Thrips are one of the most common seedling pests of cotton in most cotton growing districts. Their damage is highly visible but in warmer cotton regions is often cosmetic as damaged crops will recover with no yield loss or delay in maturity. Control is only needed in years of high numbers – usually when spring conditions are warm and dry – forcing thrips onto cotton, which is often the only available host. In cooler regions the risk from damage is greater, as plants have less time in which to recover and grow. Prophylactic control, using treated seed or other at planting applied insecticides, may be justified. Use of foliar applied insecticides against thrips on seedlings must be balanced carefully against the negative effects on the beneficial complex as disruption may encourage outbreaks of Spider mites and Aphids. Note that thrips themselves are important predators of Spider Mite eggs and can severely retard the development of mite populations.
The most common species infesting cotton are Tobacco Thrips (Thrips tabaci Lindeman), Tomato Thrips (Frankliniella schultzei Trybom) and Western Flower Thrips (Frankliniella occidentalis). Tobacco Thrips are the most common species on seedling cotton, but numbers tend to decline naturally by late November in most regions. Both Frankliniella species are only occasional pests on seedling cotton but are more common once flowering starts. Western Flower Thrips are tolerant of or resistant to many insecticides and are poorly controlled by any of the currently available seed treatments or at planting insecticides. Later in the season this tolerance/resistance is beneficial as it survives well in sprayed fields so will continue to provide valuable control of mites. Plague Thrips (Thrips imaginis Bagnall), although previously suggested to be an important species, is found rarely in cotton.
Identification: Adults are small, cylindrical insects less than 1.5mm in length. They have two pairs of narrow wings fringed with long hairs. The nymphs are less than lmm in length and wingless. First instar larvae of all three species are pale white, almost transluscent, while second instars are pale yellow. Both are common in the terminals of unprotected seedling cotton plants. The colour of adults is variable, but in general, adults of Tobacco Thrips and Western Flower Thrips tend to be a straw coloured, while Tomato Thrips tend to be dark or almost black (though there is a pale variant that can be found in tropical areas).
May be confused with: The Predatory Six-spotted Thrips (Scolothrips sexmaculatus). Larvae of this species are always pale and semi translucent. Adults have three pairs of spots on the wings and these are visible with a hand lens. Thrips may also be mistaken for Springtails as they are similar in shape.
Lifecycle: Adult Thrips lay their eggs in slits which they cut in leaves and growing points. This produces a generation of larvae which attack the growing point and folded tip leaf where they are well concealed. The first two larvae stages are completed on the plant, then the larvae drop to the soil where they complete the last two stages. The length of the life cycle from egg to adult varies from 44 days at 11ºC to 9 days at 25ºC. Populations tend to decline at temperatures greater than 30ºC.
Host range: Adult Thrips can be found in early spring on flowers of cereal (wheat, barley), canola and legume crops and on a wide range of weeds such as mexican poppy, turnip weed and paterson's curse. Larvae may be found in flowers and on leaves. The adult Thrips then transfer to cotton when the crops and weeds hay off. In the absence of plant hosts, thrips feed on other sources of protein such as mite eggs.
Damage: Damage occurs primarily to seedling cotton but can also occur in the mid to late season. The adults and larvae will feed directly on the undersides of leaves causing ‘silvery’ damaged areas. On seedling cotton they also feed within the growing terminal and may damage the emerging leaves. This can result in mild leaf distortion (leaves slightly malformed and crinkled – 1 to 5 thrips per plant), severe leaf distortion (leaves greatly reduced in size and cupped - 5 to 20 thrips per plant) through to complete death of the terminal causing the plant to produce new shoots from auxiliary buds elsewhere on the plant (20+ thrips per plant). Extreme damage may result in yield loss and delayed maturity and may be exacerbated by cool weather that further slows plant growth. Control is warranted under these circumstances. However all Thrips species commonly found in cotton are also predators of Spider Mite eggs. In some situations Thrips populations (Frankliniella spp.) build in the mid to late season and can cause obvious damage to young leaves, especially those in the terminal area, resulting in reduced leaf size. Cotton yield can be reduced if leaf tissue loss is excessive. Generally plants are more sensitive earlier and increasingly less sensitive later. As a rough guide, leaf area loss of greater than 10 -15% could result in yield loss if it occurred before crop cutout. After cutout losses of up to 15-20% could be tolerated with low risk. It is unlikely that late season Thrips would cause sufficient damage to require control, and they play an important role in controlling Spider Mites.
Monitoring: It is only necessary to sample for Thrips between seedling emergence and around 6 true leaves. Check the total number of Thrips per plant. Thrips will only cause economic damage in high numbers (>30 per plant) or if conditions are cool. Keep an eye on the presence of nymphs, which is a sure sign that the population is reproducing. Assessing early season tip and leaf damage is just as important and thresholds for both Thrips numbers and plant damage are available in the Cotton Pest Management Guide.
Control: In warm regions, control of Thrips is rarely warranted but if it undertaken it is best to use at-planting options such as seed treatments as these have less negative effects on beneficial populations. In cooler regions, the risk that economic damage will occur is higher and thrips should be managed, preferable with a seed treatment or at-planting insecticide. Application of broad spectrum foliar insecticides to control Thrips should be carefully weighed against their predatory activity and the impact of insecticides on the other beneficials.
Natural enemies: Pirate bugs, Lacewings and Ladybeetles are all good predators of Thrips.
A Tomato Thrips. One of the most common early seedling pests in most cotton growing districts. <1.5mm Photo: L. Wilson
Early season Thrips damage is usually cosmetic and rarely affects yield or earliness. Photo: A. Bishop
Thrips feed directly on the undersides of leaves causing ‘silvery’ damaged areas. Photo: L. Wilson
In the absence of plant hosts, thrips feed on other sources of protein. These Thrips have been pictured feeding on eggs in a mite colony. 1mm Photo: L. Wilson