Submitted to: Biocontrol Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/12/2015
Publication Date: 4/15/2015
Citation: Moran, P.J. 2015. Effect of water deficit on generation time and reproduction of the gall wasp, Tetramesa romana, a biological control agent of giant reed (Arundo donax). Biocontrol Science and Technology. 25:859-872.
Interpretive Summary: The giant grass known as arundo or giant reed is a non-native, invasive weed in the dry southwestern U.S., from Texas to California, where it forms dense thickets of stems in river floodplains and along the shores of reservoirs and ponds. Giant reed plants consume water that is then unavailable for irrigation, industrial or household use. This giant grass also burns easily and regrows quickly after fires, displacing native small plants and trees that would normally grow near water. Dense thickets of giant reed also alter river flow patterns and discourage recreational use of waterways. The USDA-Agricultural Research Service has pioneered the use of biological control to reduce the many negative impacts of giant reed. Biological control involves the release and evaluation of insects from the native range of the weed to decrease the ability of the weed to grow in areas where the weed has invaded. ARS scientists released a wasp that 'stings' only the tips of giant reed shoots, and not other plants, animals, or people. The wasp lays eggs inside the giant reed shoot tip, and the shoot tip then swells and produces a 'gall', which is like a plant 'tumor'. The eggs hatch into larvae and feed on the tissue inside the gall. A new generation of adults chew their way out of the galls two months later to begin the life cycle again. This study examined the effect of drought on on the arundo wasp by exposing giant reed shoots in the greenhouse to water deficit, by giving them only 1/4 of the normal amount of water. The leaves wilted and the height of the giant reed shoots was 41% less and tissue water content 1.7% less in drought-treated plants than in plants receving full watering. Under these conditions of plant 'stress', the wasps produced as many galls and offspring as on fully-watered plants. However, the adult wasps took 2 to 4 additional days to begin emerging (observed as the time of first appearance of exit holes on the shoot) and 5 to 7 days longer on average to emerge as adults when the galls were removed from the shoots and placed in moist boxes in the lab. Therefore, the rate at which the wasp can increase its population was about 40% lower on giant reed shoots exposed to waer deficit stress.The results have two main implications. First of all, for mass-rearing of the wasp, well-watered giant reed shoots should be used. Secondly, when releasing the wasp in the field for biological control of giant reed, soil water content and rainfall patterns should be taken into account in determining if the field releases are successful.
Technical Abstract: Water deficit stress can reduce the reproductive performance of galling insects, but its effects on a galling insect introduced for biological control of a perennial grass weed have not previously been examined. 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 and in California for control of giant reed (Arundo donax (L)) (Poaceae). In one study, water deficit was imposed for 7 to 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 water deficit-treated pots, 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 to 4 days longer and adult wasp generation time 5 to 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 wasp infestation had no effect on galling success or reproduction. The results suggest that water deficit stress during mass-rearing of T. romana delays adult emergence and reduces the rate of population increase, even without affecting adult fertility. Variable drought conditions in the field should be considered when evaluating T. romana establishment and impact.