Thrips species usually cannot be recognized without mounting specimens onto microscope slides. Techniques are best considered under two headings: those appropriate for routine identifications, and those required for archival and taxonomic reference purposes. However, preparation techniques are largely dependent on the care with which specimens are first collected and preserved.
Thrips may be collected into a fluid known as AGA, a mixture of 10 parts of 60% ethyl alcohol with 1 part of glycerine and 1 part of acetic acid. This mixture helps to distend the body of most thrips and keeps the body parts supple. However, 80–95% ethanol followed by storage in a freezer can yield good microscope slides, and specimens preserved this way are useful for DNA studies.
Thrips may be beaten from flowers, leaves and dead twigs. A small but heavy garden trowel is a convenient beating implement. The most convenient beating tray for thrips is a plastic Australian Barbeque tray with a finger grip at the side. The feet of thrips adhere to the plastic surface, and the thrips can be picked off with a small brush or grass stem into the collecting fluid in tubes.
The best tubes for field use are plastic Ependorf Tubes, as used in molecular biology laboratories for centrifuging, because these have screw tops that include a sealing ring, and neither leak nor break. Ensure that each tube contains a label, written in pencil.
Specimens stored in alcohol should be kept in the dark, preferably at temperatures well below 0°C, to prevent loss of color.
The following method is rapid and thus relatively inexpensive. Slides prepared in such media are not 'permanent', but can remain useable for years. This method is recommended for all routine identification work, and is particularly appropriate for larvae and for small pale adults.
The cover slip used should be small — large cover slips crush specimens and need more mountant. The traditional method of mounting a specimen onto a slide and then placing the cover slip onto this with forceps is more difficult and often introduces bubbles of air.
The objective is to prepare specimens onto slides with their shape and color retained in a condition as close as possible to the natural, living state but with the body cleared so that surface detail is visible. This ideal is difficult to achieve, and a compromise must be adopted.
Most specimens should be macerated gently to reveal fine details of body sculpture and minute setae. A few specimens should be prepared for study without maceration in order to preserve their natural color.
Specimens can be manipulated with fine micro-pins, mounted in sealing wax on match sticks. Use a pair of such pins, one straight but the other with the apex bent. A simple lifting tool to move specimens from one dish to another can be made from a small loop of fine wire. Alternatively, alcohols can be changed in dishes using a fine glass pipette. The most appropriate dishes to use are 'excavated blocks'—glass blocks 15mm high and 40mm square with a median excavation of about 5ml volume, and with a glass lid to prevent evaporation.
The objective of maceration is to remove the body contents. This is done by soaking the specimens in a weak NaOH solution for an appropriate period—NaOH solution seems to cause less damage to the body surface than KOH solution. The length of the period of treatment must be determined by experiment.
Students are recommended to experiment with very weak NaOH solution (2%) for longer periods, such as overnight (for black specimens even longer), because a long, slow process is more easily controlled than attempting to get results quickly. Maceration should always be carried out at room temperature; heating causes damage to setae and the body surface. Note that this contrasts with the techniques used for preparation of aphid and coccid specimens.
Alcohols and clove oil will absorb water from the atmosphere if not protected, particularly under warm humid conditions. The objective of the dehydration schedule is to remove water, then to render the specimens translucent with clove oil. Clearing can be improved by massaging each specimen gently with the back of the bent needle.
To facilitate this process it is best first to prepare a small mounting block. This is done by fixing to the center of a microscope slide a 2mm deep layer of 1 inch square white card. Mark the center of this with crossed lines, and then cover it securely with plastic tape to provide a clean, shiny surface.
An insect specimen is of limited value if it is not labelled with its original data.
*Hoyers Mountant must be prepared, as it is not available commercially; CMC 10 Mountant is an excellent alternative available from Masters Company, Inc., 890 Lively Blvd. Wood Dale, IL 60191, (630) 238-9292, firstname.lastname@example.org