Alfalfa

Thrips in Forage Alfalfa

Frank B. Peairs and Sue Blodgett

Onion thrips and western flower thrips are known to damage alfalfa regionally.  Both species have multiple generations each year, populations can build up quickly if conditions are favorable.  Large populations are often associated with hot, dry conditions.  

Identification (and life cycle/seasonal history)

Thrips are small, slender-bodied insects with two pairs of narrow fringed wings.  Female western flower thrips are about 1 mm long and are yellow to dark brown in color.  The smaller male is pale yellow and has a narrower and more pointed abdomen.  Adult female onion thrips are similar in size and are yellow with brownish blotches on the thorax and abdomen. Antennae are gray with the first segment lighter than other segments. Males are rare. 

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 A. N. Sparks, Jr. and T.X. Liu.  2001.  Thrips on onions:  Biology and management.  Texas Agricultural Extension Service L-5397.  http://williamson-tx.tamu.edu/IPM/ThripsonOnions.pdf and http://www.coopext.colostate.edu/TRA/PLANTS/thripsid.html).

The life cycles of these two species are similar.  The adults overwinter in protected areas such as under plant debris.  They become active during warm days in spring and are present until cold weather in fall.  Adult females insert their eggs into tissues of many different plant species.  Upon egg hatch there are two active, feeding stages (Instar I and II larvae) that resemble the adult in general shape, but are smaller and lack wings.  Thrips larvae are pale yellow and feed on developing leaf tissue.  Two additional, non-feeding larval stages are found in the soil.  A generation is completed in about three or four weeks during the summer.   

Plant Response and Damage

Thrips feed by puncturing the epidermis with a stout mandible, puncturing deeper mesophyll cells with a fine pair of stylets (maxillae) and sucking out the cell contents.  This feeding is sometimes described as "rasping-sucking", but is more accurately described perhaps as "puncture, poke and suck".   As chlorophyll is removed from feeding, the damaged area appears puckered, silvery or whitened, and may have yellow flecking.  Thrips also excrete tiny dark tar spots which can be useful when diagnosing onion thrips feeding injuries.

In alfalfa, they feed on developing leaf tissue. Scarring occurs at feeding sites, which in turn, cause the leaves to distort as they emerge.  Damaged leaves also may be curled or distorted, with a cupped or puckered appearance.   Heavy infestations of western flower thrips 7-10 days before cutting have resulted in leaf drop and losses in quality in western Colorado.

Sampling/Surveying/Timing of sampling

Reliable scouting methods have not been developed for thrips in alfalfa.  A combination of sweep net sampling and visual inspections for leaf damage is a good way to determine the presence of thrips in an alfalfa crop.  Also, buds and flower can be tapped into a white cup or onto a piece of white paper or cardboard.

Economic thresholds

There is no consensus on the importance of onion thrips and western flower thrips in alfalfa, although onion thrips are considered to have more damage potential.  Research based economic thresholds are not available.

Management Approaches

 Chemical Control

If a thrips treatment is contemplated, it is best to cut as soon as possible and treat the regrowth if the infestation persists.  Thrips are very difficult to control in alfalfa, excellent coverage is important and two applications may be required for satisfactory results.  Species identification is very important.  Onion thrips are more susceptible to pyrethroid insecticides (lambda-cyhalothrin in the table below), while western flower thrips is more susceptible to organophosphate and carbamate insecticides (carbaryl in the table below).  Dimethoate is labeled on alfalfa and has been used to control thrips, with results similar to the products below.  However, since thrips are not on the dimethoate label, it is not included in the product list.

Product List for thrips in alfalfa:

Insecticide	Product per Acre (Fl oz. or oz. product)	Preharvest Interval , remarks
carbaryl1,2	See labels	7 days. Most formulations are Extremely Hazardous to Bees! Do not apply to alfalfa in bloom. Sevin XLR+ is safe for bees if applied at <1.5 lbs ai/acre when no bees are in the field. Do not apply more than once or more than 1 7/8 lb per cutting
CobaltR1	19-38	7 days after application of 13 fl oz, 14 days after application of 26 fl oz, 21 days after application of rates above 26 fl oz per acre. Extremely Hazardous to Bees! Do not apply when bees are present.
lambda cyhalothrinR1,2	2.56-3.84	1 day forage, 7 days hay.  Extremely Hazardous to Bees! Advisable to move bees during application and allow 3 (low rate) or 5 (high rate) days before re-introduction of bees.  Not for western flower thrips.  Do not apply more than 3.84 oz./acre/cutting or more than 15.36 oz/acre/season
MSRR,1	24-32	21 days.  Extremely Hazardous to Bees! Do not apply to alfalfa in bloom. Do not apply more than 2 times/season.
ProaxisR,1	2.56-3.84	7 days for hay. Avoid application when bees actively foraging. Bee hazard when evening or morning dew. Do not apply more than 0.24 pt /A per cutting.  Do not apply more than 0.96 pt/A per season.  Advisable to move bees during application and allow 3 (low rate) or 5 (high rate) days before re-introduction of bees
RRestricted use pesticide 1Labeled for chemigation 2Generic active ingredient, several formulations.
The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.

Categories: Alfalfa, Insects, Onion thrips, Western flower thrips

Date: 5/20/2008