80 

 (1) reduced to a non-problematic level and replaced by a monoculture of 

 water lettuce, ( Pistia strati otes L.) and (2) plots 2 and 3 still con- 

 tained problematic levels of waterhyacinth and were in need of 

 retreatment. 



Table 3-4 presents a cost comparison for use of a combination of 

 GAo and 2,4-D for control of waterhyacinths on the St. Johns River, 

 Florida, as conducted by the U.S. Army Corps of Engineer's Jacksonville 

 District. Based on an average of 3642 ha of waterhyacinths controlled 

 per year, the normal application rate of 2,4-D, 2.24 kg/ha, results in 

 an annual herbicide cost of $27,096. With the use of GA 3 at 8.0 g/ha 

 and a ten-fold reduction in the amount of 2,4-D required for an equiva- 

 lent level of control (0.224 kg/ha), as suggested by Pieterse and Roorda 

 (1982), the annual herbicide costs would be $35,947 or 32.7 percent 

 higher than the current rate. Based on the results of the field appli- 

 cation at Lake Dexter, Florida (Table 3-2), the lowest combination of 

 rates of GA^ and 2,4-D which would provide a level of control (number 

 per sq m) not significantly different from 2.24 kg/ha, 2-4-D was 23.5 

 g/ha and 1.12 kg/ha, respectively. This option would result in an 

 annual herbicide cost of $111,117 or 310.1 percent higher than the 

 current rate. 



Summary 

 The results of the field test of various treatment rates of 2,4-D 

 and GAo indicated that on an operational basis, GA3 does not enhance the 

 effect of 2,4-D on water hyacinths to a significant degree. The 

 additional costs of using GA3 and 2,4-D at the relative levels suggested 

 by Pieterse and Roorda (1982) are not justified from an economic stand- 

 point at the current market prices of GA^ and 2,4-D. 



