Growth of alligator weed (Alternanthera philoxeroides (Mart.) Griseb. (Amaranthaceae)) and population development of Agasicles hygrophila Selman & Vogt (Coleoptera: Chrysomelidae) in northern New Zealand

Carol A. StewartA,C, R. Bruce ChapmanA and Chris M.A. FramptonB

A Ecology and Entomology Group, PO Box 84, Lincoln University, Canterbury, New Zealand.

B Centre for Computing and Biometrics, PO Box 84, Lincoln University, Canterbury, New Zealand.

C Current address: 25 Goring Road, Balmoral, Auckland 1003, New Zealand.


Alligator weed (Alternanthera philoxeroides) is a problem aquatic weed in New Zealand. Little is known about the population ecology of its introduced biological control agents, Agasicles hygrophila and Arcola malloi. A study of alligator weed and A. hygrophila populations conducted at a Northland site in New Zealand found that alligator weed dry weights peaked in late January 1994 and late December 1994 and declined as a result of feeding by increasing populations of A. hygrophila. Aquatic alligator weed was present throughout the 1994 winter. Internal stem diameters probably did not limit A. hygrophila pupation, as the majority of stem diameters measured were above 1.21 mm, the width required for female pupation within the stem.

Peak A. hygrophila populations occurred during February but the density of A. hygrophila was too low or the damage too late in the growing season to provide control, despite considerable defoliation. It was concluded that A. hygrophila is unlikely to cause a reduction in alligator weed in New Zealand even in conjunction with Arcola malloi. A. hygrophila was estimated to have a requirement of 277 degree days above the 13.3°C lower temperature development threshold to develop from egg to an ovipositional adult. Three generations of A. hygrophila were predicted to occur at Whatatiri in the 1994-95 season.


Plant Protection Quarterly (2000) 15 (3) 95-101.