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Biocontrol introduction

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

Import notes:

Cameron et al. (1989) - two consignments totalling 208 adults were received in October 1981 and January 1982 from CSIRO, Australia. The beetles were collected from the field near Sydney. In both cases beetles reproduced rapidly and completed a second generation within two months.



Release details:

Cameron et al. (1989) - 32,250 adults and 2,500 larvae were released at 26 sites in Northland and Auckland.


Cameron et al. (1989) - beetle populations generally began increasing immediately following releases except at sites affected by stock feeding, herbicide sprays or floods. Where releases were made early in summer, population increases of 10-20 times often occurred by the onset of colder weather in autumn. Survival over the winter period was variable and appeared to be dependent on weed condition, which was affected by frequency of floods and frosts. Beetle populations became established most quickly at sites least affected by these factors, principally northern sites and in ponds and dams. Agasicles hygrophila has spread rapidly throughout Northland.

Winks (2007b) - well established throughout Auckland and Northland and at least one site in the Waikato.

Impacts on target:

Cameron et al. (1989) - realised or potential savings from the actions of A. hygrophila in Northland are considered significant; the beetle has reduced the need for mechanical or chemical weed control, helped maintain water flow in rivers and streams, slowed the pugging of farm dam edges and prevented deterioration of dam water quality and made more water surface available for other wildlife.

Winks (2007b) - 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 (2007b) - 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. (2018) - 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, 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 https://doi.org/10.1080/00288250909509795

Landcare Research (2007a). New Zealand Arthropod Collection (NZAC) Biological Control Voucher Collection. Landcare Research website [Updated 2020] https://www.landcareresearch.co.nz/tools-and-resources/collections/new-zealand-arthropod-collection-nzac/databases-and-holdings/new-t2-landing-page/

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. (2018). Making weed biological control predictable, safer and more effective: perspectives from New Zealand. BioControl 63: 427–436 (first published online 8 Aug 2017) https://doi.org/10.1007/s10526-017-9837-5 https://link.springer.com/article/10.1007/s10526-017-9837-5

Paynter QE, Fowler AH, Gourlay AH, Haines ML, Harman HM, Hona SR, Peterson PG, Smith LA, Wilson-Davey JRA, Winks CJ, Withers TM (2004). Safety in New Zealand weed biocontrol: A nationwide survey for impacts on non-target plants. New Zealand Plant Protection 57: 102-107 https://journal.nzpps.org/index.php/nzpp/issue/view/vol57

Winks C (2007b). Alligator Weed Beetle. In The Biological Control of Weeds Book (Landcare Research) https://www.landcareresearch.co.nz/discover-our-research/biosecurity/weed-management/using-biocontrol/the-biological-control-of-weeds-book/