Although thrips are one of the most common early seedling pests in most cotton growing districts, their damage is usually cosmetic and rarely affects yield or earliness. Severe damage, resulting in plant tip loss or seedling death, tends to occur only under extreme infestation levels or in combination with cool weather. Control is warranted under these circumstances. However all thrips species commonly found in cotton are also predators of spider mite eggs. So control of thrips to prevent damage should be carefully weighed against their predatory activity and the impact of insecticides on other beneficials.
The most common species infesting cotton are tobacco thrips,
Thrips tabaci Lindeman and tomato thrips,
Frankliniella schultzei (Trybom). Plague thrips,
Thrips imaginis Bagnall, although previously suggested to be an important species, is found rarely in cotton. Adults are small, cylindrical insects less than 1.5 mm in length, and colours range from yellow to grey to black. They have two pairs of narrow wings fringed with long hairs. The nymphs are less than l mm in length, pale yellow and wingless.
T. tabaci is the most abundant in the early season and is usually a pale straw colour.
F. schultze can be abundant on young cotton but more typically dominates once flowering has commenced. This species can vary in colour but in most cotton regions is typically a dark almost black colour.
In the 2001/02 cotton season, the western flower thrips,
Frankiliniella occidentalis (Pergande), was recorded in cotton for the first time. This species was accidentally introduced into Australia in the early 1990's. It is similar in appearance to
F. schultzei exept that it is paler in colour (see picture). This makes it easy to distinguish from
F. schultzei later in the season but hard to distinguish from
T. tabaci in the early season. The western flower thrips causes similar damage to cotton as our native species and also feeds on spider mite eggs. The strains introduced to Australia have resistance to a range of insecticides, including many organophosphates and carbarmates.
Damage Thrips are pests only during the seedling stage of growth. They infest the underside of cotyledons, young leaves and also the growing point of the plant. They use their mouthparts to pierce the leaf surface and suck out plant juices. Damaged leaves take on a silvery, bleached appearance and younger leaves become distorted in shape. Under most circumstances, this form of damage is cosmetic, plants recover quickly and control is not warranted.
Thrips infestations tend to be serious under cool, dry conditions when seedlings are unable to grow away from the damage. Severe damage can result in destruction of the growing point ('tipping out') leading to extensive lateral branching. Seedling death can be associated with extreme infestations or a combination of desiccation by thrips and damping off diseases. If damage to this extent is allowed to occur, heavy losses of plant stand may necessitate replanting the crop, while severe multiple branching may cause a maturity delay of 7 to 14 days. This delay, if compounded by damage from other pests, may cause a yield loss. Analysis of the effects of thrips on the yield and maturity of cotton over the past 6 years has shown that the risk of yield loss occuring is about 1 year in 10 in warm regions; but is higher at 1 year in 2 in cool regions. Overall, the risk of delayed maturity was low with delays of less than 4 days most common. Only extreme populations (70 thrips per plant) caused delays in excess of 7 days. This information can be used to help make decisions on the need for prophylactic insecticide treatment to the seed or at planting.
Lifecycle
Adult thrips may swarm over cotton and cause damage to young leaf tissue for only a few days before disappearing, but it is more common for them to lay eggs in slits which they cut in leaves and growing points. This produces a generation of nymphs which also damage seedlings. Nymphs particularly attack the growing point and folded tip leaf where they are well concealed. The nymphs pupate in the soil. The length of the life cycle from egg to adult varies from 44 days at 11degrees C to 9 days at 25 degrees C. Populations decline at temperatures greater than 30 degrees C.
Large numbers of thrips have been observed in early spring on flowers of cereal crops (wheat, barley) and on weeds such as Mexican poppy, turnip weed and Paterson's curse. The thrips then transfer to cotton when the cereal crops and weeds dry out or hay off. A wet winter followed by a cool, dry spring appears most favourable for thrips abundance, prolonging the flowering period of weeds and therefore the supply of pollen. Cotton crops adjacent to cereal crops are especially at risk from thrips and should be monitored closely for signs of thrips damage. In the absence of pollen on seedling cotton, thrips feed on other sources of protein such as eggs or larvae of some cotton pests, particularly mites. Destruction of thrips populations with broad spectrum insecticides is one of the most important contributors to mite populations reaching damaging levels.
Another thrips species, the six spotted thrips, Scolothrips sexmaculatus (Pergande), is a predatory species and feeds mainly on mite eggs and motile stages. The nymphs of S. sexmaculatus are difficult to distinguish from other thrips.
Thrips adults are highly mobile, and nymphs are very small and difficult to find, therefore counts may vary with the time of day. The damage potential also varies according to crop vigour and temperature. Seedlings will tend to grow away from infestation if the temperatures are warm. On the other hand, thresholds may need to be lowered in adverse conditions such as cold weather, where herbicide damage to plants has occurred, or where damping-off disease is widespread. For more information see the Cotton Pesticides Guide or CottonLOGIC.
Thrips control
Seed dressings can reduce early damage but will degrade over 14 to 21 days post planting. By then leaf area should be sufficient for above-ground applications of systemic insecticides to be effective if necessary. Systemic granular insecticides, applied at planting, provide longer control, but in some districts they are not necessary for thrips control due to low numbers or good growing conditions. If using seed dressings or granular insecticides be aware that adult thrips will continue to be found in the crop as they move from cereals and weeds. These adults will feed and die, causing no damage. The best indicator of declining control is the presence of thrips nymphs which indicates that the pesticide is no longer effective.
Due to the capacity to recover from some damage, thresholds for thrips must include both pest abundance and damage levels - which should both be exceeded before control is warrented.
Thrips Thresholds in Different Phases of IPM Guidelines
| |
Planting to flowering (1 flower per m) |
Flowering to Cut-out (1 open boll per m) |
Cut-out to Harvest |
| Adults and Larvae/Plant |
10 |
no threshold |
no threshold |
| Damage (Reduction in leaf area) |
80% |
no threshold |
no threshold |
