A review of mate‐finding Allee effects in insects: from individual behavior to population management

Abstract

Like other animals and plants, insects may find it difficult to survive and reproduce in small populations, to the extent that their long-term persistence may be jeopardized. The Allee effect is a theoretical framework that formalizes this decrease in survival or reproduction in small populations, and the resulting decrease in population growth and persistence. Mating failure in low-density populations is likely to generate an Allee effect and, therefore, has a major effect on the functioning of small populations. Here, I review mate-finding Allee effects in insect species, and their consequences for individual mating success, population dynamics, and population management. I focus, in particular, on the comparison of theoretical expectations with observational data. Several studies have reported some degree of mating failure at low density. However, almost none of the datasets available allow comparison with the predictions of classical mate-searching models. A few studies at the population level have reported the co-occurrence of mating failure at low density and a demographic Allee effect, but no study has yet clearly demonstrated a causal relationship between mating failure and lower rates of population growth. Thus, although the theoretical development of management tactics based on Allee effects is considered promising, the current lack of evidence supporting this strategy limits its potential relevance. I call here for a more rigorous approach to the study of mate-finding Allee effects and propose new approaches for this purpose.

The Life History Traits of the Arctiine MothPareuchaetes insulata, a Biological Control Agent ofChromolaena odoratain South Africa

“…Protandry presents two advantages in that it: (i) maximizes copulation opportunities for males (Wiklund & Fagerström 1977;Bulmer 1983) and; (ii) minimizes the pre-reproductive period of females because they emerge when most males are available for mating (Fagerstrom & Wiklund 1982;Carvalho et al 1998;Larsen et al 2013). In small populations (environments where mate location is difficult) and in short-lived insects such as P. insulata, reproductive asynchrony, expressed as excess protandry, its absence, or its reverse (protogyny), can lead to 'matelessness' and can be a mechanism for Allée effects which can negatively affect population growth or result in the extinction of species (Calabrese & Fegan 2004;Calabrese et al 2008;Larsen et al 2013;Fauvergue 2013). In this context, the absence of protandry or presence of protogyny in P. insulata could be a clear disadvantage for adults and could impair their mating success to the extent that their long-term persistence may be jeopardized, as females may have dispersed before the emergence of males.…”

Mass action in two-sex population models: encounters, mating encounters and the associated numerical correction

“…If the case is that individual mass implies non-negligible (relative) size, then the simulations cannot be used as we set them, and new computational experiments together with a re-assessment of the whole model have to be done. In contrast to the crude assumptions made for mass action, we refer the reader to the interesting papers (Gurarie & Ovaskainen, 2011, 2013, where the authors present a refined continuous timecontinuous space theoretical framework that shows how encounter rates depend on the interplay of spatial distributions, scales of movement, individuals densities and inherent dynamics. However, we believe the simple model presented here can inform the theoretical modelling of ephemeral (short-lived) mating encounters for insects, important in pest control management (Boukal & Berec, 2008;Fauvergue, 2013;Gordillo, 2015).…”

“…The quantitative law is deduced directly from geometrical abstractions using the mean of a Poisson process that models the number of collisions a particle receives from others and where simultaneous collisions are allowed (Hutchinson & Waser, 2007;Kauzmann, 2012). At relatively low population densities, the same idea is used as a phenomenological approach to approximate the encounter rates between males and females, with the birth rate taken proportional to the product of their densities (Bazykin, 1998;Fauvergue, 2013). We remark that two-sex population models become relevant when sexual dimorphism in vital rates is present, which has been observed in several species (Caswell, 2001).…”

Host-Parasitoid Dynamics and the Success of Biological Control When Parasitoids Are Prone to Allee Effects

“…However parasitoids released into the field often fail to establish [5], [6]. The most pervasive determinent of establishment failure is initial population size [7], [8], so that the Allee effect has been hypothesized as a likely cause of these introduction failures [7], [8], [9], [10], [11].…”

“…Several theoretical studies have investigated how mating difficulties may translate into demographic effects [19], [20]. Although firm evidence of causal relations between mate-finding Allee effects and establishment success in insect populations is still relatively rare [11], mating failure at low density is often considered to be an important cause of demographic Allee effects in invading species [21], [22], [23], [24], [25].…”