17.1.2. Passive collecting
Many insects live in microhabitats from which they are difficult to extract — notably in leaf litter and similar soil debris or in deep tussocks of vegetation. Physical inspection of the habitat may be difficult and in such cases the behavior of the insects can be used to separate them from the vegetation, detritus, or soil. Particularly useful are negative phototaxic and thermotaxic and positive hygrotaxic responses in which the target insects move away from a source of strong heat and/or light along a gradient of increasing moisture, at the end of which they are concentrated and trapped. The Tullgren funnel (sometimes called a Berlese funnel) comprises a large (e.g. 60 cm diameter) metal funnel tapering to a replaceable collecting jar. Inside the funnel a metal mesh supports the sample of leaf litter or vegetation. A well-fitting lid containing illuminating lights is placed just above the sample and sets up a heat and humidity gradient that drives the live animals downwards in the funnel until they drop into the collecting jar, which contains ethanol or other preservative.
The Winkler bag operates on similar principles, with drying of organic matter (litter, soil, leaves) forcing mobile animals downwards into a collecting chamber. The device consists of a wire frame enclosed with cloth that is tied at the top to ensure that specimens do not escape and to prevent invasion by scavengers, such as ants. Preieved organic matter is placed into one or more mesh sleeves, which are hung from the metal frame within the bag. The bottom of the bag tapers into a screw-on plastic collecting jar containing either preserving fluid or moist tissue paper for live material. Winckler bags are hung from a branch or from rope tied between two objects, and operate via the drying effects of the sun and wind. However, even mild windy conditions cause much detritus to fall into the residue, thus defeating the major purpose of the trap. They are extremely light, require no electric power and are very useful for collecting in remote areas, although when housed inside buildings or in areas subject to rain or high humidity, they can take many days to dry completely and thus extraction of the fauna may be slow.
Separating bags rely on the positive phototaxic (light) response of many flying insects. The bags are made from thick calico with the upper end fastened to a supporting internal ring on top of which is a clear Perspex lid; they are either suspended on strings or supported on a tripod. Collections made by sweeping or specialized collections of habitat are introduced by quickly tipping the net contents into the separator and closing the lid. Those mobile (flying) insects that are attracted to light will fly to the upper, clear surface, from which they can be collected with a long-tubed aspirator inserted through a slit in the side of the bag.
Insect flight activity is seldom random, and it is possible for the observer to recognize more frequently used routes and to place barrier traps to intercept the flight path. Margins of habitats (ecotones), stream lines, and gaps in vegetation are evidently more utilized routes. Traps that rely on the interception of flight activity and the subsequent predictable response of certain insects include Malaise traps and window traps. The Malaise trap is a kind of modified tent in which insects are intercepted by a transverse barrier of net material. Those that seek to fly or climb over the vertical face of the trap are directed by this innate response into an uppermost corner and from there into a collection jar, usually containing liquid preservative. A modified Malaise trap, with a fluid-filled gutter added below, can be used to trap and preserve all those insects whose natural reaction is to drop when contact is made with a barrier. Based on similar principles, the window trap consists of a window-like vertical surface of glass, Perspex, or black fabric mesh, with a gutter of preserving fluid lying beneath. Only insects that drop on contact with the window are collected when they fall into the preserving fluid. Both traps are conventionally placed with the base to the ground, but either trap can be raised above the ground, for example into a forest canopy, and still function appropriately.
On the ground, interception of crawling insects can be achieved by sinking containers into the ground to rim-level such that active insects fall in and cannot climb out. These pitfall traps vary in size, and may feature a roof to restrict dilution with rain and preclude access by inquisitive vertebrates (Fig. 17.1). Trapping can be enhanced by construction of a fence-line to guide insects to the pitfall, and by baiting the trap. Specimens can be collected dry if the container contains insecticide and crumpled paper, but more usually they are collected into a low-volatile liquid, such as propylene glycol or ethylene glycol, and water, of varying composition depending on the frequency of visitation to empty the contents. Adding a few drops of detergent to the pitfall trap fluid reduces the surface tension and prevents the insects from floating on the surface of the liquid. Pitfall traps are used routinely to estimate species richness and relative abundances of ground active insects. However, it is too rarely understood that strong biases in trapping success may arise between compared sites of differing habitat structure (density of vegetation). This is because the probability of capture of an individual insect (trappability) is affected by the complexity of the vegetation and/or substrate that surrounds each trap. Habitat structure should be measured and controlled for in such comparative studies. Trappability is affected also by the activity levels of insects (due to their physiological state, weather, etc.), their behavior (e.g. some species avoid traps or escape from them), and by trap size (e.g. small traps may exclude larger species). Thus, the capture rate (C) for pitfall traps varies with the population density (N) and trappability (T ) of the insect according to the equation C = TN. Usually, researchers are interested in estimating the population density of captured insects or in determining the presence or absence of species, but such studies will be biased if trappability changes between study sites or over the time interval of the study. Similarly, comparisons of the abundances of different species will be biased if one species is more trappable than another.
Many insects are attracted by baits or lures, placed in or around traps; these can be designed as “generic” to lure many insects, or “specific”, designed for a single target. Pitfall traps, which trap a broad spectrum of mobile ground insects, can have their effectiveness increased by baiting with meat (for carrion attraction), dung (for coprophagous insects such as dung beetles), fresh or rotting fruit (for certain Lepidoptera, Coleoptera, and Diptera), or pheromones (for specific target insects such as fruit flies). A sweet, fermenting mixture of alcohol plus brown sugar or molasses can be daubed on surfaces to lure night-flying insects, a method termed “sugaring”. Carbon dioxide and volatiles such as butanol can be used to lure vertebrate-host-seeking insects such as mosquitoes and horseflies.
Colors differentially attract insects: yellow is a strong lure for many hymenopterans and dipterans. This behavior is exploited in yellow pan traps which are simple yellow dishes filled with water and a surface-tension reducing detergent and placed on the ground to lure flying insects to death by drowning. Outdoor swimming pools act as giant pan traps.
Light trapping (see section 17.1.1 for light sheets) exploits the attraction to light of many nocturnal flying insects, particularly to the ultraviolet light emitted by fluorescent and mercury vapor lamps. After attraction to the light, insects may be picked or aspirated individually from a white sheet hung behind the light, or they may be funneled into a container such as a tank filled with egg carton packaging. There is rarely a need to kill all insects arriving at a light trap, and live insects may be sorted and inspected for retention or release.
In flowing water, strategic placement of a stationary net to intercept the flow will trap many organisms, including live immature stages of insects that may otherwise be difficult to obtain. Generally, a fine mesh net is used, secured to a stable structure such as bank, tree, or bridge, to intercept the flow in such a way that drifting insects (either deliberately or by dislodgement) enter the net. Other passive trapping techniques in water include emergence traps, which are generally large inverted cones, into which adult insects fly on emergence. Such traps also can be used in terrestrial situations, such as over detritus or dung, etc.
(After an unpublished drawing by A. Hastings)