Reproduction in Risky Environments: The Role of Invasive Egg Predators in Ladybird Laying Strategies

Abstract

Reproductive environments are variable and the resources available for reproduction are finite. If reliable cues about the environment exist, mothers can alter offspring phenotype in a way that increases both offspring and maternal fitness (‘anticipatory maternal effects’—AMEs). Strategic use of AMEs is likely to be important in chemically defended species, where the risk of offspring predation may be modulated by maternal investment in offspring toxin level, albeit at some cost to mothers. Whether mothers adjust offspring toxin levels in response to variation in predation risk is, however, unknown, but is likely to be important when assessing the response of chemically defended species to the recent and pervasive changes in the global predator landscape, driven by the spread of invasive species. Using the chemically defended two-spot ladybird, Adalia bipunctata, we investigated reproductive investment, including egg toxin level, under conditions that varied in the degree of simulated offspring predation risk from larval harlequin ladybirds, Harmonia axyridis. H. axyridis is a highly voracious alien invasive species in the UK and a significant intraguild predator of A. bipunctata. Females laid fewer, larger egg clusters, under conditions of simulated predation risk (P+) than when predator cues were absent (P-), but there was no difference in toxin level between the two treatments. Among P- females, when mean cluster size increased there were concomitant increases in both the mass and toxin concentration of eggs, however when P+ females increased cluster size there was no corresponding increase in egg toxin level. We conclude that, in the face of offspring predation risk, females either withheld toxins or were physiologically constrained, leading to a trade-off between cluster size and egg toxin level. Our results provide the first demonstration that the risk of offspring predation by a novel invasive predator can influence maternal investment in toxins within their offspring.

Invasive Egg Predators and Food Availability Interactively Affect Maternal Investment in Egg Chemical Defense

“…One explanation may be found in the trend for a greater number of eggs to be laid in the presence of predator cues, reflecting results from previous studies where A. bipunctata laid a larger number of eggs in response to the same stimulus (Paul et al, 2015). This suggests that females may have been constrained from any concomitant increase in investment in egg alkaloid level (Smith and Fretwell, 1974), resulting in the lack of increase in egg alkaloid level when aphids were absent but predator cues were present (i.e., P+/A−).…”

“…The A. bipunctata eggs used for the trial were collected from culture and frozen at −80 • C for 12 months. Quantification of egg (-)-adaline content is destructive, but within-female repeatability of egg (-)-adaline content and egg mass is high (Paul et al, 2015). The values of egg (-)-adaline and egg mass from previously analyzed eggs (Paul et al unpublished data) were therefore used as proxy values for the females that laid them and used to select remaining unanalysed eggs for the cannibalism trials.…”

Effect of natural predators on Queensland fruit fly, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae) control by sterile insect technique (SIT)

“…For example, fruit fly ( Drosophila sp.) and ladybird beetle (Adalia bipunctata) females altered timing of egg laying and total egg number under conditions of simulated predation risk . Similarly, we found that female Q‐flies laid fewer eggs when predators were present.…”

“…The presence of predators is known to influence the egg‐laying behaviour of a number of prey species . For example, fruit fly ( Drosophila sp.)…”

“…The presence of predators is known to influence the egg-laying behaviour of a number of prey species. [58][59][60] For example, fruit fly (Drosophila sp.) and ladybird beetle (Adalia bipunctata) females altered timing of egg laying and total egg number under conditions of simulated predation risk.…”