Location: Exotic and Invasive Weeds Research
Title: Effect of nitrogen fertilization on growth of Arundo donax and on rearing of a biological control agent, the shoot gall-forming wasp Tetramesa romana Authors
Submitted to: Biocontrol Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 8, 2013
Publication Date: N/A
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. Arundo 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 arundo 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 arundo. 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 arundo shoots, and not other plants, animals, or people. The wasp lays eggs inside the arundo shoot tip, and the shoot tip then swells and produces a 'gall', which is 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 one or two months after their mother laid them as eggs inside the shoot tip. ARS scientists are developing a technique to produce many thousands of wasps per month on greenhouse arundo plants, to release large numbers of wasps into the field. This study examined the effect of fertilizing arundo shoots in the greenhouse with three increasing levels of urea pellets (a form of nitrogen, an essential element for plant growth) to make the plants more vigorous, so that they would produce bigger galls and more wasps. The urea-fertilized arundo plants increased their nitrogen content by less than 1% and water content by three to four percent, and grew 1.4-fold taller, compared to unfertilized arundo plants. The number of arundo wasps produced per gall and per parent female were not affected by urea fertilization. However, the time required from egg-laying to emergence of the new generation of adults was four to five days shorter from arundo plants that received the two higher doses of urea (15 and 30 grams per pot), compared to plants that received no urea or only the lowest dose (7.5 g per pot). Under field conditions, urea fertilization did not affect the number of shoots galled by the arundo wasp in small plots (about 10 square feet), probably because the urea was spread out to other plants outside the plots by the tangled roots of the field arundo plants. The results can be applied to improve mass-production of the arundo wasp on greenhouse plants.
Technical Abstract: Nitrogen augmentation often leads to increased feeding and/or reproduction by herbivorous insects, but little is known about the effects on insects that gall grasses. The shoot tip-galling wasp Tetramesa romana has been released for biological control of the giant grass arundo (Arundo donax) in the U.S. and Mexico. Rhizomes are fertilized with urea as part of a mass-rearing protocol, and localized fertilization could enhance field establishment. In a greenhouse study, arundo rhizomes were fertilized with urea pellets at rates equivalent to 1,000 kg (low), 2000 kg (moderate), and 4,000 (high) kg N per ha-1, applied one week after planting (2/3 of total rate) and after wasp infestation. Total nitrogen content of ungalled stems was 0.60–0.65% higher under low and moderate fertilization compared to unfertilized pots, and shoot water content was elevated 3–4% by all urea levels. Fertilization with moderate urea increased relative growth rate by 1.4-fold based on length gain across all shoots compared to shoots in unfertilized pots, but did not affect final dry biomass of shoots or roots. Fertilization did not affect number and duration of female shoot tip alighting and probing events. Percentage shoots galled, progeny production per shoot, rate of increase of wasp populations and wasp size were not affected by fertilization, but adult generation time was reduced by 4–5 days on shoots in pots provided moderate and high urea, and more adults and immatures remained after four weeks in galls on shoots that received no urea than on galls on fertilized shoots. Fertilization did not affect gall density per m shoot length or per wasp released into field plots. Variable greenhouse conditions and rhizome connectivity in the field likely diluted treatment effects. Urea nitrogen addition can increase the efficiency of mass-rearing of the arundo wasp by reducing generation time and increasing adult emergence.