1. Numerous studies demonstrate that given the dichotomous choice of predator-free habitats vs. habitats containing predators, prey choose predator-free habitats when foraging for food or ovipositing. However, predation risk is rarely dichotomous in nature, and very few studies have assessed whether prey can quantify predation risk when selecting habitats. 2. It was shown previously that gravid females of the mosquito Culiseta longiareolata, when simultaneously offered pools with multiple choices of densities of the predator Notonecta maculata, oviposited more in the zero-predator density pools but oviposited less frequently and similarly across all other densities. This flat oviposition response across various Notonecta densities was in contrast to a decrease in mosquito immature survival with increasing Notonecta density. Here, we reconsider this question with the same species but with a different experimental design; rather than experimentally assessing multiple predator densities simultaneously, we offered only pairwise choices on any given night. Specifically, we offered ovipositing Culiseta females all pairwise combinations from no, low and high predation risk (0, 1 and 4 Notonecta per pool). 3. Overall oviposition was lower when mosquitoes could only choose pools containing Notonecta (1 or 4). In all pairwise comparisons, more females chose pools of lesser predation risk. Thus, gravid females of this species, and probably many other species, can quantify predation risk, and not only assess presence or absence of predation risk, when choosing oviposition sites. 4. This is the first demonstration that an ovipositing female of any species can quantify risk of predation. We suggest, based on statistical and behavioural factors, that pairwise comparisons, and not simultaneous multiple-choice experiments, are the experimental design of choice to adequately test this ability.

Individuals of different sex, size or developmental stage can compete differently and hence contribute distinctively to population dynamics. In species with complex life cycles such as insects, competitive ability is often positively correlated with larval developmental stage. Yet, little is known on how the development and survival of early-instars is influenced by interference from late-instar larvae, especially at low densities when exploitative competition is expected to be negligible. Furthermore, the specificity and mechanisms by which interference competition operates are largely unknown. We performed two complementary experiments aiming to quantify the competitive effects of late instar Ochlerotatus caspius on early instar larvae at low densities and under high resource supply rate. The first experiment examined the net effect of interference by 4^th on 1^st instar O. caspius larvae, relative to the effect of 1^st instars on themselves. The second experiment examined the effect of species-specific, non-physical interference competition (i.e., cage larvae) by 4^th on 1^st instar O. caspius larvae at low or high densities. Specifically, we compared the responses of O. caspius larvae raised in the presence of caged con- or hetero-specific, Culiseta longiareolata, with that of larvae in the empty-cage control group. As expected, interference from late instar larvae had a net negative effect on the development rate of first instars. In contrast, the presence of caged con-specifics (non-physical interference) accelerated the development rate of O. caspius, however, this pattern was only evident at the low density. Notably, no such pattern was detected in the presence of caged hetero-specifics. These results strongly suggest the existence of species-specific growth regulating semiochemicals.