Rainfall and host reproduction regulate population dynamics of a specialist seed predator

Abstract

1. Mast seeding is a widespread resource pulse caused by synchronized and intermittent production of a large seed crop by plant populations. The effects of masting on wildlife have been well documented in granivorous vertebrates, but less is known about its impact on population dynamics of insects.2. This study investigated, over 6 years, variation in abundance of a specialist weevil (Curculio elephas) preying on holm oak (Quercus ilex) acorns.3. An immediate bottom‐up effect of seed production on weevil larval abundance was detected, which was driven by an increase in realised fecundity and aggregation at seed‐rich trees. Moreover, trees producing on average more and larger acorns sustained larger weevil populations. However, no correlation was found between current and previous year adult abundance, suggesting that C. elephas did not capitalise on the reproductive bottom‐up effect.4. It was rainfall, not masting, that most strongly shaped the temporal variation in insect abundance. Rainfall facilitates emergence after diapause at underground earth cells and was also responsible for among‐tree synchronisation in adult weevil population dynamics.5. In spite of their trophic specialisation, not only food availability, but also weather affects weevil numbers. The present results indicate that moving beyond bottom‐up effects is required to better understand complex systems that involve masting plants and insects that consume their seeds.

The specialist of a specialist: the natural history of the predispersal seed predator weevil Hemicolpus abdominalis (Coleoptera: Curculionidae)

“…Post & Forchhammer, 2008). Considering such an isolated Cerrado patch, it is possible that even small environmental fluctuations could affect the population density and, therefore, the species persistence in this area (Lande, 1993;Fagan et al, 2001;Bogdziewicz et al, 2020). Since fruit set in T. formosa is entirely dependent on pollination by long-tongued hawkmoths, any environmental factor negatively impacting the pollinator fauna (Amorim et al, 2014) would also indirectly affect the weevil population density by decreasing the fruit availability.…”

“…The population dynamics of the host plant may also affect the insect population dynamics and its long-term persistence (e.g. Gripenberg & Roslin, 2005;Bogdziewicz et al, 2020). Moreover, in cases where the host plant presents a wide spatial range, the distribution and local prevalence of specialised insects will be subject to a broader range of biotic and abiotic variables.…”

Effects of Longer Droughts on Holm Oak Quercus ilex L. Acorn Pests: Consequences for Infestation Rates, Seed Biomass and Embryo Survival

“…C. elephas distribution is circum-Mediterranean, and it is very common in evergreen oak forests located in dry areas. Adults emerge from late August onwards favored by late summer storms that soften the soil surface [11,12,38] (Figure 2).…”

“…The first result from the consequences of environmental variables (e.g., rainfall, temperature) on insect fitness (e.g., insect development speed, reproduction rate, winter survival, etc.) [1,2,7,[10][11][12]. Indirect effects are generally mediated by plants and include changes in food availability, plant community and/or structural composition, phenological mismatches between insects and their host plants, etc.…”

“…Furthermore, in silvopastoral systems like the Iberian oak savannahs, these pests provoke economic losses, as acorns are a key food source for free range livestock [25]. Previous studies have shown that the lower summer rainfall predicted by climatic models may reduce weevil population sizes [11,12,26,27]. However, it remains unknown whether it will change the species composition of acorn-feeding guilds (i.e., relative prevalence of Curculio and Cydia).…”