Cannibalism in Two Subtropical Lady Beetles (Coleoptera: Coccinellidae) as a Function of Density, Life Stage, and Food Supply M. H. Bayoumy & J. P. Michaud
Journal of Insect Behavior ISSN 0892-7553 J Insect Behav DOI 10.1007/s10905-015-9510-8
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Author's personal copy J Insect Behav DOI 10.1007/s10905-015-9510-8
Cannibalism in Two Subtropical Lady Beetles (Coleoptera: Coccinellidae) as a Function of Density, Life Stage, and Food Supply M. H. Bayoumy 1 & J. P. Michaud 2
Revised: 20 March 2015 / Accepted: 11 June 2015 # Springer Science+Business Media New York 2015
Abstract Cannibalism is an important factor influencing both immature survival and adult reproductive success in aphidophagous lady beetles. This study employed three series of laboratory experiments to characterize life stage-specific cannibalism responses of Coccinella undecimpunctata L. and Cydonia vicina nilotica Mulsant to 1) different conspecific densities, with and without prey, 2) other life stages, and 3) various densities of prey, Myzus persicae Sulzer. All larval instars of both species cannibalized more in the absence of prey than in its presence at all conspecific densities, but in general, cannibalism increased with conspecific density only in the absence of prey, and more strongly for third and fourth instar C. undecimpunctata than for their C. vicina nilotica counterparts. Adults contributed the most cannibalism of any life stage, and eggs were the most vulnerable. In addition to cannibalizing their own and earlier instars, second and third instar C. undecimpunctata sometimes cannibalized third and fourth instars, respectively, and fourth instars occasionally ate pupae. Larvae of C. vicina nilotica were only preyed upon by the same or later stages and pupae, by adults, not fourth instars. A relative vulnerability index was calculated for each life stage based on its net vulnerability to cannibalism by all life stages and plotting these indices revealed species-specific patterns of diminishing vulnerability to cannibalism as a function of life stage. Relative species vulnerability to cannibalism, considering all life stages, was higher for C. undecimpunctata (0.55) than for C. vicina nilotica (0.45). Finally, linear regression was used to characterize the change in propensity for cannibalism between same-instar larval pairs as a function of prey density, which enabled determination of a theoretical upper prey threshold for each larval instar, i.e., the prey
* J. P. Michaud
[email protected] 1
Faculty of Agriculture, Economic Entomology Department, Mansoura University, Mansoura 35516, Egypt
2
Agricultural Research Center-Hays, Kansas State University, 1232 240th Ave, Hays, KS 67601, USA
Author's personal copy J Insect Behav
density beyond which no cannibalism would be expected. In both species, regressions for third and fourth instars did not intercept the X-axis, suggesting that some cannibalism by these stages was inevitable within the range of prey densities tested. Keywords Coccinella undecimpunctata . Cydonia vicina nilotica . density-dependence . Myzus persicae . relative vulnerability . size disparity
Introduction Cannibalism, the killing and eating of conspecifics, has profound implications for the population dynamics of many species (Rudolf 2008), especially predators (Fox 1975; Polis 1981; Elgar and Crespi 1992; Persson et al. 2003). Because conspecifics share common resources, cannibalism represents a form of intraspecific interference competition that may or may not yield additional fitness benefits in terms of improved survival, growth and development (Ho and Dawson 1966; Osawa 1992; Michaud and Grant 2004; Hofmann and Kia-Hofmann 2012), or adult reproductive performance (Al-Zubaidi and Capinera 1983; Sonleitner and Guthrie 1991). The most obvious driver of cannibalism behavior is immediate food scarcity (Polis 1981), although it can be also observed when spatial resources are limiting, such as feeding sites (e.g., Story and Robinson 1979; Hart 1987) or overwintering sites (e.g., Hatchett et al. 1975; Tarpley et al. 1993). There are also potential costs of cannibalism (Nishimura and Isoda 2004), including the loss of inclusive fitness if relatives are consumed (Agarwala and Dixon 1993; Joseph et al. 1999) and the risk of injury or disease transmission (Elgar and Crespi 1992; Pfennig et al. 1998). However, the ubiquity of cannibalism has led many to infer that the benefits of eliminating a competitor while simultaneously obtaining a meal are usually more than sufficient to offset these costs (Polis 1981; Elgar and Crespi 1992). At the individual level, opportunities for cannibalism will vary with conspecific encounter rates, and the latter serve as a reliable indicator of the intensity of conspecific competition. Thus, local density of conspecifics is usually a key factor influencing cannibalism rates (Nakakita 1982; Cottrell and Yeargan 1998a; Michaud 2003). The eggs of lady beetles (Coleoptera: Coccinellidae) are a highly nutritious food source and, consequently, frequent targets of both cannibalism and intraguild predation (Pienkowski 1965; Cottrell and Yeargan 1998b; Michaud and Belliure 2000; Osawa 2002; Cottrell 2005; Michaud and Jyoti 2007). Gravid coccinellid females will sometimes lay eggs for their own immediate consumption when short of food (Ponsonby and Copland 1998; Santi and Maini 2007; Ferrer et al. 2011). Neonate larvae can derive significant physiological benefits by cannibalizing sibling eggs that are either infertile or late to hatch (Brown 1972; Osawa 1992, 2002) and it has been suggested that females may manipulate opportunities for egg cannibalism within clutches so as to maximize their own fitness (Michaud and Grant 2004; Perry and Roitberg 2005). Other studies have demonstrated preference for, or greater suitability of, conspecific versus heterospecific coccinellid eggs (Michaud 2002; Cottrell 2004; Ware et al. 2008). Since cannibalism is one of the primary sources of coccinellid egg mortality in field studies (e.g., Michaud and Belliure 2000; Osawa 2011) females avoid oviposition on plants with larval residues (Doumbia et al. 1998; Ruzicka 2003; Michaud and Jyoti 2007; Martini et al. 2009). Eggs and young coccinellid larvae are also vulnerable to cannibalism by adults (Agarwala and
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Dixon 1992; Schellhorn and Andow 1999; Cottrell 2005) and larval coccinellids are well known to cannibalize one another in interactions that are usually asymmetric (large eating small) and density-dependent (Michaud 2003; Pervez et al. 2006). Because cannibalism in coccinellids tends to be size-structured among larvae, and stage-structured among life stages of differential vulnerability, a thorough analysis of the behavior should consider all possible interactions between life stages while varying food availability and conspecific density as independent factors. Although cannibalism has been reported and studied in many species of Coccinellidae (Hodek and Evans 2012), it has not yet been explored in Coccinella undecimpunctata L. or Cydonia vicina nilotica Mulsant, two species valued for their contribution to biological control of aphids in agricultural crops of Egypt and other countries in northern Africa and the Middle East. The present study had three objectives. The first was to quantify levels of larval cannibalism in each instar as a function of larval density and test for interactions with availability of prey, Myzus persicae (Sulzer). The second was to estimate life stagespecific propensities and vulnerabilities to cannibalism by observing pairwise interactions among all life stages in the absence of prey in order to calculate a life stage-specific index of vulnerability to cannibalism, and then use these values to calculate a relative vulnerability index for each species. The third was to determine how cannibalism between sameinstar larval pairs would vary as a function of prey density.
Materials and Methods Insect Colonies Adults of C. undecimpunctata and C. vicina nilotica were collected from a peach orchard infested with green peach aphid at Mansoura University, Egypt in November 2012. Stock colonies were held in a climate-controlled growth room at 20.0±1.0 °C, a photoperiod of 16:8 (L:D) h, and 75±15 % RH. Green peach aphids, M. persicae, were reared on potato, Solanum tuberosum L. in a greenhouse to serve as prey for both stock colonies and experiments. Pairs of beetles (n=30 of each species) were isolated in plastic Petri dishes (6.0 cm diam × 2.0 cm ht) and provisioned ad libitum with a diet of M. persicae for 3 days, whereupon males were transferred to separate dishes and females permitted to oviposit in isolation. Eggs were collected daily and, once every 3 days, males were reintroduced to females for a six h period to maintain fertility. After eclosion, first instar larvae were isolated in Petri-dishes (as above) and supplied with ad libitum aphids refreshed daily until they pupated. Due to some field hybridization between the subspecies Cydonia vicina isis Crotch (black elytra) and C. vicina nilotica (tan elytra), an additional generation was bred so that all individuals of the former (bearing the dominant allele for melanism) could be removed and the experiments performed only with tan individuals. Maternal lineages were tracked to ensure that no individuals were mated with siblings. Stage-Specific, Density-Dependent Larval Cannibalism To examine how larval cannibalism responds to changes in density (i.e., conspecific encounter rate), newly-molted larvae (
