Author: Gary L. Hein & Frank B. Peairs[1]
Thrips can be serious problems in years where a warm, dry spring is experienced. This allows for thrips to build up in surrounding areas before moving into bean fields when their host plants mature.
Identification (and life cycle/seasonal history)
Two kinds of thrips, onion thrips, Thrips tabaci Lindeman, and western flower thrips, Frankliniella occidentalis (Pergande), attack dry beans in the region. Thrips are small, active, cigarette-shaped, yellow-to-brown insects. Onion thrips are most commonly associated with furrow irrigated beans grown in close proximity to winter wheat. Infestations commonly occur as area wheat and grasses mature, and the onion thrips disperse in search of new food sources. Western flower thrips will be present later when bean are flowering, and they tend to congregate in the bean flowers.
It is difficult to tell these two species apart without the aid of a good microscope. The shape of the antenna and the arrangement of the setae (hairs) behind the head are the most reliable characteristics to use to separate these two species (see http://txspace.tamu.edu/bitstream/handle/1969.1/87053/pdf_1476.pdf?sequence=1 and http://wci.colostate.edu/shtml/ot.vs.wft.shtml).
Plant Response and Damage
Onion thrips feed by puncturing the bean leaf and sucking up the plant juices that ‘bleed’ from the plant. A yellow spotting of the leaves will occur at the feeding sites. Extensive thrips feeding results in leaf cupping and distortion that is made more severe by plant stress (low moisture and high temperature). Non-stressed crops often outgrow the damage with little loss in yield. Western flower thrips feed in developing flowers and can cause flower and pod abortion. Pod abortion is worsened by increased plant stress. Both of these thrips can transmit tobacco streak virus (Red Node).
Management Approaches
Consider treatment if there are more than 15 onion thrips per plant and damage (leaf cupping) is observed. Tap the plant on white cardboard or in a white container and then count the thrips that have been knocked off. Onion thrips infestations and damage are often more severe at field edges so be sure to assess the entire field before making a treatment decision.
Five flower thrips per blossom can reduce the number of pods per plant. Although this is the point where pod loss starts, economic losses may not occur until higher flower thrips levels are reached. To sample for flower thrips, drop blossoms into a container of rubbing alcohol to drive the thrips out so that they can be counted easily. Both species of thrips are more prevalent during dry periods, and populations are often reduced by rainfall or sprinkler irrigation. Onion thrips have not been difficult to control, but flower thrips may prove difficult to reach with contact insecticides.
Species identification is very important. Onion thrips are more susceptible to pyrethroid insecticides (e.g., lambda-cyhalothrin in the table below), while western flower thrips is more susceptible to organophosphate and carbamate insecticides (carbaryl in the table below).
Due to constantly changing labels, laws and regulations, the sponsors of this web site assume no liability for the suggested use of any chemicals noted in the management section. Contact your local pest management personnel for current information on labeled pesticides that can be used for the intended plant disease, insect pest or weed.
