Evaluating the impact of a biological control agent Carmenta mimosa on the woody wetland weed Mimosa pigra in Australia

Summary

This study assessed the impact of Carmenta mimosa (a stem-mining moth) introduced into northern Australia as a biological control agent of the invasive mimosa, Mimosa pigra. Comparisons between sites with and without the moth (spreading slowly from original release sites) were made. Litter fall, seed banks, vegetation cover, density and age structure of mimosa stands were compared using data collected at sites where C.mimosa was present and where it was initially absent.

Study sites: Previous surveys identified 10 sites on the Finniss River catchment (five with Carmenta mimosa and five without) in November 2000. In July 2001, eight more sites (four with C.mimosa and four without) were established on the Adelaide River catchment. Sites included both open floodplain and paperbark forest, to test whether agent impact varied with habitat.

Methodology:  Five litter trays (fine plastic mesh, 60 cm circumference and 40 cm deep, attached to steel fence posts 1.5 m above the soil surface) were placed, at least 10 m apart, just inside the edge of a closed canopy mimosa stand at each site. Indications of fire (ash and burnt vegetation) were noted each year. In 2001, 25 plants were labelled per site, to monitor survival. Litter samples were collected annually, during the dry season after most of the litter fall had occurred. Tray contents were dried, separated into leaves, woody material, seeds, pods, and inflorescences, and weighed.

C.mimosa abundance was assessed by randomly selecting 10 of the 25 marked plants per site and counting the number of larval frass holes. If fewer than 10 marked plants survived at a site, additional plants were sampled. Neurostrota gunniella (another moth with stem-mining larva) stem damage was quantified by removing 10, 50 cm long stem samples (one per plant) and counting the number of frass holes.

Acanthoscelides puniceus (a bruchid beetle) seed feeding was quantified by examining 100 seeds per litter sample (or all seeds if <100 were in a tray) and counting those containing holes made by emerging adult beetles. Coelocephalapion pigrae ( a flower-feeder) abundance was sampled by beating 10 stem tips from the nearest plants to each litter tray, against a 21 cm diameter funnel with a 7 cm diameter, 7.5 cm deep plastic jar containing 70% alcohol, to collect and preserve insects.

In 2002, plant densities and percentage cover were estimated at each site along transects (selected at random) within the mimosa. Plants growing within the sample area (initially a 0.5 × 0.5 m quadrat located at the start of the transect) were counted until at least 30 mature (at least 50 cm tall) plants were sampled. If there were fewer than 30 mature plants, additional quadrats placed were along the transect. Numbers of seedlings and mature plants, and the cover of mimosa, litter, bare soil and other vegetation at ground level, were recorded. Seed bank was sampled by taking 15 soil cores (7 cm diameter to 5 cm depth) from beneath the canopy at each site, with seeds extracted by sieving and counted. In 2003, similar transects were conducted. Mature plants were also sampled destructively; the age of each was determined according to the number of growth rings.

As the litter trays were initially located just inside each stand, it was possible to measure the approximate distance the stands had advanced or retreated during the study period.

C.mimosa feeding damage was conspicuous at several sites, with many mimosa plants heavily defoliated, and with dead and dying branches riddled with holes created by larvae. Seed rain varied between habitats, being significantly lower in paperbark woodland in all years. In 2001 and 2003 there was a significant negative correlation between C.mimosa and seed rain. In 2002 and 2003, the impact of C.mimosa was greater in paperbark woodland compared with open floodplain habitats. In open floodplain sites without C.mimosa, seed rain averaged 9,013 seeds/m², almost identical to prior to control agent introductions. At the highest level of C.mimosa infestation, seed rain was 336 seeds/m². No correlations were found between C.pigrae abundance and seed rain, or N.gunniella abundance and seed rain.

The impact of habitat and C.mimosa on litter fall was only evident in 2003, when litter fall was significantly lower in paperbark woodland.

Seed consumption by Acanthoscelides puniceus: Overall, around 6% of seeds were consumed by A.puniceus. In 2002, seed consumption was higher in open floodplain habitats (10.5%) compared with paperbark woodland (1%).

Mimosa seed banks: Overall, seed banks averaged 3,710 (± 755) seeds/m². C.mimosa presence had a significant negative impact on seed banks, but impact varied between habitats. No difference was apparent between C.mimosa treatments in paperbark woodland sites, where seed banks were significantly lower than in open floodplains.

Competing vegetation: Percentage cover of competing vegetation varied between years and was positively correlated with C.mimosa abundance, whilst the number of mimosa seedlings/m² was negatively correlated with competing vegetation cover.

Stand densities and age structure: Densities of all plants, seedlings and mature plants were significantly lower where C.mimosa was present. Seedlings exhibited the greatest reduction (95%), with mature plants being reduced by 70%. In all of the expanding sites, large numbers of seedlings were present. Sites where mimosa was contracting were typified by seedling absence.

Survival: Mimosa survival declined over time. Only 45% of originally tagged plants were alive after 2 years. About 10% were assumed destroyed in fires. Eleven sites were affected by fire (five with C.mimosa present). Eight sites burnt once, two sites burnt twice and one site burnt every year. There appeared no effect of these fire on C.mimosa survival.

Mimosa and agent expansion: The frequency of sites where mimosa stands expanded, contracted or remained stable, appeared to vary according to C.mimosa presence.

Conclusions: This study suggests that Carmenta mimosais is effective at preventing Mimosa pigra stand regeneration. In contrast to previous studies, no impact on mimosa could be attributed to the flower feeding Coelocephalapion pigrae or the stem-mining moth Neurostrota gunniella, whilst Acanthoscelides puniceus consumed up to around 10% of seeds; whilst they are unlikely to suppress dense mimosa thickets, they may reduce the rate of mimosa spread.

 

Note: The compilation and addition of this summary was funded by the Journal of Applied Ecology (BES). If using or referring to this study, please read and quote the original paper, this can be viewed at: http://blackwellpublishing.com/submit.asp?ref=0021-8901