John Bowden

An analysis of factors affecting catches of insects in light-traps

Analysis of published data on catches of insects in light-traps with a variety of light sources and of different designs showed that all conformed to the previously proposed model describing the functioning of a light-trap: catch = constant × where W = trap illumination and I = background illumination. Light-trap catches in differing cloud conditions and in open and woodland situations also varied as predicted by the model. A table of correction factors for different amounts of cloud cover is provided. The results are discussed in relation to use of light-traps and interpretation of light-trap data.

The influence of moonlight on catches of insects in light-traps in Africa. Part II. The effect of moon phase on light-trap catches

Nightly light-trap catches of insects, covering periods of 2–5 years, from two sites in Africa within 10° of the equator are examined in relation to the regular changes in night illumination of the lunar cycle. For several species average log catches at different phases of the moon are almost linearly related to (log) night illumination, catches of some species, such as Isoptera and Bostrychidae, increasing and of others, such as Marasmia trapezalis (Gn.) (Pyralidae), Lampyridae and Dorylus spp. (Formicidae), decreasing with moonlight. Relative catches of M. trapezalis and Bostrychidae varied by a factor of 30:1 between no moon and full moon. Analysis of whole-night catches gives some evidence on the pattern of insect activity through the night, identifying Syntomis monothyris (Hmps.) (Ctenuchidae) and Stemorrhages sericea (Dru.) (Pyralidae) in particular as early morning fliers. Evidence on how night illumination affects catch, and on the times of night when illumination has most effect, is consistent for the two sites and for different years. However, any adjustment of nightly catches to those expected under standard conditions of illumination can only be approximate. Although most of the differences between catches at different moon phases are accounted for by night illumination, many factors influence catch on an individual night, and moonlight is a major factor only for certain species. A hypothesis about how a light-trap may affect insect behaviour allows changes in catch of some species over the lunar cycle to be explained by the influence of background illumination on trap effectiveness.

The influence of moonlight on catches of insects in light-traps in Africa. Part I. The moon and moonlight

An account is presented of the distribution and amounts of moonlight in latitudes near the equator. This includes a Table on the amount of moonlight for each hour of the night throughout a standard lunar cycle, applicable to any locality between 10°N and 10°S, and a Table of standard groups of moon phase which can be used at any locality irrespective of latitude. The construction of these Tables is described in detail and their use briefly discussed. A method is described which enables light-trap catch records to be arranged for analysis directly against moon phase.

Light-trap and suction-trap catches of insects in the northern Gezira, Sudan, in the season of southward movement of the Inter-Tropical Front

Catches in light-traps adjoining cotton were obtained at the time of seasonal southward movement of the Inter-Tropical Front (ITF) in October, and during most of the following two months. Taxa studied were mostly Orthoptera and moths, many associated with sorghum, others long-distance migrants. Suction-trap catches at three heights up to 50 ft were obtained for short periods in October and November, and aircraft catches at 250 ft were also available on two days. Suction-trap catches of grass-feeding Homoptera suggest that displacement of these insects was associated with changes in wind direction marking movement of the ITF in October. The exact form of the displacement system in relation to the front cannot be reconstructed from catches at a single place, but it seems likely that proximity of the front at or soon after the time of a brief period of crepuscular activity stimulates insects to take flight and rise to 50 ft or more so that they are displaced. In many taxa, light-trap catches showed a regular pattern of increase, with only slight nightly fluctuations from a logarithmic trend, following full moon. Other increases were superimposed on this pattern at times when the ITF passed north of the trap site, and in some taxa particularly when it was far north. The pattern of change after full moon, shown most clearly in taxa with source populations close to the trap, was related to the moons influence on the range of trap effectiveness. But various qualitative variations suggest that, in addition, aspects of behaviour or development may have adaptive relationships to the lunar cycle; variations include differences between taxa, particularly in timing of catch changes, and increasing proportion and decreasing maturity of females of certain taxa at the time of the regular increases in catch.

The relationship between light- and suction-trap catches of Chrysoperla carnea (Stephens) (Neuroptera: Chrysopidae), and the adjustment of light-trap catches to allow for variation in moonlight

Analysis of catches of Chrysoperla carnea (Steph.) in a light-trap and a suction trap at Rothamsted, southern England, showed that the light-trap catch varied as predicted from a model proposed to describe the functioning of a light-trap: catch = constant × , where W = trap illumination and I = background illumination. After adjustment to allow for changes in illumination during the flight period of C. carnea , the light-trap catch was very similar to the suction-trap catch. For C. carnea , a light-trap provides as unbiased a sample as a suction trap, but because of variation in trap effectiveness with variation in illumination, light-trap catches obscure changes in activity and abundance. Although similar studies are desirable to confirm this for other species, it is suggested that in light-trap studies catches should be adjusted to allow for changes in illumination during flight or trapping periods.

Studies of elemental composition as a biological marker in insects. II. The elemental composition of apterae of Rhopalosiphum padi (L.) and Metopolophium dirhodum (Walker) (Hemiptera: Aphididae) from different soils and host-plants

Energy-dispersive X-ray spectrometry was used to make quantitative determinations of the elemental composition of plasma-ashed apterous individuals of Rhopalosiphum padi (L.) and Metopolophium dirhodum (Walker). R. padi was reared on 24 plant–soil combinations and M. dirhodum on nine plant-soil combinations. Analyses were done for 12 elements: Na, Mg, Al, P, S, Cl, K, Ca, Mn, Fe, Cu and Zn. Principal components analyses of individuals showed no distinction, in either aphid species, between insects reared on any plant–soil combination, whether all elements, elemental sub-sets, soil sub-sets or host-plant sub-sets were considered. When group means (mean for all individuals from a particular plant-soil combination) were used in principal components analyses, five groups of R. padi could be distinguished from the other 19, although neither set could be further separated. Four of the five distinguishable groups were from plants that grew particularly poorly, and the fifth was from plants in a soil in which all host-plants grew less well than in other soils. There were no distinct separations between groups of M. dirhodum , though there were slight indications that soils were distinguishable. In both R. padi and M. dirhodum , elemental differences involved the minor elements, particularly Mn, Fe, Cu and Zn. The uniformity of elemental content in the apterae of both aphid species may be a consequence of genetic uniformity within clones of parthenogenetically reproducing species.

Photoperiod, dormancy and the end of flight activity in Chrysopa cornea Stephens (Neuroptera: Chrysopidae)

The flight activity of Chrysopa carnea Steph. was studied during 1970–77 using 21 suction traps sited between 50 and 58°N. in northwestern Europe. Flight continues until late October and early November, well beyond the time (late August) when reproductive diapause is initiated at these latitudes. Flight ends as an all-or-none response to changing daylengths, the most northern populations having a shorter critical photoperiod, about 10 h 20 min, between morning and evening civil twilights, than the most southern ones, the critical photoperiod of which is about 10 h 45 min. By contrast, the critical photoperiod for induction of reproductive diapause is longer in northern populations. At latitudes less than 30° N. there is no winter dormancy but there is sometimes aestivation during the hottest period of the summer. In C. carnea , uniquely, winter dormancy has two distinct components, reproductive diapause and loss of flight activity, each controlled independently by photoperiod.