(1) Sweep net and sweeping
For the taxonomist, sweeping is probably the most rewarding way of collecting chalcids since a relatively good diversity of species can be collected in relatively short time. If material is collected into alcohol (see below) then a good day's collecting will provide as much material as an average Malaise trap in four months.
The sweep-net described here is based on the design first drawn up 50 years ago by Dr Z. Boucek who is well known for his exemplary work on chalcidoids. The frame has been adapted so that it can be dismantled easily for travel. The best frame is constructed of aluminium and takes the form as shown in Fig 1 and Fig. 2. The triangular head increases the surface area in contact with the ground when sweeping grassland or it allows easier penetration into dense vegetation (the normal round head is very poor).
Fig. 1 Construction of suitable sweep-net frame and bag for collecting Chalcidoidea and other microhymenoptera
The long handle allows the net to be used far away from the body, making sweeping underneath low, overhanging bushes easier, and also extends the area covered by each individual sweep. The netbag should be constructed of durable material which allows the easy passage of air (eg silkscreen), but has a small enough mesh to prevent the escape of smaller specimens. Canvas must be avoided because it obstructs the passage of air which does not allow insects to enter the net easily. The material for the netbag should be as translucent as possible to lessen the light differential between the inside and outside of the net and thus, in turn, lessen the number of insects that will escape if the contents of the bag are examined. The netbag should not be deeper than arm's length (ie about 60cm) and should have a well-rounded bottom (fig. 00). A net back that is too long will tend to flap around during sweeping and damage the insects inside, whilst one that is too short cannot be folded over the frame to prevent escaping when the net is not in use. One that is too tapered will make removal of specimens more difficult. The top of the bag which fits around the frame is best made of tough material such as canvas. The bag can be folded over the frame and sewn in position or the canvas can be sewn in such a way to allow the frame to be threaded through the hem (Fig. 1).
Fig. 2 Details of construction of head of sweep-net frame
The net can be adapted to allow a removable screen to be added in order to screen out larger debris such as leaves. The size of the mesh can vary from about 4-10mm. Surprisingly, the 0.4mm mesh will allow insects up to 7mm long to pass through. This facilitates emptying the net whether specimens are aspirated directly or the entire contents are emptied into alcohol (see below).
When sweeping choose an area where the vegetation is as diverse as possible. For sweeping grassland, place a side of the triangular head flat on the ground and press firmly downwards with the hand that is further down the shaft and hold the end of the shaft with the other hand. Using the latter mostly as a pivot (although it moves from one side of the body to the other), pull the sweep-net around in a large semicircular arc, pressing the net as firmly as possible into the ground for the total duration of the arc. The sweep should be steady but not too fast. At the end of the arc take one or two steps forward, turn the net over and reverse the sweep. Failure to keep the net close to the ground for as much of the arc as possible will greatly reduce the catch since many chalcidoids inhabit grass within a few centimeters of the soil. Sweep for 20-50 'arcs' and then empty the net.
When sweeping bushes, trees or other vegetation, it is best to sweep for not more than 5 minutes using long, smooth, regular strokes, taking care to avoid areas swept in the previous stroke or two. Sweeping for longer or too vigorously will result in damage to the insects in the net.
The net can be emptied in a number of ways:
a. Using an aspirator (see below)
To empty the net, flip the contents into the bottom and then, facing into the sun, quickly place the opening of the net over your head and with one hand extend the net bag horizontally outwards, or very slightly upwards, in the direction of the sun. Most of the insects will be attracted towards the more brightly lit closed end and will also tend to move upwards, ie towards the closed end of the net. The effect can be increased by standing in the shade of a tree with the closed end of the bag in sunlight. Using this method few chalcidoids will escape because they are generally slow moving. However, many of the Diptera, which are faster moving, will manage to find their way out of the net. Do not empty the net by looking down into it since most of the insects will escape. This method can be quite uncomfortable, especially in hot, humid conditions but has the advantage that insects can be collected alive (if required) and that it is not necessary to carry any other piece of equipment whilst collecting. Using the free hand to manoeuvre the aspirator (Fig. 3), suck up the specimens as quickly as possible. After a few minutes shake the debris from the end of the net to about the middle. This makes it easier to collect the insects. In the past some collectors have put the entire contents of the net, including debris, into a killing bottle or fumigate with a killing agent in some other method. Whilst this can be advantageous if "collecting" time is short the resulting material can be difficult to sort through subsequently and might result in damage to specimens.
|Fig. 3 Construction of a suitable aspirator for collecting small insects|
b. Using a light box
A good light box is about 50x30x30cm and constructed entirely of perspex or some other transparent material. An old fish tank would suffice or two large sandwich boxes stuck together to form a larger box with a hole cut in bottom of one (the hole should be large enough to allow easy access for an aspirator).
Place debris in the box and lay it on its side. Use the box in a darkened area with strong light coming from one direction (eg in an outside doorway or window) preferably with a pale background (eg white wall or sky) or that small insects can be seen easily. Emerging insects will be attracted towards the stronger light and can be collected with an aspirator as they walk up the walls of the box. Even with a large opening in the side (which is kept towards the darker aspect) escape will be negligible with a good light differential.
This quick and simple method will produce many species which would otherwise have been missed in the sweep net, either because of their small size or because they feign death.
c. Using a separation bag
A separation bag uses the same principle as a light box (see Masner & Gibson, 1979). It consists of tripod holding a 30cm diameter circular frame that holds a strong canvas bag, the top of which is closed off by a circular perspex or glass lid. A hole about 12-15cm long is cut in the side of the bag about half way up. This is closed by a zip or Velcro®. The contents of the sweep net bag are emptied into the canvas bag and after a few minutes the emerging insects can be collected off the side of the bag or the lid using an aspirator introduced through the side of the bag.
d. Direct into alcohol
This is by far the most efficient method of collecting chalcidoids and is especially efficient for smaller specimens that might be missed if the net is emptied using an aspirator as described below. Using this method the entire contents of the net are dumped into a polythene bag containing 70% alcohol (or perhaps saturated salt solution). To minimize the escape of insects when transferring into a polythene bag, start with one that contains no alcohol. Dampen the sides of the polythene bag by spraying with a little alcohol then empty the contents of net by everting the end of it into the polythene bag. Most, if not all, of the insects will get stuck to the sides of the bag thus preventing them from escaping. Wash the insects and debris into the bottom of the bag with as little alcohol as possible. Tuck the top of the bag into a belt around your waist. Repeat, using the same bag for at most one hour then start again with a fresh polythene bag.
This is a device for collecting smaller insects (Fig. 3, above) by sucking through one tube and collecting insects through the other. The entry tube can be made of class or good quality perspex to allow the insects to be seen they are collected. A bottle can be used for the collecting chamber rather than a tube as shown. This allows more specimens to be collected into a single container. When not sucking up insects place an index finger over the end of the tube, or place a small cork or piece of tissue in the end of the tube to prevent escape. Do not suck up spiders, aphids, small grasshoppers, etc. as these produce moisture that may cause smaller specimens to stick together or to the side of the tube. Also avoid ichneumonids, braconids, ants and moderately large aculeates (eg sphecids) as these may bite off heads and antennae. If a 75x25mm tube is used with the aspirator, collect a maximum of 300-400 chalcidoids or other microhymenoptera before changing the tube. A crumpled piece of tissue (about 10cm square) in the tube prevents 'sweating' and also gives insects a refuge from inadvertently collected ants, spiders, etc. No more than 50 specimens should be collected into tubes without tissue paper. The catch can be killed by placing a piece of tissue in the entry tube of the aspirator and dipping it in ethyl acetate. Push the paper a little way into the entry tube and close off with a small cork. Leave this to stand for about 30 minutes to kill the insects.
Last updated 07-Jun-2004 Dr B R Pitkin