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Size, not temperature, drives cyclopoid copepod predation of invasive mosquito larvae

Citation

Russell, Marie; Qureshi, Alima; Wilson, Christopher; Cator, Lauren (2021), Size, not temperature, drives cyclopoid copepod predation of invasive mosquito larvae, Dryad, Dataset, https://doi.org/10.5061/dryad.j3tx95x9p

Abstract

During range expansion, invasive species can experience new thermal regimes. Differences between the thermal performance of local and invasive species can alter species interactions, including predator-prey interactions. The Asian tiger mosquito, Aedes albopictus, is a known vector of several viral diseases of public health importance. It has successfully invaded many regions across the globe and currently threatens to invade regions of the UK where conditions would support seasonal activity. We assessed the functional response and predation efficiency (percentage of prey consumed) of the cyclopoid copepods Macrocyclops albidus and Megacyclops viridis from South East England, UK against newly-hatched French Ae. albopictus larvae across a relevant temperature range (15, 20, and 25ºC). Predator-absent controls were included in all experiments to account for background prey mortality. We found that both M. albidus and M. viridis display type II functional response curves, and that both would therefore be suitable biocontrol agents in the event of an Ae. albopictus invasion in the UK. No significant effect of temperature on the predation interaction was detected by either type of analysis. However, the predation efficiency analysis did show differences due to predator species. The results suggest that M. viridis would be a superior predator against invasive Ae. albopictus larvae due to the larger size of this copepod species, relative to M. albidus. Our work highlights the importance of size relationships in predicting interactions between invading prey and local predators.

Methods

Tire data:

We measured the temperature of rainwater collected in six used car tires that were divided between an urban London area and a nearby suburban area. These temperature data provided rough guidelines for our choice of the three temperatures to be tested in our functional response and predation efficiency experiments.

Functional response data:

Adult non-gravid female copepods, identified by larger relative size, were removed from their culture and each was placed in a Petri dish (diameter: 50 mm, height: 20.3 mm) holding 20 mL of spring water. The copepods were placed in three different controlled environments set to 15 ± 1, 20 ± 1, and 25 ± 1°C, all at a 12:12 light/dark cycle, to begin a 24 h starvation and temperature-acclimation period for the predators. Each combination of copepod species (M. albidus or M. viridis) and temperature had 28 copepods, each one held in its own Petri dish; there were 168 copepods in total. 

Groups of 87 newly-hatched Ae. albopictus larvae were each pipetted into a small plastic tub containing 150 mL of spring water to strongly dilute any residual food from the hatching media. Each tub of 87 larvae was then split into seven Petri dishes each containing 20 mL of spring water and the following larval densities: 1, 2, 4, 8, 16, 24, and 32. All larvae were counted into dishes at room temperature and then divided into the three different controlled environments, allowing a 12 h temperature acclimation period before predator introduction. For each of six combinations of copepod species and temperature, there were 35 Petri dishes containing a total of 435 newly-hatched Ae. albopictus larvae, with a total of 2,610 larvae used across all predator treatments and controls. This sample size allowed for four replicates of each larval density and one predator-absent control at each density, for each combination of copepod species and temperature.

The next day, the copepod predators held at three different temperatures were introduced to larval Petri dishes of matching controlled temperatures. The copepods were removed after a 6 h period of predation, which follows the schedule of similar experiments. Each copepod was preserved in 80% ethanol so that its body size could later be measured and matched to the larval count data. Immediately following the removal of the copepods, the number of surviving larvae in each Petri dish was counted and recorded. In addition, the body lengths of the 10 gravid females selected from each species of copepod, as well as the body lengths of all copepods included as predators in the functional response experiments, were measured from the front of the cephalosome to the end of the last urosomite.

Predation efficiency data:

Ae. albopictus larvae were hatched, and any residual food was diluted in spring water following the same procedure used for the functional response experiments. Adult non-gravid female copepods were each placed in a Petri dish for a 24 h period of starvation and acclimation to three different temperature settings: 15 ± 1, 20 ± 1, and 25 ± 1°C, all at a 12:12 light/dark cycle. The largest non-gravid copepods of each species were selected to minimize the risk of selecting males or immature stages. The Petri dishes containing larvae were split into the three different temperature settings 12 h prior to the introduction of copepod predators, and there was a 6 h period of predation. At the end of the 6 h period, the copepods were removed, and each was stored individually in 80% ethanol so that predator body lengths could later be measured. The number of surviving larvae in each Petri dish was recorded immediately after removing the copepods.

At each of the three temperature settings, there was a total of 24 Petri dishes, each containing 24 larvae (1,728 larvae across all temperatures); the 24 dishes were divided into three groups (n = 8): one with M. albidus predation, one with M. viridis predation, and one as a control. Every predator treatment was matched to a control that had been held at the same temperature. Each temperature setting had a total of 192 control larvae. A total of 24 M. albidus and 24 M. viridis copepods were used in this experiment.

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