Neochetina eichhorniae Warner and N. bruchi Hustache, biological control agents of Eichhornia crassipes (Martius) Solms-Laubach, are usually incapable of flight but occasionally develop indirect flight muscles enabling dispersal. This reportedly alternates with oögenesis and is reversible. We examined host quality as a possible explanation for the transitions between these 2 states by allowing populations of the 2 species to develop on plants differing in nutritive quality and then examining the status of their ovaries and flight muscle development. The leaf nitrogen content of the plants increased directly with fertilizer treatment levels but herbivory by the weevils changed the pattern of variation. Neochetina eichhorniae suppressed overall nutritive quality while still enabling tissue nitrogen levels to increase with fertilizer treatments. Neochetina bruchi, however, negated these effects and tissue nitrogen levels failed to correlate with fertilizer treatments. As a result, herbivore intensity (the number of weevils per plant) and the proportion of the populations that responded in one way or the other (either oögenesis or flight muscle development) differed between the 2 species. Very few N. eichhorniae responded in the lowest fertilizer treatment and none produced flight muscles. This increased in the intermediate treatments to about an 80% response with most individuals reproductive. At higher levels, the overall response declined somewhat with an increasing proportion becoming dispersive. Very few N. bruchi developed flight muscles except in the highest fertilizer treatment. The frequency of reproductive N. bruchi varied little across fertilizer treatments, tracking host quality instead. We conclude that transitions from reproduction to dispersal in these 2 species are not in response to low nutritive quality of the plant tissue and require adequate nutrition to occur. Host quality, however, is affected by a multitude of factors, including the intensity of herbivory, which complicates interpretation of nuanced responses.
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