Larvae of the aquatic dipteran Hydrella pakistanae are used as a biological control agent for the invasive aquatic macrophyte Hydrilla verticillata. We developed a method to estimate the physiological age of the adult female flies as one method to evaluate the field populations of hydrilla. While chronological age is based on the length of time an organism lives, physiological age is based on number of eggs laid and may be influenced by environmental factors such as climate and nutritional supply. Changes in the appearance of follicular relics in ovaries with oviposition provided a reliable method to estimate fecundity, and were used to establish a total of seven physiological age classes: three nulliparous and four parous. Changes that occur in the fat body were used to assist in classification of nulliparous females or those with low egg counts. Although the exact number of ovipositions cannot be determined, researchers can easily distinguish flies that have laid relatively few eggs from those that have laid many. This information enables researchers to evaluate whether H. pakistanae is laying enough eggs to maintain populations sufficient to control hydrilla. The technique can probably be applied to estimate fecundity of other aquatic insect orders.
Species 1: Diptera Ephydridae Hydrellia pakistanae
Species 2: Hydrocharitales Hydrocharitaceae Hydrilla verticillata (hydrilla, waterthyme)
Keywords: age-grading, follicular relics
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