Substratum Requirements for Oviposition, Seasonal Egg Densities, and Conditions for Egg Eclosion in Ranatra montezuma (Heteroptera: Nepidae)

Abstract

Egg densities of Ranatra montezuma Polhemus were monitored in the near-thermally constant environment of Montezuma Well, Yavapai County, Ariz., during January 1987 to December 1988. Seasonal egg frequency showed a multimodal pattern with three peaks during the year. R. montezuma females selected floating and soft, decaying plant substrata over firm green plant substrata for egg oviposition in almost 100% of the laboratory trials. Gram penetration load readings were taken with a texture analyzer to compare plant substrata with and without eggs, and to examine the suitability of plant substrata in the field. The relationship between waterfowl feeding on the vegetation and egg oviposition is discussed. The average incubation time for eggs was 20.6 d at 21°C. No eggs hatched when continually submerged or when dried for 24 h. Eighteen percent of the eggs hatched when subjected to a 6-h drying period compared with 67.7% in control treatments in which the eggs were floating. Less than 1% of the eggs hatched at 10°C, whereas 36.4 and 76% of the eggs hatched in paired treatments at 15°C and 21°C, respectively. Even though annual water temperature in Montezuma Well is constantly warm (21 ± 4°C), Ranatra eggs are absent from late November to early February. We propose that low air temperatures (>10°C) limit oviposition during the winter months because females must lay eggs in substrata at the air-water interface.