The importance of illumination level in the daily onset of flight activity in nocturnal moths

Abstract

ABSTRACT. The daily onset of flight activity in nocturnal moths from Denmark and Florida was investigated in natural and experimental reductions of illumination. The daily cycle of illumination is described, especially the variation of the rate of change during the evening. Different species commence activity (‘onset’) at different mean light intensities. These means occur around the termination of twilight, at between 0 and –2.50 log lux. In a given species the illumination at mean onset is constant, and the dispersion of onsets in the population is constant with respect to the illumination. As a consequence, the dispersion with respect to time increases, the longer the duration of twilight. The temporal dispersion, therefore, depends upon season and latitude. A model describing the number of onsets in relation to the decrease of illumination is presented. At the physiological level it is proposed that a preparatory process precedes the actual onset of activity. This is observed as a latency between the light change and the activity onset. It is presumed to represent a change, from an inhibiting to a disinhibiting output from a controlling CNS centre, brought about by the reduction in illumination increasing the sensitivity of the centre to a ‘sensitizer’. This might represent the Zeitgeber effect of illumination reduction. The results indicate that during a gradual decrease of illumination, the latency process is initiated by a certain low illumination and that it proceeds further, the lower the illumination. The faster the rate of decrease, the faster is the rate of the process, so that it is completed at about the same time as a threshold illumination for overt activity is reached, irrespective of the rate of decrease. However, at very fast reductions of illumination, the physiological process cannot keep pace with the reduction, and activity onset occurs at a lower illumination than normal.

Observations of the flight behaviour of the army worm moth, Spodoptera exempta, at an emergence site using radar and infra‐red optical techniques

“…This irradiance value was very similar t o the -2~ lo-' Wm-' nm-' found in our 1979 study (Riley et al, 1981). In photometric units appropriate for human vision, this illumination corresponds t o about 2.0 lux, and is close to the value for the onset of flight in Deilephia porcellus and Noctua (=Tryphaena) pronuba moths (Dreisig, 1980). Dawn take-off occurred at much lower and more varied irradiance levels ( Wm-2 nm-' was reacheda value not disimilar t o that at which flight was initiated at dusk.…”

Separating selection by diurnal and nocturnal pollinators on floral display and spur length in Gymnadenia conopsea

“…This could reflect a decreasing relative contribution from nocturnal pollinators with increasing latitude, as low nocturnal temperatures may limit insect activity, and light conditions during night and day become increasingly similar. The daily onset of flight activity in nocturnal moths often coincides with the termination of twilight (Dreisig 1980), and it has been shown that illumination in the subarctic is continuously at levels that normally inhibit the activity of moths (Dreisig 1981). Experimental manipulations of the pollination environment along a latitudinal gradient could reveal whether the relative contribution from diurnal and nocturnal visitors varies with latitude, or in a more mosaic pattern in response to variation in the composition of the pollinator community within different geographic regions.…”

The impact of artificial light at night on nocturnal insects: A review and synthesis

“…ALAN may also prolong the foraging activity of diurnal and/or crepuscular competitors, which generally become more active with increasing illumination (Kempinger, Dittmann, Rieger, & Helfrich‐Forster, ; Kronfeld‐Schor et al, ). Initiation of foraging activity in moths (Dreisig, ; Eaton, Tignor, & Holtzman, ; Riley, Reynolds, & Farmery, ) and crickets (Campbell, ) is triggered when ambient light intensity declines to a species‐specific level. Foraging activity of nocturnal and crepuscular bees is both initiated and inhibited by specific light levels (Dyer, ; Kelber et al, ).…”