Barley Thrips

Limothrips denticornis Haliday

Description
Adult barley thrips are very small, less than 2 mm long. To the naked eye they appear slender, dark colored and shiny. Females have feathery wings while the males are wingless. The wings lay flat along the back and are visible as pale or silver strips. Wingless larvae are pale yellow, white or green with a slightly darker head capsule and red eyespots. They have two pupal stages. The prepupal stage lasts a few hours to one day; the pupal stage lasts from two to six days.

Host plants
Barley thrips overwinter in headlands and in shelterbelts where they prefer Kentucky bluegrass and brome plants. During the growing season barley thrips can be found in barley grown throughout western Canada.

Overwintering
In the fall, females migrate from the crop into overwintering sites. Males and late maturing larvae remain on the crop and are killed by starvation and early frosts. Only the adult females overwinter in debris in fields and along headlands, especially near shelterbelts, in sod or under bark. They have also been collected from forest litter, leaf mould, insect galls and bird nests.

Spring appearance
In early spring, thrips leave their overwintering sites by launching themselves from the tip of a leaf or grass awn. They fly short distances or are carried long distances on wind currents. There is often a flight in early spring to grass hosts, and another flight later in spring to cereal hosts.

Once in a crop, flight is limited to very short distances between host plants. Females mate in the fall but require approximately six weeks in the spring for the eggs to mature. Eggs are laid, either scattered or in short rows, in an incision usually made alongside the leaf veins. Upon hatching, the larvae feed for approximately seven days, shedding their skin twice.

Adult male barley thrips mate with the pupal stage of the females. Females are already mated when they emerge from the pupae. After two or three days in the pupal stage adults emerge and feed until the crop begins to mature.

Feeding damage
Both immature and adult barley thrips feed by rasping or scraping away the leaf surface and sucking out the cell contents. This feeding causes the plants to take on a whitened or bleached look.

Economic Importance

Damage assessment
Barley thrips infest barley at a very early stage and remain feeding on the inside of the leaf sheath. At the beginning of head formation, intensive feeding produces heads that appear white or silvery and may produce thin, shrivelled grains. Often there is no seed development at the top and bottom of the head and intermediate grains are shrivelled. Yield losses may also occur if feeding in the sheath is intensive enough to kill the flag leaf. Without the flag leaf, kernels do not fill properly and seed weight is reduced.

Sampling methods
Sample for barley thrips from when the second node is visible until the head has completely emerged from the boot. Most thrips will be found under the top two leaf sheaths. Break the plant at the second node from the top and run your thumb nail between the edges of the sheath at the collar. Carefully unroll the sheath from the stem.

Thrips populations can be estimated by counting the number of thrips found on the top two leaf sheaths of stems. Roughly divide the field into nine equal sections and take one stem at random from each section. If the total number of adults from the stems is less than 34, no insecticide treatment is recommended. However, if the number of adults is greater than 87, an insecticide application is recommended.

For thrips numbers between 34 and 87, re-sample the field taking one stem from each of the nine sections. As shown in the table, this process is repeated until a decision can be reached or until the number of stems sampled reaches 63. If a decision cannot be made, re-sample the field after a day or two. In practice, only a small percentage of fields should require sampling more than 18 stems before a decision can be reached.

Summary of the Barley Thrips Sequential Sampling Plan

• Divide the field into 9 roughly equal parts
• Take one stem from each part
• Count the thrips found within the top two leaf sheaths
• Consult the decision table
  

Management Strategy

For chemical controls consult the current Alberta Agriculture publication Crop Protection guide or the insecticide selector.

The most viable alternative to insecticidal control is breeding plants for thrips resistance. At present, there are no barley varieties with documented resistance to thrips.