Eggs
The majority of thrips species are so small and cryptic that, except when present in very large numbers, many inspectors and commercial operators may fail to see them. Adults and larvae are able to hide in concealed places on plants such as beneath plant hairs, within tight buds, enclosed in developing leaves, or underneath the calyx of fruits. Eggs are laid concealed within plant tissues. Casual inspection may thus not reveal the presence of thrips, and even insecticide treatment may be ineffective because the chemical fails to contact the hidden thrips. Effective detection methods have yet to be deployed by most quarantine inspection systems, reliance usually being placed on inspection for feeding damage and simple beating to reveal thrips. However, adult and larval thrips can be extracted from plant material within two or three minutes if a sample is placed in a small Tullgren Funnel using turpentine as an irritant rather than light;the living thrips then run down into a glass tube at the bottom of the funnel where they are readily observed and counted.
Infestation levels in glasshouse crops are usually monitored by means of blue or yellow sticky traps. One shade of blue is particularly attractive to flying adult thrips and is widely used for monitoring the species (Brødsgaard, 1989a). Pheromone lures that attract males and females are now available to increase the sensitivity of monitoring at low levels of infestation or in easily damaged crops (Hamilton et al., 2005). Thrips can also be monitored by extracting thrips from flowers and recording their numbers or the percentage occupancy of flowers (Navas et al., 1994;Steiner and Goodwin, 2005). Western flower thrips adults are easily carried into glasshouses by wind, as well as on the clothes or in the hair of working personnel, thus making re-infestation from surrounding weeds a constant probability. Indeed, weed control around a crop, whether inside a glasshouse or on surrounding land, is the first measure to be adopted in any control strategy. Thrips are also easily carried on equipment and containers that have not been properly cleaned, and infestations in sterile laboratories with filtered air are usually due to thrips being carried in on the clothes and hair of workers. Nationally and internationally, F. occidentalis is readily transported to new areas on all types of planting material as well as on cut flowers, both commercial and domestic (Vierbergen, 1995).
Related invasive species
- Frankliniella occidentalis
Related Farm Practice
- Breeding
- Damage
- Hosts
- Pests
- Transmission
- Host plants
- Feeding
- Economic valuation
Related location
- Italy
- Kenya
- Europe
Since the 1970s Frankliniella occidentalis has successfully invaded many countries to become one of the most important agricultural pests of ornamental, vegetable and fruit crops globally. Its invasiveness is largely attributed to the international movement of plant material and insecticide resistance, both of which have combined to foster the rapid spread of the species throughout the world (Kirk and Terry, 2003). Individuals are very small and they reside in concealed places on plants;thus are easily hidden and hard to detect in transported plant material. They reproduce rapidly and are highly polyphagous, breeding on many horticultural crops that are transported around the world.
The symptoms of infestation by F. occidentalis vary widely among the different plants that are attacked. On roses or gerberas with red flowers, or on dark Saintpaulia flowers, feeding damage is readily visible as white streaking. This type of damage is less apparent on white or yellow flowers, and these commonly tolerate very much higher thrips populations with no visible symptoms. Severe infestation leads to deformation of buds if the feeding occurs before these start opening. Capsicums and cucumbers that have been attacked whilst young, show serious distortions as they mature. Leaf damage is variable, but includes silvering due to necrotic plant cells that have been drained of their contents by thrips feeding, malformation due to uneven growth, and a range of spots and other feeding scars. Eggs laid in petal tissue cause a 'pimpling' effect in flowers such as orchids. Egg laying on sensitive fruits such as table grapes, tomatoes and apples leads to the spotting of the skin of the fruit, which reduces the aesthetic value of the fruit. It can also lead to splitting and subsequent entry of fungi. However, the most serious effect of thrips feeding is due to the transmission of tospoviruses into susceptible crops, such as tomatoes, capsicums, lettuce or Impatiens. At least five different tospoviruses are known to be transmitted by western flower thrips and more may well be discovered: Tomato spotted wilt virus (TSWV), Impatiens necrotic spot virus (INSV), Groundnut ringspot virus (GRSV), Chrysanthemum stem necrosis virus (CSNV) and Tomato chlorotic spot virus (TCSV) (Whitfield et al., 2005). These viruses are acquired by the first-instar or early second-instar larvae when feeding on an infected plant, and are then transmitted only later when these larvae develop into the mobile adults;it is not possible for an adult to acquire and then transmit any of these viruses (Moritz et al., 2004). Virus symptoms vary considerably among plants, ranging from the disastrous wilting and collapse of lettuce plants, through a range of leaf mottling and distortions, to ring-spotting on tomato and capsicum fruits. These virus attacks can lead to the total loss of certain crops (see reviews in Kuo, 1996). F. occidentalis also transmits a carmovirus (Pelargonium flower break virus, PFBV) and may transmit an ilarvirus (Tobacco streak virus, TSV) (Jones, 2005).
F. occidentalis is a highly polyphagous species with at least 250 plant species from more than 65 families being listed as 'hosts'. Unfortunately, the term 'host plant' is poorly defined in the literature on thrips. Plant species have sometimes been listed as 'hosts' simply because adults have been collected from them. The concept of 'host plant' is best restricted to those plants on which an insect can breed, and for many of the 250 plants from which F. occidentalis has been recorded there is little or no evidence of successful breeding. However, the association of adults with various plants has economic importance when viruliferous adults feed on susceptible plants. In its native range of the western USA, this thrips species can be found in large numbers on a very wide range of native plants, from lowland herbs to alpine shrubs and forbs. As a pest it is found both outdoors and in glasshouses, and it attacks flowers, fruits and leaves of a wide range of cultivated plants. These include apples, apricots, peaches, nectarines and plums, roses, chrysanthemums, carnations, sweet peas, Gladiolus, Impatiens, Gerbera and Ranunculus, peas, tomatoes, capsicums, cucumbers, melons, strawberries, lucerne, grapes and cotton. In northern Europe it is found particularly on glasshouse crops, such as cucumbers, capsicums, chrysanthemums, Gerbera, roses, Saintpaulia and tomatoes. In southern Europe it is extremely damaging to many field crops, including capsicums, tomatoes, strawberries, table grapes and artichokes, and at least in southern Italy, it has become a dominant member of the thrips fauna in wild flowers. Similarly, in Kenya the species has become a dominant member of the wild thrips fauna near agricultural fields. In contrast, in Australia it has not been found breeding on any native plant species. A further complication in considering its pest status is that in some areas this thrips species is an important predator of plant-feeding mites, such as on cotton in California, and it is then regarded as a beneficial (Trichilo and Leigh, 1986).