Effect of water deficit on generation time and reproduction of the gall wasp Tetramesa romana,a biological control agent of giant reed (Arundo donax)

Water deficit stress can reduce the reproductive performance of galling insects, but has not previously been studied in the context of mass-rearing of a galling agent on a perennial grass. The effects of water deficit were examined for the wasp Tetramesa romana Walker (Hymenoptera: Eurytomidae), released in the Lower Rio Grande Basin of Texas and Mexico in 2009 and in northern California in 2010 for control of giant reed (Arundo donax (L)) (Poaceae). In one study, water deficit was imposed for 7–10 weeks during gall maturation, ending when adult progeny began to emerge. Aboveground water content was reduced by 1.7% and culm height by 41% in pots receiving one-fourth of normal watering (soil saturation), indicating that water deficit stress occurred. Water deficit did not affect proportion of culms successfully galled or number of progeny produced. However, time to first exit hole appearance was 2–4 days longer and adult wasp generation time 5–7 days longer on galls on water deficit–stressed compared to control culms, thereby reducing the wasp's intrinsic rate of increase. Water deficit imposed only during parent wasp oviposition had no effect on galling success or reproduction. Water deficit thus has a negative effect on rearing by delaying adult emergence and reducing the rate of population increase, even without affecting adult fertility. Mass-rearing should involve well-watered plants, and variable drought conditions in the field should be considered when evaluating T. romana establishment and impact.