Plants and animals use a wide variety of environmental cues for their biological activities, such as flowering, migration, navigation, foraging, hibernation, aestivation among others1–8. Daylight and ambient temperature from the Sun are critical for many biological activities of organisms and behavior in many animals 4,6–7,9. Daylight is a major abiotic predictor of blooming in plants 10. Some animals use the Sun’s magnetic field for migration, foraging and navigation. For instance, bees use the Sun’s position for their diurnal movement and navigation between nests and foraging grounds11. The nocturnal bees use the dim light of the Moon and the Stars for their daily navigation 12–15. Large-scale spatial or astronomical factors, such as solar and lunar eclipse, despite are rare, can affect the environment and biological activities of animals 5,16. Because these events are rare, their effects on bioactivities of animals and plants have been investigated less.
The solar eclipse occurs when the Moon gets between the Earth and the Sun, and the Moon casts a shadow over the Earth 17. Its magnitude can be varied considerably based on the angular diameter of the moon and the sun that are coinciding at their centers18. It lasts for a few minutes to many hours, but the duration of annular event can be lasted only up to a few minutes. During the solar eclipse, the air temperature and light intensity drop 7,9,19–20.
Because organisms’ response to weather conditions (especially temperature and light) is different among species, their response to solar eclipse can be different 7,9. Behavioral responses to total solar eclipse have been observed in fishes 21, copepods 22, seabirds23, antelope 16, and cattle 24. Animals responded differently during the annular and total solar eclipses. For instance, the antelope ground squirrels’ locomotor activities increased during partial solar eclipse25; Hamadryas baboons, on the other hand, were less active during solar eclipse26; the captive chimpanzees took a gesture towards the sun during the annular solar eclipse27; European ground squirrel was non-responsive during partial eclipse28; hippopotamus and many birds in Mana Pools National Park behaved as though it was dusk, and baboons stopped feeding5. Wild chimpanzees, however, showed no change in their foraging behavior during an annular solar eclipse in Mahale Mountains National Park in Tanzania29. In the Phylum Arthropoda, the effect of solar eclipse has been studied in desert cicadas30, orb-weaving spiders4 and crickets, katydids, butterflies, moths, and honeybees 31. The latter study found that the activity of crickets, katydids, butterflies, moths and honeybees went abnormal during the 1932 total solar eclipse.
Bees are generally sun-loving insects and use the Sun for navigation32. While most bees opt to fly during bright sunny warm days, some manage to fly in very dim light conditions—including moonlight and even starlight12–13,33−35. Even though some standard and informal studies on the flight activities of Apis spp has been carried out during the period of solar eclipse 9,36–39, limited information is available on the effect of solar eclipse on pollinators in foraging ground (but see, Galen et al. 2018). Galen et al.9 found that the flight activity of honeybees, bumblebees, and some solitary bees dropped suddenly during the totality phase of the 2017 annular solar eclipse in the U.S. Divan37 showed that the stingless bees reduced their foraging activity and disoriented during the totality phase of solar eclipse in Karnataka. In Dehradun, the activity of Apis dorsata was not affected during a low magnitude partial solar eclipse occurred during 0737 h to 0920 h36. Apis mellifera stopped visiting foraging ground during the annular phase of solar eclipse38.
On December 26, 2019, parts of south India experienced an annular solar eclipse (Fig. 1). It had a magnitude of about 97% 18. The annular event of the eclipse was visible from parts of Saudi Arabia to parts of Indonesia through parts of south India (Fig. 1). In Kerala, the annular event was total and visible in and around Cheruvathur (12.18 N; 75.14E) in Kasaragod district with its highest magnitude. The annularity in the west coast of India, where the study took place, was during 0925-0930h. However, the region had experienced partial eclipse during 0805h through 1130h Indian Standard Time (Fig. 2). Our observations were made in Kannapuram and south Kanhangad, which located about 15 km south and north of Cheruvathur as a crow flies, respectively. The study sites were non-cloudy and non-windy on the day of eclipse and during our observation. The darkness experienced by the sites was only during the eclipse period.
In this study, we report the effect of solar eclipse on the visitation rate of bees to four plant species – Cleome rutidosperma (Cleomaceae), Hygrophila schulli (Acanthaceae), Mimosa pudica (Fabaceae), and Urena sinuata (Malvaceae).