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Biocontrol introduction

Target pest: Alternanthera philoxeroides (Caryophyllales: Amaranthaceae), alligator weed

Agent introduced: Agasicles hygrophila (Coleoptera: Chrysomelidae), alligator weed beetle


1981, 1982

Import source:

South America via USA and Australia

Import notes:

Cameron et al. (1989) - Agasicles hygrophila was imported from South America to USA to control A. philoxeroides in south-eastern states. It was imported into Australia as biocontrol from USA in 1976. Two consignments totalling 208 adults were received in New Zealand 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) - between December 1981 and March 1984, 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)/Gourlay (2021c) - 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)/Gourlay (2021c) - 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.

Paynter (2024) - factors influencing the success of weed biocontrol agents released and established in New Zealand were investigated. Each agent’s impact on the target weed in New Zealand was assessed as ‘heavy’, ‘medium’, ‘variable’, ‘slight’ or ‘none’, where a ‘heavy’, ‘medium’ or ‘variable’ impact have all been observed to reduce populations or percentage cover of their target weed in all or part of their respective target weed ranges in New Zealand. Results showed that: (i) agents that are highly damaging in their native range were almost invariably highly damaging in New Zealand; (ii) invertebrate agents with a closely related ‘native analogue’ species are susceptible to parasitism by the parasitoids that attack their native analogues and failed to have an impact on the target weed, and (iii) agent feeding guild helped predict agent impact - in particular, agents that only attack reproductive parts of the plant (e.g., seed and flower-feeders) are unlikely to reduce weed populations. Damaging impacts of A. hygrophila, a defoliating beetle, have been reported in its native range, it does not have a New Zealand native ecological analogue and its impact in New Zealand is assessed as ‘variable’.

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) in 'General comments' section] 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)/Gourlay (2021c) - occasionally causes minor damage to Alternanthera sessilis [subsequently described as A. nahui - see Heenan et al. (2009) in 'General comments' section] and A. denticulata.

Paynter et al. (2018) - rare minor spillover feeding on A. denticulata and A. sessilis [subsequently described as A. nahui - see Heenan et al. (2009) in 'General comments' section]. Non-target plants were not recognized as present in NZ when A. hygrophila was introduced. Retrospective testing indicated predictable spillover attack.

General comments:

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. [See entries in 'Impacts on non-targets' section pertaining to these species.]


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.

Gourlay H (2021c). Alligator weed beetle: Agasicles hygrophila. The Biological Control of Weeds Book - Te Whakapau Taru: A New Zealand Guide (Landcare Research) [Update of Winks (2007b)] https://www.landcareresearch.co.nz/discover-our-research/biodiversity-biosecurity/weed-biocontrol/projects-agents/biocontrol-agents/alligator-weeb-beetle/

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 (2024). Prioritizing candidate agents for the biological control of weeds. Biological Control, Volume 188, January 2024, Article Number 105396 https://doi.org/10.1016/j.biocontrol.2023.105396

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: Agasicles hygrophila. The Biological Control of Weeds Book - Te Whakapau Taru: A New Zealand Guide (Landcare Research) [Updated 2021 - see Gourlay (2021c)] https://www.landcareresearch.co.nz/discover-our-research/biodiversity-biosecurity/weed-biocontrol/projects-agents/biocontrol-agents/alligator-weeb-beetle/