Family Aeolothripidae banded thrips

Aeolothrips intermedins Bagnall

This generally common species is associated with various members of the Asteraceae, Brassicaceae, Fabaceae and Linaceae, including crops such as field bean, linseed and oilseed rape, but is of little or no pest status. Second-instar nymphs are at least partly predacious.

BIOLOGY

Adults occur from May to August, and nymphs develop from July onwards. Fully grown nymphs

Fig. 180 Forewing of Aeolothrips intermedius (x50).

Fig. 180 Forewing of Aeolothrips intermedius (x50).

spin cocoons in the soil or amongst leaf litter (in which they then spend the winter), adults emerging in the spring. There is just one generation annually.

DESCRIPTION

Adult 2mm long, black-bodied; forewings white, banded with black (imparting a striking banded appearance to the insect) (Fig. 180) and with the vein around the tip pale (cf. Aeolothrips tenuicornis, below). Nymph white to creamish-white.

Aeolothrips tenuicornis Bagnall Banded-wing flower thrips

This common species is also associated with Asteraceae, Brassicaceae, Fabaceae and Linaceae, and has a similar life-cycle to that of the previous species. Adults are distinguished from those of Aeolothrips intermedius (above) by the dark colour of the marginal vein at the tip of the forewing.

Family THRIPIDAE

Aptinothrips rufus (Haliday) Grass thrips

This widely distributed and often abundant thrips is associated mainly with grasses. It will also breed on cereals, such as barley, oats and wheat, but is not an important pest of these crops.

BIOLOGY

Adults of this mainly parthenogenetic species overwinter in tufts of dead grass, amongst turf and in other sheltered situations. They become active in the spring and eventually deposit eggs. Populations of adults and nymphs are at their

Fig. 181 Outline of the head and pronotum: (a) Aptinothrips; (b) Chirothrips.

greatest from July to September, and there are up to three overlapping generations annually. The egg stage is moderately protracted and lasts for almost 3 weeks. Nymphs feed for about 2 weeks and adults appear just over a week later, after very brief propupal and pupal stages.

DESCRIPTION

Adult 0.8-1.0mm long, yellowish-brown, narrow-bodied, apterous; head longer than wide (Fig. 181a); pronotum more or less quadrate; antennae 6-segmented. Nymph pale yellowish-orange.

NOTE Other species are also of significance on cultivated grasses, at least in continental Europe. These include Anaphothrips obscurus Miiller and Aptinothrips stylifer Trybom.

Chirothrips spp.

Members of the genus Chirothrips breed on wild and cultivated grasses and, in continental Europe, sometimes reach pest status. The thrips are readily distinguished from members of other grass-infesting genera (e.g. Aptinothrips, above), as the head is distinctly smaller than the pro-notum (Fig. 181b). Also (but not in the meadow foxtail thrips, Chirothrips hamatus Trybom). the second antennal segment is asymmetrical, usually having an outwardly directed apical projection. The grass flower thrips {Chirothrips manicatus Haliday) is a very polyphagous species and that most frequently encountered.

Frankliniella occidentals (Pergande) Western flower thrips

In the 1980s, this polyphagous non-indigenous thrips appeared in considerable numbers on glasshouse plants in England and various other parts of Europe, including France, Germany, the Netherlands and Scandinavia, having been introduced accidentally from abroad, mainly on chrysanthemum cuttings. Significant infestations have occurred on aubergine, cucumber, tomato and various other plants (including many ornamentals) and attempts to eradicate the pest have not proved successful. In warmer parts of Europe, this pest is also now established on various outdoor hosts, including fruit trees and vines. Adults and nymphs cause distortion, silvering and speckling of leaves and flowers (Plates lc and Id), and damage from even a relatively small number of individuals is often extensive. The thrips are also implicated in the transmission of important plant viruses, such as tomato spotted-wilt virus.

BIOLOGY

This species breeds continuously under suitable conditions, and the life-cycle is completed in 2-3 weeks at normal glasshouse temperatures. There are two nymphal instars, after which individuals drop to the ground to complete their development in the soil. Although adults and nymphs are sometimes observed on the exposed surfaces of leaves and flower petals, they are of secretive habit and occur more frequently on the underside of leaves, or hidden within the shelter of flowers and beneath bud scales.

DESCRIPTION

Adult 1-2mm long, pale yellow to brownish-yellow; antennae 8-segmented; forewings with a complete row of setae along veins I and II (cf. onion thrips, Thrips tabaci, p. 93). Egg 0.2mm long, pearly-white. Nymph golden-yellow; eyes reddish.

Frankliniella intonsa (Trybom) Flower thrips

This generally common, polyphagous species is sometimes damaging to outdoor broad-leaved plants. Damage is usually restricted to the leaves and is of only minor significance. However, on strawberry (in common with certain species of thrips that may feed within the flowers at blossom time) this pest has been implicated in the development of distorted fruitlets, especially on late-season cultivars. Individuals are distinguishable from Frankliniella occidentalis (above) on the basis of microscopical features.

Kakothrips pisivorus (Westwood) Pea thrips

This widely distributed thrips is a generally common pest of legumes, especially pea and broad bean crops growing in allotments and private gardens. The surface of infested tissue becomes silvery and flowers sometimes fail to develop; plants and pods are also malformed. Heavily infested plants are severely stunted and crop yields may be reduced. Most damage tends to occur under dry conditions during June and July; late-sown or late-maturing crops are particularly susceptible. In the British Isles, attacks are often especially common in the south and east of England.

BIOLOGY

Adults occur from May to July, when they infest the flowers, foliage and pods of pea and bean crops. Eggs are inserted in rows along the stamen sheaths or in other floral tissue; they hatch in about 10 days. Nymphs are very active and are usually most numerous in June or July. Individuals feed for up to 3 weeks before becoming fully grown. They then enter the soil, where they overwinter. The new adults appear after nymphs have passed through brief propupal and pupal stages. There is a single generation each year.

DESCRIPTION

Adult female 1.5-2.0 mm long, blackish-brown to black, and somewhat flattened; antennae 8-segmented, the third segment yellow; legs mainly brown, with yellow tarsi; forewings dark brown but clear basally; abdomen distinctly pointed posteriorly. Nymph yellow when young, later becoming orange; abdomen with a dark brown tip in the second instar.

Limothrips cerealium Haliday Grain thrips

This thrips is generally abundant on cereals (especially wheat and oats) and grasses, and often causes concern. Developing grains of heavily infested florets become shrivelled and discoloured; infested florets may also become blind. Germination of seed is affected adversely and, on barley, attacks can result in poor malting quality of the grain. Damage caused to crops, however, is rarely extensive and the pest is usually of only minor significance. Mass nights of adult females in summer (these are the familiar 'thunderflies') often constitute a nuisance, especially when the insects invade glasshouses, homes, gardens and amenity areas. The thrips readily gain entry through seemingly impenetrable joints or cracks, and are often then implicated in the triggering of burglar-, fire- and smoke-alarms. In continental Europe, infestations of Limothrips on cereal crops often occur in association with Haplothrips aculeatus (p. 94), an uncommon species in the British Isles.

BIOLOGY

Adult females overwinter away from host plants, often sheltering beneath the bark of conifer trees. They become active from late May onwards and migrate to host plants where eggs are laid, each placed in a small slit cut into the plant tissue. The eggs hatch within 2 weeks. The nymphs then feed within the shelter of leaf folds, beneath leaf sheath, or in the developing flowers, and pass through two instars. They are fully grown in about 2 weeks and then enter brief propupal and pupal stages (within the ear or a leaf sheath) before the appearance of the new adults. Mating takes place before the short-lived males die. The females then migrate in vast masses, at about harvest time, before taking up their winter quarters. There is typically one generation annually.

DESCRIPTION

Adult female 1.6-1.8mm long, black to brownish-black, with fully developed wings; head as large as pronotum (prothorax); antennae 8-segmented; pronotum transversely rectangular; legs yellowish. Adult male similar to female but smaller and apterous. Egg 0.3 mm long, whitish-yellow. Nymph whitish or yellowish. Pupa whitish to pale yellow; eyes reddish.

Limothrips denticornis Haliday Barley thrips

This often common and widely distributed species is also associated with various species of Poaceae, especially barley and oats. Adults are most readily distinguished from those of Limothrips cerealium (above) by the asymmetrical third antennal segment, which is drawn outwards into a distinct, thumb-like prominence.

Taeniothrips inconsequens (Uzel) Pear thrips

Infestations of this often common thrips occur on various deciduous trees, especially pear but also apple and plum. The thrips feed within the flower buds; sap often seeps from the damaged tissue. Later, they attack the petals, stamens and styles of open flowers, to produce brownish patches and distortion; young leaves are also attacked. Heavy infestations lead to reduced fruit set and to russeting of fruitlets.

BIOLOGY

Adults of this mainly parthenogenetic, univoltine thrips appear from January or February onwards. They then feed in the swelling and opening buds of host trees. Eggs are laid during May, either in leaf veins or in the blossom stalks (pedicels), and they hatch about 10 days later. The nymphs feed for several weeks and eventually, when fully grown, enter the soil. Each forms a small earthen cell within which the propupal and pupal stages occur. Individuals eventually moult into new adults and these emerge in the following spring.

DESCRIPTION

Adult 1.2-1.7 mm long, dark brown; legs pale; wings greyish; antennae 8-segmented. Nymph whitish to yellowish-white; eyes dark red.

Thrips angusticeps Uzel

Field thrips

The field thrips is a widespread and locally common pest of various agricultural and horticultural crops, including field bean, linseed, pea, red beet and sunflower. Infested leaves are often discoloured and distorted. On pea crops, which are particularly susceptible, most damage occurs in April or May before seedlings are 5-10 cm high; plants then become stunted and the leaves puckered and blotched with yellow (cf. damage caused by pea thrips, Kakothrips pisivorus, p. 91). Infestations are especially common following brassica seed crops that have been infested during the previous season.

BIOLOGY

Adult thrips with reduced wings (i.e. brachy-pterous adults) overwinter in the soil. They emerge in the early spring and then invade various hosts. A generation of nymphs develops on the young leaves and within the growing points of the plants, which eventually gives rise to fully winged (macropterous) adults; these migrate in late May or early June to summer hosts, including various brassicaceous plants (Brassicaceae). Nymphs produced on these hosts develop into the overwintering generation of brachypterous adults.

DESCRIPTION

Adult 1.0-1.3 mm long, mainly brown or dark brown; antennae 7-segmented, with a pale third segment; legs mainly brown with yellow tarsi; forewings with a complete row of setae along vein II and an incomplete row of setae along vein I. Nymph translucent-whitish.

Thrips tabaci Lindeman Onion thrips

This often abundant pest occurs on many cultivated plants, including leek, onion and various glasshouse-grown vegetables and ornamentals; the thrips are directly harmful and often cause extensive silvering of affected tissue. Outdoor attacks are especially severe in hot, dry weather. Although formerly considered an important vector of plant viruses, this no longer appears to be so. Cucumber and tomato: infested foliage and fruits become mottled, and young cucumber leaves may develop noticeable window-like patches; the thrips can also transmit spotted-wilt virus. Leek and onion: infested foliage turns silvery, and leaves may subsequently twist and curl.

BIOLOGY

This thrips is parthenogenetic. Eggs, usually about 30 per female, are laid in plant tissue and hatch within 1-2 weeks. Nymphs feed on host plants for up to 2 weeks and then enter the soil to complete their development. New adults appear about a week later. Large populations may develop on glasshouse-grown plants, and breeding is continuous so long as conditions remain favourable. Outdoors, females usually overwinter in the soil and two or more generations of adult females and nymphs occur from May onwards.

DESCRIPTION

Adult female 1.0-1.3 mm long, greyish-yellow to brown; forewings pale brownish-yellow, with line of setae along vein I incomplete; antennae 7-segmented, yellowish-brown (cf. western flower thrips, Frankliniella occidentalis, p. 91). Egg whitish and elliptical. Nymph whitish to pale yellowish-orange; eyes red.

Thrips simplex (Morison) Gladiolus thrips

This species, which was introduced into Europe from Southern Africa, is an important pest of glasshouse-grown gladioli. The thrips infest the corms and, later, cause a streaking of the devel oping leaves and flower stems. Direct damage to flowers results in silver flecking; affected tissue eventually turns brown. Breeding is continuous under suitable conditions (temperatures above 10°C) and, on outdoor-planted hosts, is favoured by hot, dry conditions. Adult thrips are 1.5 mm long and dark brown, with 8-segmented antennae; the nymphs are yellowish or orange.

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