YCA surveys in the Vallée de Mai suggest that baiting has reduced numbers of ants |02 March 2020

The yellow crazy ant (YCA) is an invasive alien species and causes significant impacts on the native wildlife of the Vallée de Mai. The Seychelles Islands Foundation (SIF) responded to this threat by setting up a dedicated YCA taskforce earlier last year, who have carried out two rounds of baiting in the forest in an attempt to reduce the numbers of YCA. In December we surveyed the numbers of ants using pitfall traps, to assess the impact of baiting on YCA distribution and abundance.

The YCA taskforce used a more ergonomic pitfall trap design for this survey in the Vallée de Mai, which was treated with AntOff (the ant bait). Another survey was carried out in Fond Peper without AntOff treatment for comparison. The resulting 75 samples (50 from the Vallée de Mai and 25 from Fond Peper) were then sorted in December.

The results showed that YCA now occupy 60% of the Vallée de Mai in very low numbers (0–9 individuals) which is the smallest distribution since 2015 and the lowest numbers since they were first discovered in the site. YCA appear to have returned to the distribution from 2010; they are found in highest abundance only in the north-east of the forest.

On the other hand, the results showed the highest numbers of YCA in Fond Peper since pitfall surveys began. The substantial decrease of YCA in the Vallée de Mai strongly suggests that the two fipronil treatments this year were highly effective in reducing YCA numbers and bolsters our confidence in further reducing them with another baiting deployment in March.

The Fond Peper survey shows us how abundant YCA could have been had no action been taken to arrest their increasing population. It also highlights the potential for rapid reinvasion if control efforts are not sustained. The high numbers of YCA in Fond Peper are also of major conservation concern given the site’s importance for endemic caecilians, geckos, skinks, black parrots, snails, slugs and many other invertebrates.

Populations of other invasive alien ants such as Technomyrmex albipes, Nylanderia bourbonica and Odontomachus simillimus have also decreased substantially since baiting started in 2019. This is noteworthy as if we manage to locally eradicate YCA in the future, the absence of a competitor may make way for another ant species to invade Vallée de Mai, or indeed allow native ants to return.

Contributed by SIF