with Arundo donax, actual reduction of Arundo from the insect release will not likely strictly
occur in the form of fewer acres, but rather in the form of a reduction in the density and height of
the plant, as well as possibly some modest acreage reduction. This study used reduced acres,
however, as a proxy for reduction in Arundo biomass. This is an assumption of convenience for
the analysis, assuming the analytical results are comparable to realty.
USDA scientists provided data of estimated infested acres for 2002 and 2008, with a total
growth rate of 15% over the time period (Yang 2008). Distributing the growth equally among
the years suggests an annual growth rate of 2.36%. This yearly rate is used to forecast expected
growth 50 years into the future, which is the base used to estimate impacts of Arundo control
scenarios.
In 2007, a natural occurrence of a wasp (one of the four insects selected for biological
control) was discovered near Laredo (Goolsby 2008). A study of the impact of the natural
occurrence of the wasp indicates its impact is minimal. Although the impact is minimal, the
effect of the natural spread of the wasp is included in establishing the base from which impacts of
the introduced biological agents are estimated.
Potential Water Saved
The Arundo water consumption rate found in the literature is applied to the reduced
acreage of giant reed to assist in the projection of the amount of potential water saved. However,
water use by native replacement vegetation is considered to realize an estimate of net water
savings. The resulting net water saved is allocated to an irrigated composite acre based on
specific crop usage levels specified in the Texas AgriLife Extension Service crop enterprise
budgets for the region (Texas AgriLife Extension Service 2007). The resulting calculations