Species-specific reduction of survival and aphid consumption in temperate ladybirds exposed to prolonged and intensified heat waves
Description
In this study, we tested how insect predators responded to various heat wave regimes (prolonged duration, increased temperature), derived from realistic meteorological data. In five ladybird species (Coccinella septempunctata, Ceratomegilla undecimnotata, Harmonia axyridis, Hippodamia variegata and Propylea quatuordecimpunctata), survival was monitored over 12 days and aphid consumption was measured every 3 days.
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We established three temperature regimes to simulate current and projected summer conditions in Central Europe based on realistic meteorological data. The ‘control’ temperature regime equals to average ambient July temperatures (ranging from 13-27°C daily), recorded between 2014 and 2023 by the meteorological station in Prague-Ruzyně (Crop Research Institute). The ‘heat wave’ regime (ranging from 17-39°C daily) represents the average heat wave temperatures recorded during the same 10-year period (i.e., we used several days with the highest temperature for each year and averaged those days). The ‘extreme heat wave’ regime represents the same temperature course as the ‘heat wave’ regime, but temperatures were increased by 5°C (ranging from 22-44°C daily). For all regimes, we used computer-instructed climatic chambers that adjusted the temperature in 1-hour steps and were set to long-day photoperiod 16L: 8D, which is very close to natural summer conditions in Central Europe. To avoid heat shock from abrupt temperature fluctuations during the manipulation of ladybirds (see below), temperature in the climatic chambers room was set to 28°C, which is close to late morning and evening temperatures in the ‘heat wave’ temperature regime. During the experiment, survival was monitored twice a day over a 12-day period. Every day at 7 a.m., we monitored ladybird survival and provided them with water through a small wet tissue. At 7 p.m., we monitored ladybird survival again, rehydrated the tissue, and fed ladybirds with fresh Ephestia kuehniella eggs and small pieces of apple. We cleaned the Petri dishes and changed the wet tissues every three days. In addition, we assessed predation rates every three days. Thus, data on aphid consumption was collected in three occasions (days 3, 6, and 9), resulting in different heat wave durations from the beginning of the experiment, to assess both the immediate and cumulative effects of heat waves on each species performance. To measure aphid consumption, we prepared Petri dishes containing 10 live aphids (Hyalopterus pruni) of similar size, and then 16 ladybirds were tested simultaneously. On each day, 8 individuals of each temperature regime and species combination (4 individuals per sex) were placed in a Petri dish with the aphids, and predation was recorded through direct 15-minute observations. During this time, we counted the number of aphids consumed and also measured handling time, defined as the time it takes a ladybird to consume an aphid, from capture to completion of feeding. Only the handling time of the first aphid consumed in each trial was included in the analysis. For the remaining 8 ladybirds, predation was measured indirectly. These individuals were also placed in separate Petri dishes with 10 aphids, and after 15 minutes, we counted the number of aphids left to estimate the number consumed. In total, 240 ladybirds were repeatedly measured in the behavioral experiment.
Institutions
- Ceska Zemedelska Univerzita v PrazePraha, Praha