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Cereal Leaf Beetle Parasitoid Has Arrived in Skagit County!

Volume 5 Issue 6

Beverly Gerdeman and Hollis Spitler, WSU NWREC

The cereal leaf beetle (CLB), Oulema melanopus, (Coleoptera, Chrysomelidae), was first detected in Washington State in 1999 and is a pest of cereal grains, grass forage/seed crops, and other grass-host species.
Adult CLB feeding damage seen as slits in the leaves of grasses from a mass emergence along a drainage ditch in Skagit County, 20 April 2015.

Figure 1. Adult CLB feeding damage seen as slits in the leaves of grasses from a mass emergence along a drainage ditch in Skagit County, 20 April 2015.

The pest has a single generation in Washington State.  Damage is primarily due to the larvae, which feed on the leaf surface membrane, which contains chlorophyll and can impact grain quality and yield. Adults emerge in the spring, feed and mate.  Adult damage appears as slits in the leaves (Fig 1). The adults begin to lay eggs about 10 days later on the upper leaf surface (Fig. 2). Freshly laid eggs are orange in color, then turn dark before hatching in 4 – 23 days depending on temperature.  Young larvae are light yellow but soon turn black due to a coating of slime and feces.  This black slime easily wipes off onto your pants as you wander into the fields and indicates their presence.  Larvae feed for 3-4 weeks then shed their slimy coating and drop to the ground to pupate in earthen cells before emerging as adults 2-3 weeks later.  Emerging adults do not mate and enter a summer dormant period before seeking overwintering sites as temperatures and light begin to drop.
Two larval CLB feeding and 2 eggs, characteristically laid end to end, on the upper surface of the leaf, May 17, 2016.

Figure 2. Two larval CLB feeding and 2 eggs, characteristically laid end to end, on the upper surface of the leaf, May 17, 2016.

Biological control can reduce CLB populations by 70-90% but will not eradicate the pest.  In 2002, first releases of the exotic parasitoid wasp, Tetrastichus julius, the primary parasitoid of CLB, were made in collaboration with USDA APHIS from Montana and Pennsylvania.  CLB have become an increasing problem in Skagit County in the past 3 years and plans were being made for release of T. julius in 2016.  However a background survey of CLB larvae taken on 16 May from wheat and oat fields at the WSU NWREC station in Mount Vernon, revealed parasitoids were already present.  A 65% parasitization rate was determined (13 of 20 parasitized).  The parasitoid wasp, T. julius is present in Skagit County grains but the distribution of T. julius in the county remains unknown.  To determine if CLB larvae are infested, collect ~20 larvae and individually crush them in a 70% alcohol solution to observe the contents under a hand lens or dissecting microscope.  If parasitized, the wasp larvae will wiggle and appear as small, solid yellow, sickle-shaped,moving objects among the CLB guts (Fig. 3).
 Five wasp larvae are visible in the crushed CLB larva, observed May 17 at WSU NWREC.

Figure 3. Five wasp larvae are visible in the crushed CLB larva, observed May 17 at WSU NWREC.

Presence of these tiny parasitoid wasps should reduce CLB populations to non-damaging levels in the next couple of years.  If populations of the parasitoid continue to rise, Skagit County may serve as an insectary source for regions lacking the parasitoid.  For assistance in learning how to dissect the larvae, contact Bev Gerdeman at WSU NWREC.  Diana Roberts (WSU Regional Extension Specialist, , Spokane County) and Colin Park (USDA APHIS PPQ, Portland, OR) were instrumental in developing the parasitoid wasp release program in Washington State.

For more information refer to extension bulletin (PDF)