Barley Thrips

Limothrips denticornis Haliday

Insect Life Cycle

• Description
  Adult barley thrips are very small, less than 2 mm long. To the naked eye they appear slender, dark coloured and shiny. Females are winged and males are wingless. The wings lay flat along the back and are visible as pale or silver strips. The larvae are wingless, pale yellow, white or green with a slightly darker head capsule and red eyespots. They have two pupal stages. The first ("prepupal") stage lasts only a few hours to one day; the pupal stage lasts from two to six days.
  
• Host plants
  Barley thrips overwinter in sod along roadsides and in shelterbelts. They prefer Kentucky bluegrass and brome sod in shelterbelts. During the growing season barley thrips can be found in barley grown throughout western Canada. In the fall, females migrate from the crop into overwintering sites. Males and late maturing larvae remain on the crop but are killed by starvation and early frosts.
  
  
• Overwintering
  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 birds' nests.
  
  
• Spring appearance
  In early spring, thrips leave their overwintering sites by launching themselves from the tip of a leaf or grass awn and flying short distances or being carried long distances on wind currents. Research shows that there is often a flight in early spring, probably 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 to mature eggs. Eggs are then 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. Therefore, females are already mated when they emerge from the pupae. After two or three days adults emerge and feed until the crop begins to mature.

• 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
  According to work done in North Dakota, thrips populations can be estimated by counting the number of thrips found on the top two leaf sheaths of stems collected randomly from a field. Sample during the period when the second node is visible and the completion of the boot stage. Divide the field into roughly nine equal sections and take one stem at random from each section. If the total number of adults from the none 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, an additional sample of one stem from each of the nine sections is required. As shown in the table, this process is continued until a decision can be reached or until the number of stems sampled reaches 63. If a decision cannot then 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
  

Decision Table for Barley Thrips Sequential Sampling Plan

# Samples	Don't Treat	Treat
9	33	88
18	83	160
27	135	229
36	188	298
45	243	365
54	298	431
63	353	497



• Economic threshold
  Since susceptibility of the crop varies according to plant variety, weather conditions and management practices, it is difficult to predict losses due to thrips damage. However, work in North Dakota indicates that the threshold for damage is approximately 4 adults per stem. This threshold has not yet been validated in Alberta.
  
  Dimethoate and methomyl are registered for thrips control in barley. Due to the current value of barley, however, spraying may not be warranted except for seed growers who find extremely high populations or who have experienced problems in the past and are currently finding levels above the threshold.