Abstract
Despite its deleterious impact on farming and agriculture, the physiology and energetics of insect migration is poorly understood due to our inability to track their individual movements in the field. Many insects, e.g. monarch butterflies, Danaus plexippus (L.), are facultative migrants. Hence, it is important to establish whether specific insect populations in particular areas migrate. The polyphagous insect, Helicoverpa armigera (Hübner), is especially interesting in this regard due to its impact on a variety of crops. Here, we used a laboratory-based flight mill assay to show that Helicoverpa armigera populations clearly demonstrate facultative migration in South India. Based on various flight parameters, we categorized male and female moths as long, medium or short distance fliers. A significant proportion of moths exhibited long-distance flight behavior covering more than 10 km in a single night, averaging about 8 flight hours constituting 61% flight time in the test period. The maximum and average flight speeds of these long fliers were greater than in the other categories. Flight activity across sexes also varied; male moths exhibited better performance than female moths. Wing morphometric parameters including forewing length, wing loading, and wing aspect ratio were key in influencing long-distance flight. Whereas forewing length positively correlated with flight distance and duration, wing loading was negatively correlated.
Highlights
Migratory insects undertake journeys ranging from a few meters to thousands of kilometers over land and water [1]
Not much difference was observed with respect to average and maximum flight speed among the moths tested (Fig 3A and 3B)
Time spent in flying by female longrange fliers exceeded medium-range and short-range fliers (Fc (2, 51) = 80.93, P < 0.01)
Summary
Migratory insects undertake journeys ranging from a few meters to thousands of kilometers over land and water [1]. Facultative migration in Helicoverpa armigera noctuid moths regularly migrate between their summer and winter ranges separated by thousands of kilometers, undertaking nocturnal flights at altitudes of hundreds or thousands of meters [9,10,11,12]. These migrations are either obligate (i.e. independent of environmental factors and in habitats that support a single generation; [13]), or facultative (i.e. mainly depending on environmental cues experienced during development; [14, 15]). Insects are readily amenable to experimental manipulation and their migration can be studied using laboratory-based assays
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