Thrips in small grains cereal crops

Thrips (used for both singular and plural) are members of the Order Thysanoptera. Even more confusing, there is also a genus of thrips named Thrips. That is, all Thrips are thrips but not all thrips are Thrips!

Thrips are characterized by small size (the largest species is only 2 mm as adults; the smallest is 0.6 mm), long slender bodies, and fringed wings (winged and wingless adults exist in some species). Males are smaller than females.

Figure 1: Adult thrips on barley leaf showing off fringed wings neatly folded over its abdomen. Photo: Sheila Elder, Saskatchewan, Canada

Adult thrips are generally relatively weak flyers and employ a‘clap and fling’ technique. The animal claps the leading edges of its wings together at the end of the upstroke then rotates the wings around the trailing edges, flinging them apart. Many small insects use this technique to promote air circulation and generate lift quickly. Pigeons also use this technique for their noisy flight initiations. For small insects, the viscosity of the air has a much greater effect than on larger animals. Fringed wings reduce drag associated with this effect.  

There are about 6,000 species of thrips worldwide with 147 described species in two suborders in Canada, including 28 non-natives. Recent molecular work indicates that there may be as many as 255 additional as-yet-undescribed species in Canada. The most common and broadly distributed family is the Thripidae, followed by the Phlaeothripidae and Aeolothripidae. Other families are far less represented.

Although some species are important for pollination and a few are predators of other small insects, some are pests in crops. They have unique, asymmetrical mouthparts characterized by a greatly reduced right mandible. Their feeding is described as ‘rasping-sucking’: they scrape the surface of plant tissue and ingest fluid flowing from the wound. When feeding on actively growing plant tissue, growth reductions and distorted growth may be observed and yield loss can occur. When they feed on more mature tissue, silver leaf scars can occur that reduce the quality and marketability of some crops. Thrips are also important vectors of topsoviruses.

One suborder of thrips lays very small eggs (0.08 mm to 0.2 mm) singly in slits in plant tissue; the other lays eggs on plant surfaces. Eggs hatch into nymphs: juveniles resemble adults but are not sexually mature and have no wings. There are two juvenile feeding stages, followed by two non-feeding stages: pre-pupa and pupa.   

The barley thrips, Limothrips denticornis, was first reported in North America in 1923 in New York. In its native Europe and Asia, it can be found on a wide variety of grass species but is a minor pest and only on rye. In North America, it is generally more important on barley, though it can be found on winter wheat, durum, winter rye, corn, and triticale. Adults are small (1.1 mm to 1.8 mm), elongate, and dark brown to black. These thrips lay eggs on upper leaf sheaths and each female can produce 100 eggs. Juveniles are smaller and lighter coloured. Barley thrips overwinter as adults and move to winter grasses in the spring. They are somewhat stronger flyers than many thrips species, but are still limited by their size. In Northern Europe, cereal thrips, including L. denticornis, have been reported to appearin large numbers ahead of thunderstorms. This may be associated with the warm conditions that precede these events, but it has also been suggested that they are sensitive to the electrical fields associated with storms.    

Another cereal thrips, Limothrips cerealium, has also been reported in Canadian small grains cereals and was reported in 1928 to be responsible for 10 per cent losses in oats in Canada.

Thrips feeding on cereals can result in tissues appearing bleached. When numbers are high and feeding is intense, kernels can be shriveled. Severe flag leaf feeding can result in kernels filling improperly and reduced kernel weight.

Figure 2: Thrips nestled at the base of leaf. Photo: Sheila Elder, Saskatchewan, Canada

Scouting for barley thrips should be done from first sign of flag leaf until the head is completely emerged from the boot. Barley thrips can be found on stems but are more commonly under the top two leaf sheaths. Because thrips are relatively weak flyers, there may be greater concentrations in protected field edges. Greatest damage has been reported in dryland cropping areas after prolonged drought.

Economic thresholds:

Threshold (thrips/stem) = (Cost of control per acre / expected $ value per bushel) / 0.4

.Sample at least 50 stems from different parts of the field. One adult thrips per stem can cause a loss of 0.4 bushels per acre. This usually translates to an action threshold for barley and oats of 7 – 8 thrips/stem prior to head emergence but greater precision can be achieved by using the formula. The action threshold is the number of insects detected that can cause enough damage to justify the expense and effort of applying control. Numbers lower than this do not warrant control. Only apply control prior to the completion of heading.

Thresholds for cereal thrips have been determined for barley and oats but effects on other cereals crops in North America are less well understood. Work in Europe indicated comparable damage per thrips in rye, triticale, and winter barley. Recent reports of barley thrips in durum also suggest a risk of damaging effects, but these are not as well understood. A report from Germany indicated that, despite some relatively high thrips numbers, there was no correlation between barley thrips and damage. However, there is also evidence from Europe of the importance of long crop rotation to thrips damage control in wheat.