Biology of Thrips

By Joe Funderburk

There are about 5000 described species of thrips (insects in the Order Thysanoptera) [1,2]. Most feed on fungi and live in leaf litter or on dead wood.  The species that feed on higher plants occur mostly in the Family Thripidae. This family includes the important pest species.  Some reproduce in flowers and feed on the cells of the flower tissues, on pollen grains and on small developing fruits. Many of the flower-living species are facultative predators. Other species primarily feed on leaves. Some species are obligate predators on small insects and other arthropods.  Some of the most common pest species feed on a wide range of plants and even prey on mites.

The life history of a thrips involves an egg, two larval stages, and the nonfeeding stages of the propupa and pupa. The eggs of flower thrips and leaf-feeding species are inserted into plant tissue by means of a serrated ovipositor (that is a specialized egg-laying structure). In warm weather, life cycles take 21 days or less.

Thrips are haplodiploid, that is males have half the number of chromosomes (the haploid number) that are found in the females (the diploid number) [3]. The males develop from unfertilized eggs. Males usually are smaller than females.

Long-distance dispersal has been recorded for a few species of thrips [4]. This is possible only under unusual circumstances as small organisms dehydrate rapidly and must often probe the plants and uptake fluids to survive. Most flower thrips disperse over a series of short flights. The frequency and duration of flight  varies with the species and gender and is influenced by the weather, by the suitability of food, and possibly by crowding. The eastern flower thrips (Frankliniella tritici) and the Florida flower thrips (Frankliniella bispinosa) are highly dispersing, moving rapidly between flowers [5]. Adults of these species can rapidly recolonize a crop treated with an insecticide resulting in an 'apparent' rather than a 'real' lack of control. Adults of the western flower thrips (Frankliniella occidentalis) disperse much less frequently. Mass flights of flower thrips are typical  following senescence of the abundant spring flowers, hence the origin of the lay name 'thunder flies'.

The feeding apparatus of thrips is unique. Only one mandible is present and another mouth structure forms a stylet or tube through which food is drawn [6]. Larvae and adults use a similar punch and suck feeding technique. The single mandible punches a hole in the plant surface through which the stylet is inserted.

Thrips induce a range of symptoms in plant tissue by their feeding [7]. On small fruits, feeding results in deformity.  Some species cause similar damage to leaves.  Silvering is common, due to air entering cells from which the contents have been removed, and on fruits this leads to scarring and corky tissue development. Very large populations of thrips can induce premature flower loss, and can reduce available pollen below critical levels.

The western flower thrips lay eggs in the small fruits in the flower. Eggs are laid individually resulting in a small dimple sometimes surrounded by a white halo on mature fruit. For fruits such as tomato, this dimpling can result in cullout of individual fruits or even downgrading of the crop.  In grapes, these dimples or 'halo spots' serve as the entry for microorganisms and subsequent fruit rot. 

Flower thrips develop slowly in the winter in the southern US [8,9,10], and overwinter as pupae in the soil under plant litter in the northern US. Numbers increase rapidly in the spring on the abundant wild plant hosts. As spring flowers senesce, crop fields become 'islands' for aggregating populations. Natural enemies become abundant later in the spring, and populations of flower thrips are low in the summer and fall. Natural enemies include the minute pirate bugs (true bugs of the order Hemiptera, family Anthocoridae, genus Orius) and entomopathogenic ( that is, insect pathogenic) nematodes that are specialized parasites of thrips (order Tylenchida, family Allantonematidae, genus Thripinema) [11]. The importance of other natural enemies of thrips is not well understood. Examples include the bigeyed bugs (order Heteroptera, family Lygaeidae, genus Geocoris) and predatory thrips (order Thysanoptera, family Aeolothripidae, genera Aeolothrips and Franklinothrips).

Thrips can be collected from a plant, single flower, or leaf by beating the vegetation over a small, white plastic tray (such as an artist's palate or barbecue plate) using a small trowel or large knife. Thrips are sufficiently stunned by this beating for them to adhere to the plastic momentarily before running away. They can be counted or removed with a fine brush into collecting vials. The preserving medium of choice is 70% alcohol. Samples of flowers, leaves, and other plant structures can be placed directly in jars of alcohol and the thrips extracted. Presence/absence sampling programs have been developed to estimate thrips densities, and management decisions are based on the proportion or flowers infested.

The small size of thrips makes reliable identification a challenge. Persons are easily trained to distinguish the adults of the key pest species under a stereomicroscope in the lab. This is suitable for researchers, but it is time consuming and may be impractical for scouts. Scouts are aided by a knowledge of the species of thrips for which a crop is host. If more than one occurs commonly on the crop in their geographic region, the species may be differentiated by color or some other easily-observed characteristics [1]. A CD rom was developed for the identification and biology of thrips in the southern US by Gerald Moritz, David Morris, and Laurence Mound. The identification software used, LucID, is particularly user-friendly. All of the thrips, and all of the character states to be considered when attempting an identification, are fully illustrated with photomicrographs, and these pictures are all manipulated with the software to give images with great depth of field. This system is very useful to researchers or scouts with all levels of experience.

For information, contact Joe Funderburk.

References cited
1. Moritz, G., D. Morris, and L. Mound. 2001. Thrips ID Pest thrips of the world. CSIRO Publishing, Collingwood, Australia.

2.  Mound, L.A. 1997.  Biological diversity. Thrips as Crop Pests (ed. T. Lewis), pp. 197-216. CAB International, Wallingford, UK.

3.  Crespi, B.J. 1993.  Sex ratio selection in Thysanoptera. Evolution and Diversity of Sex Ratio in Insects and Mites (eds. D.L. Wrensch and M. Ebbert), pp. 214-234. Chapman and Hall.  

4.  Mound, L.A., and R. Marullo. 1996. The thrips of Central and South America: an Introduction. Memoirs of Entomology 6: 1-488.

5.  Ramachandran, S., J. Funderburk, J. Stavisky, and S. Olson.  2001. Population abundance and movement of Frankliniella species and Orius insidiosus in field pepper.  Agricultural and Forest Entomology 3: 129-137.

6.  Heming, B.S. 1993. Structure, function, ontogeny, and evolution of feeding in thrips (Thysanoptera). Functional Morphology of Insect Feeding (eds. C.W. Schaefer and R.A.B. Leschen), pp.3-41.  Thomas Say Publications in Entomology, Entomological Society of America. Lanham, Maryland.

7.  Childers, C.C. 1997. Feeding and oviposition injuries to plants. Thrips as Crop Pests (ed. T. Lewis), pp. 505-538. CAB International, Wallingford, UK.

8.  Toapanta, M.A., J.E.Funderburk, R.J. Beshear, S.M.Olson, and T.P. Mack. 1996. Abundance of Frankliniella spp. (Thysanoptera: Thripidae) on winter and spring host plants.  Environmental Entomology 25: 793-800.

9.  Toapanta, M.A., J.E. Funderburk, and D. Chellemi. 2001. Development of Frankliniella species (Thysanoptera: Thripidae) in relation to microclimate temperatures in vetch.  Journal of Entomological Science 36: 426-437.

10.  Chellemi, D.O., J.E. Funderburk, and D.W. Hall. 1994. Seasonal abundance of flower inhabiting Frankliniella species (Thysanoptera: Thripidae) on wild plant host. Environmental Entomology 23: 337-342.

11.  Loomans, A.J.M., T. Murai, and I.D. Greene. 1997. Interactions with hymenopterous parasites and parasitic nematodes. Thrips as Crop Pests (ed. T. Lewis), pp. 355-397. CAB International, Wallingford, UK.