated that weevil humidity response would not differ significantly between the moving air at the tops of the plants and the still air of the duff.