Target pest: Alternanthera philoxeroides (Caryophyllales: Amaranthaceae), alligator weed
Agent introduced: Agasicles hygrophila (Coleoptera: Chrysomelidae), alligator weed beetle
Import source: South America via Florida and Australia
Release details: 32,250 adults and 2,500 larvae released at 26 sites in Northland and Auckland
Establishment: Winks (2007) - well established throughout Auckland and Northland and at least one site in the Waikato.
Impacts on target: Winks (2007) - controls alligator weed in many lakes and ponds, not able to control the weed in regularly flooded running water, terrestrial infestations or in areas that get frosted. New Zealand conditions are often marginal for the beetles. Landcare Research (2017e) - it is estimated that the biocontrol agents (A. hygrophila and the moth Arcola malloi) are saving around $505,000 per year in Auckland and Northland, with a resulting benefit to cost ratio of 101:1.
Impacts on non-targets: Paynter et al. (2004) - surveys record A. hygrophila only feeding on target plant, although lab tests indicated the naturalised exotic Alternanthera sessilis [subsequently described as the indigenous A. nahui - see Heenan et al. (2009) below] should be an acceptable food plant. But A. hygrophila is restricted to very humid sites and only controls floating mats of alligator weed, and A. sessilis does not form floating mats. Winks (2007) - occasionally causes minor damage to Alternanthera sessilis [subsequently described as A. nahui] and A. denticulata. Heenan et al. (2009) - New Zealand plants previously assigned to A. sessilis described as a new species, A. nahui, indigenous to New Zealand. It is uncertain whether A. denticulata should be considered indigenous or naturalised. Paynter et al. (2017) - rare minor spillover feeding on A. denticulata and A. sessilis. Non-target plants were not recognized as present in NZ when A. hygrophila was introduced. Retrospective testing indicated predictable spillover attack.
Cameron PJ, Hill RL, Bain J and Thomas WP (1989). A Review of Biological Control of Invertebrate Pests and Weeds in New Zealand 1874-1987. Technical Communication No 10. CAB International Institute of Biological Control. DSIR Entomology Division. 424p.
Heenan PB, de Lange PJ, Keeling J. (2009). Alternanthera nahui, a new species of Amaranthaceae indigenous to New Zealand. New Zealand Journal of Botany 47 (1): 97-105
Landcare Research (2007). New Zealand Arthropod Collection (NZAC) Biological Control Voucher Collection. http://www.landcareresearch.co.nz/resources/collections/nzac/holdings/biological-control-voucher-collection
Landcare Research (2017e). Alligator weed: a financial snapshot. Weed Biocontrol: What's New? 81: 4 http://www.landcareresearch.co.nz/publications/newsletters/biological-control-of-weeds/issue-81
Paynter Q, Fowler SV, Groenteman R. (2017). Making weed biological control predictable, safer and more effective: perspectives from New Zealand. BioControl (First online: 8 Aug 2017). https://doi.org/10.1007/s10526-017-9837-5
Paynter, Q.E., Fowler, A.H., Gourlay, M.L., Haines, M.L., Harman, H.M., Hona, S.R., Peterson, P.G., Smith, L.A., Wilson-Davey, J.R.A., Winks, C.J. and Withers, T.M. (2004). Safety in New Zealand weed biocontrol: A nationwide survey for impacts on non-target plants New Zealand Plant Protection 57: 102-107
Winks C (2007). Alligator Weed Beetle. In The Biological Control of Weeds Book (Landcare Research) http://www.landcareresearch.co.nz/research/biocons/weeds/book/documents/Alligator_Weed_Beetle.pdf