Abstract
Understanding the characteristics and drivers of dispersal is crucial for predicting population dynamics, particularly in range-shifting species. Studying long-distance dispersal in insects is challenging, but recent advances in entomological radar offer unique insights. We analysed 10 years of radar data collected at Rothamsted Research, U.K., to investigate characteristics (altitude, speed, seasonal and annual trends) and drivers (aphid abundance, air temperature, wind speed and rainfall) of high-altitude flight of the two most abundant U.K. ladybird species (native Coccinella septempunctata and invasive Harmonia axyridis). These species cannot be distinguished in the radar data since their reflectivity signals overlap, and they were therefore analysed together. However, their signals do not overlap with other, abundant insects so we are confident they constitute the overwhelming majority of the analysed data. The target species were detected up to ∼1100 m above ground level, where displacement speeds of up to ∼60 km/h were recorded, however most ladybirds were found between ∼150 and 500 m, and had a mean displacement of 30 km/h. Average flight time was estimated, using tethered flight experiments, to be 36.5 minutes, but flights of up to two hours were observed. Ladybirds are therefore potentially able to travel 18 km in a “typical” high-altitude flight, but up to 120 km if flying at higher altitudes, indicating a high capacity for long-distance dispersal. There were strong seasonal trends in ladybird abundance, with peaks corresponding to the highest temperatures of mid-summer, and warm air temperature was the key driver of ladybird flight. Climatic warming may therefore increase the potential for long-distance dispersal in these species. Low aphid abundance was a second significant factor, highlighting the important role of aphid population dynamics in ladybird dispersal. This research illustrates the utility of radar for studying high-altitude insect flight and has important implications for predicting long-distance dispersal.
Highlights
An estimated three billion insects fly through a 1 km2 ‘window’ of sky in England during a typical summer month [1]
Data obtained from vertical-looking radar shows that ladybirds have a high propensity for high-altitude flight, with flight recorded up to 1118 m above ground level
Indirect estimates of the spread of H. axyridis in its invasive range vary from 105 km/ year in the U.K. [57] to 500 km/year in South Africa [58], but these estimates are influenced by anthropogenic factors and/or obtained from historical records, which may be incomplete [6]
Summary
An estimated three billion insects fly through a 1 km2 ‘window’ of sky in England during a typical summer month [1]. Knowledge of the characteristics (e.g. altitude and displacement speed, seasonal and annual trends) and drivers (e.g. prey abundance, environmental factors) of insect flight is crucial for estimating longdistance dispersal capability, predicting the dynamics, persistence and spread of insect populations, and has important applications in pest management and conservation [2,3,4,5,6] This knowledge is important in the case of invasive alien species (IAS) and those undergoing range shifts in response to global warming [1,2,3,4,5,6,7], since higher temperatures have been shown to drive increased migration in certain insects [8]. Studying the dispersal capability of H. axyridis is important given that this species has been linked to declines in indigenous ladybirds and is thought to present a threat to native biodiversity [11,12]
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