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| 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
(Fig.1). 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. |
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| Figure 1.
Egg, first and second instar larvae, propupa, pupa, adult male, and
adult female thrips |
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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. |
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| Figure 2. Deformity on nectarine from thrips feeding |
Thrips induce a range of symptoms
in plant tissue by their feeding [7]. On small fruits, feeding results in
deformity (Fig. 2). Some species cause similar damage to leaves
(Fig. 3). 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 (Fig. 4). Very large populations of thrips can
induce premature flower loss, and can reduce available pollen below
critical levels.
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| Figure 3. Deformity on peanut leaves from thrips
feeding |
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| Figure 4. Scarring and corky
tissue on nectarines resulting from thrips feeding. |
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| Figure 5. Dimpling resulting
from thrips egg-laying |
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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 (Fig. 5). In grapes, these
dimples or ‘halo spots’ serve as the entry for microorganisms and
subsequent fruit rot (Fig. 6). |
| 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). |
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| Figure 6. Halo spots on
grape from thrips egg-laying |
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| Figure 7. Sampling for
thrips |
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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 (Fig. 7). 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. |
For a table describing the commonly occurring thrips species in Florida
and the southeastern US, click HERE
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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. |
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