Scouting for Bean Leaf Beetle

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Bean leaf beetles primarily target soybeans but can also inhabit dry beans and forages.

Appearance and Life Cycle

Adults range from 4 to 9 mm in length and vary in colour but are most often yellow, orange, tan or red.  They may or may not have four square-shaped black spots on their wings and all have a small black triangle at the start of their wings.  They are often mistaken for a cucumber beetle or ladybeetle adults.  Larvae can be up to 10 mm in length and are white with brown colouring at each end of their body, as well a 3 pairs of legs.  They look very similar to corn rootworm larvae.

Bean leaf beetle have one complete life cycle per year.  Adults overwinter in grassy field edges, underneath piles of dead leaves and emerge around the end of April (when temperatures are between 10 and 13°C); they will then feed on early emerging soybeans or forages and live until approximately the end of June.  They lay lemon-shaped, orange eggs at the base of soybean and legume plants and when the larvae hatch they feed on soybean roots and nodules but are not of economic concern.  Pupation occurs in the soil and first generation adults emerge in July and feed into fall before moving into their overwintering habitat. 

Damage

Areas of southern Ontario, where soybeans are planted early and overwintering of bean leaf beetle is the most successful are 15 to 25% higher risk for damage than other parts of the province.

The overwintering generation feeds from April to mid-June; cutting holes between the main veins in leaves and/or cutting seedling plants at the base.  Damage from the first generation of beetles can be found from July to September; they cut holes between the main veins on leaves or on the surface of pods, leaving only a thin film of tissue to protect the seeds.  These pod lesions increase susceptibility to secondary seed diseases such as Alternaria.  Other damage from the first generation of bean leaf beetle can include clipped pods and bean leaf beetles can vector bean pod mottle virus causing the plant and seed to become wrinkled and mottled.

Scouting & Thresholds

·         Seedling Stage (VE-V1) – randomly select five areas in the field to sample; count beetles found in 5 – 6 meters of row at each location.  Note the average number of beetles per row.  Try not to create much disturbance when scouting as beetles may drop off plants and hide in soil cracks as you approach.  If there are 16 adult beetles per 30 cm of row and plants are clipped off at the stem, control is warranted.

·         Beyond Seedling Stage (V2-R3) – establish the percent defoliation of 20 plants in five areas of the fields and confirm he presence of adults.  30% defoliation pre-bloom (vegetative stages) warrants control, 15% defoliation R1-R4 warrants control.

·         R4-R6 Stages – visual observation of percent defoliation and damaged or clipped pods on 20 plants in five areas of the field (avoiding edges) and confirm the presence of adults. 25% defoliation or 10% of pods with feeding injury warrant control.

 

Prevention and Control Methods

Fields with a history of bean leaf beetle seedling injury should be planted later (end of May to beginning of June); after the emergence of the overwintering generation to avoid feeding on seedlings.  However, later planted fields may be more susceptible to pod feeding from first generation adults in July and August.  Therefore, fields with a history of bean leaf beetle pod injury should be planted early to avoid damage from first generation adults.

Neonicotinoid seed treatments can be used help early-season control of bean leaf beetle and lessen the impact of damage on seedlings.

Foliar insecticide treatment is recommended when defoliation threshold levels have been reached.  Talk to your local DEKALB Agronomist for recommendations.

 

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Sources: Grain Farmers of Ontario. Guide to Early Season Field Crop Pests. (verified May/15). Natasha Wright, Florida Department of Agriculture and Consumer Services, Bugwood.org