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  • Author or Editor: W. Kirk x
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Across much of northern Europe, there is a traditional link between thunderstorms and the mass appearance of thrips migrating from cereal crops. The link is reflected in the common names for thrips in seven countries. This association could be a coincidence because cereal thrips migrate in warm weather in summer when thunderstorms are common, but personal observations and an earlier study indicate that a causative link is likely. The possibility that electric fields from thunderstorms affect the behaviour of cereal thrips is discussed.

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Adult male western flower thrips form mating aggregations within which fighting can occur between pairs of males. They may be defending a strategic position for mating. An understanding of this fighting behaviour, which is part of mating behaviour, may be useful for the development of novel methods of pest management. A bioassay was developed to observe male fighting and record it on video. The effect of male density on this behaviour was then investigated. Interactions consisted of either a brief contact with instant separation and no fighting or a longer contact with mutual abdominal flicking before separation. Fights usually lasted just a few seconds, but there were also many prolonged interactions with several bouts of abdominal flicking. The rate of brief contacts without fighting increased linearly with male density as expected because of the greater number of thrips in the arena. However, the rate of fighting and the duration of fights had a maximum at an intermediate male density, with less fighting and shorter fights at higher and lower densities, presumably reflecting the changes in the costs and benefits of fighting at different male densities. Observations suggested that a pheromone may be involved during fighting.

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An accurate estimate of a population is essential for pest management. For Frankliniella occidentalis (Pergande) in strawberry, counts of thrips in flowers are commonly used as there is a strong correlation between thrips number per flower and fruit damage. The aim of this study was to look at the abundance and population structure of thrips within different flower stages and positions on the plant to test whether these affect population estimates. Adult females were found in open buds (petals showing), but were most frequent in young and mature flowers, whereas adult males were not found in buds and were most frequent in mature and senescent flowers. Twice as many adult thrips were found in mature flowers at the top of the plant compared to those at the side. Only larvae were found in closed buds (no petals showing). Larval numbers increased gradually with flower stage and peaked in senescent flowers. Numbers of adults and larvae declined after flowering. Total numbers of thrips and the ratios of females:males and of adults:larvae varied systematically with flower stage. The choice of flower stage and position for sampling could affect population estimates by as much as a factor of four.

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Adult males of the western flower thrips, Frankliniella occidentalis (Thysanoptera: Thripidae), produce an aggregation pheromone that attracts both male and female adults. This pheromone is assumed to be produced by the sternal glands that are externally evident from distinctive regions known as pore plates or areae porosae on abdominal sternites III–VII of adult males. In this paper, we investigate the structure of these glands by light microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The structure of the glands is similar to that already found in other Thrips and Frankliniella species, with a dome of secretory cells near internal cuticular ridges. Many secretory ductules, about 100 nm in diameter, pass through the cuticle to pores on the outside surface of the cuticle. Despite their small size, these pores can be seen by light microscopy. Feltwork structures are present at the inner end of the ductules. The gland structure is consistent with pheromone production.

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In no-choice experiments, the addition of sucrose to a diet of water had no effect on oviposition rate whether or not pollen was also present. However, the addition of 1% or 5% tannic acid to a diet of water decreased the oviposition rate by 68% and 86% respectively when pollen was also present. A novel design of U-tube cage allowed thrips to choose between two oviposition and feeding substrates in the presence of pollen. When thrips were offered a choice between 5% tannic acid solution and water, 88% fewer eggs were laid in the tannic acid. However, the total number of eggs laid per U-tube was not significantly different from in control U-tubes with water as the only substrate. It is concluded that tannin acts as a feeding deterrent and/or oviposition deterrent rather than as a toxin.

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The aggregation pheromone of the western flower thrips, Frankliniella occidentalis (Thysanoptera: Thripidae), has previously been identified as the monoterpenoid ester neryl ( S )-2-methylbutanoate. It attracts both male and female adults to traps in commercial glasshouses. In this paper, we investigate the rates of production of this compound at two male densities and in the presence or absence of an adult female. Solid-phase microextraction (SPME) was used to entrain live-male headspace odour and this was compared with external standards to obtain an estimate of pheromone production, which was about 100–300 pg male −1 h −1 . Significantly more pheromone was apparently produced per male at the higher density, but no firm conclusions could be drawn because the rates were close to the limits of resolution. The rate of production was not affected by the presence of a female thrips. Another male-headspace component, ( R )-lavandulyl acetate, was only detectable at the higher male density.

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The extent to which the western flower thrips pupates on or off the host plant in glasshouse crops determines the effectiveness of ground-based control methods. Glasshouse experiments with water traps, sticky traps and infra-red video recording in crops of cucumber and pot chrysanthemum, when average relative humidities were below 80%, showed that large numbers of second-instar larvae dropped to the ground to pupate. The percentage of larvae dropping rather than remaining on the plant was calculated to be 96–99% in cucumber and 92–99% in pot chrysanthemum. Experiments with water traps showed that most larvae dropped during the evening trapping period from 16.00 to 24.00 h for both crops in the glasshouse. The pattern was similar, but less marked, for pot chrysanthemum in a controlled-temperature laboratory at 25 °C and 80–95% relative humidity. Continuous infra-red video recording in a heavily infested cucumber crop showed there was a marked peak in dropping during a period of about 1–4 h each evening, with peak rates reaching 4,000 to 8,000 larvae m−2 h−1. The time of the peak drop varied between days, suggesting that an environmental cue is involved. Possibilities for exploiting this behaviour for control are discussed.

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Across much of northern Europe, there is a traditional link between thunderstorms and the mass appearance of thrips migrating from cereal crops. The link is reflected in the common names for thrips in seven countries. This association could be a coincidence because cereal thrips migrate in warm weather in summer when thunderstorms are common, but personal observations and an earlier study indicate that a causative link is likely. The possibility that electric fields from thunderstorms affect the behaviour of cereal thrips is discussed.

Restricted access