Rainfall during multiyear La Niñas caused the decline of social wasps in Northeastern Amazonia

Greenhouse gases released into the atmosphere have triggered climate change worldwide resulting in higher average temperatures and a greater frequency of warm El Niño and cold La Niña events known as the El Niño Southern Oscillation (ENSO) (Geng et al., 2023; for ENSO values see Null, 2024). In northeastern Amazonia, including French Guiana, weather data compiled over the past 30 years show that La Niña years are correlated with extreme episodes of rainfall, lower temperatures, and less solar radiation (Dejean et al., 2011; see also Gaucherel, 2004; Ponton, 2001).

The excessive rainfall of the exceptionally long and strong 1998–2001 La Niña event (34 months) caused a major decrease in Guianese wasp diversity with 70.5% of the species no longer recorded (Appendix S1: Figure S1; Table S1). Because we noted a similarity between the outcomes of all social wasps pooled and that of Polybia bistriata (Polistinae), this species served as a biological indicator on global change in French Guiana when associated with Clusia grandiflora (Clusiaceae) whose large and thick leaves protect the wasps' nests from inclement weather (Figure 1) (Corbara et al., 2009; Dejean et al., 2010, 2011, 2022).

Here, we capitalized on a second very long La Niña episode (July 2020–February 2023; 32 months) (Null, 2024) to examine its impact on the P. bistriata nests. Via this biological indicator, we report the fate of social wasps in French Guiana over 27 years (1997–2023).

The area studied is situated along the road leading to the Petit-Saut dam (5°4′5″ N, 52°59′54″W–5°4′18″ N, 53°0′19″ W) where Clusia is one of the few plants able to grow on soil scraped down to the laterite, and so remains small. Thus, depending on the year, we monitored 93–149 Clusia for wasp nests each July between 1997 and 2023 to record individuals sheltering active P. bistriata nests in order to obtain percentages to be compared with climatic data from the Enerco405 AK automatic weather station at Paracou situated in the same forest 23.5 km from the area studied.

The best GLM model obtained, according to the AIC, contained three significant climatic variables: (1) El Niño years, (2) PRS > 2700, and (3) PDS > 300 with significant effects (p = 1.02e⁻³, p = 9.07e⁻⁵, p = 3.89e⁻⁴, respectively, Wald test). PRS > 2700 had a negative effect, while El Niño years and PDS > 300 had a positive effect on the number of P. bistriata nests (Figure 2b). The other climatic variables degraded the AIC and were not significant.

A time series analysis noted an increase in the mean level of wasp nests by 0.15 in 2009 that coincides with a weak La Niña and the beginning of the 2009–2010 El Niño event, and confirmed three declines, each occurring during multiyear La Niña episodes. The first arose in 2000 (lowered the mean level by 0.3), the second in 2012 (lowered the mean level by 0.1), and the last in 2022, but its effect, although visible, is blurred by the declining number of wasp nests (Figure 2c).

Therefore, the GLM showed that the percentages of Clusia sheltering P. bistriata nests followed only roughly the ENSO variations mostly due to the influence of the Atlantic on rainfall and to the fact that La Niña or El Niño events can straddle two calendar years (Figure 2d). This was the case for the increase in wasp nests occurring in 2009 during both La Niña and El Niño events noted by the time series analysis (Figure 2c). Yet, it is noteworthy that more than 300 mm of rainfall during the dry seasons was beneficial to P. bistriata and perhaps to social wasps as a whole.

We also noted that among the nine declines in the number of wasp nests recorded using the GLM, the time series analysis confirmed only three that occurred during the 1998–2001, 2010–2012, and 2020–2023 La Niña events, the effect of the last one being less visible due to the low number of wasp nests remaining after 2012. As a result, only a few P. bistriata nests were recorded during the 2020–2023 La Niña years so that the percentages of Clusia sheltering a wasp nest were below 1% despite our choosing this wasp–plant association as a biological indicator. During this period, other wasp species were very difficult to find. As an example, in the Petit-Saut area before the 1998–2001 La Niña event, we recorded 424 nests belonging to 60 wasp species in 1997 along ≈5 km of forest edges, whereas these values declined to only 97 nests and 16 species in 2002 (Appendix S1: Table S1). Consequently, the decrease in the P. bistriata population is due to heavy rainfall whose threshold of more than 2700 mm was reached eight times out of nine during the La Niña years (Figure 2). Indeed, as a consequence of heavy rainfall during the extreme rainy seasons, the ambient air becomes so moist inside wasp nests covered by an envelope that the brood decay, something exacerbated as the wasps continue to hunt and store prey that rot in turn. Thus, the development of bacteria, fungi, and gregarine protozoa was facilitated (Bouwma et al., 2005; Dejean et al., 2010; Reason et al., 2022).

We recorded two particularly long-lasting La Niña events. During the 1998–2001 La Niña, only one value went beyond 2700 mm (i.e., 3013 mm for the 2000 rainy season), whereas during the 2020–2023 La Niña, the rainy seasons exceeded 2700 mm three times, the highest peaking at 3746 mm. Because La Niña events are predicted to be longer and more frequent (Geng et al., 2023; Wang et al., 2023), it will be difficult for social wasp populations to survive in large areas of Northern Amazonia where La Niña is correlated with strong rainfall.

Global warming worldwide caused greater temperatures that reached an increase of ≈0.9°C between 1979 and 2023 (Figure 3), contributing to the decline of many species' populations worldwide in recent decades, including insects (Finn et al., 2023; Harvey et al., 2022). Yet, this effect was limited in French Guiana as the temperatures registered in our weather station decreased slightly between 1980 and 2023 likely due to the 2020–2023 La Niña episode so that the factor temperature is not influential in this study (Figure 3).

In conclusion, the decline of social wasps is related to the greater frequency of heavy rains that are associated with La Niña events in northeastern Amazonia so that the population of the most frequent species serving as a biological indicator was almost nil in 2023. This negative effect of excessive rainfall was also noted around Brasilia and in Brazilian Amazonia and the Atlantic Forest (Carvalho et al., 2021; Raw, 2018) and likely extends to a wider Neotropical region. Because extreme El Niño and La Niña events will become more frequent with global warming (Cai et al., 2023) and because each can be correlated with heavy rains depending on the geographical area, the long-term situation for tropical social wasps could be critical.

The authors declare no conflicts of interest.