The Bean Leaf Beetle is an increasingly common insect pest in the soybean fields of Québec and Eastern Ontario.
Identification
Adults are usually yellow, tan, or red in color, with black and yellow legs. The head is black and antennae are yellow with dark tips. The body is characterized by rectangular black markings and an inverted triangle of the same color behind the neck. They measure approximately 5 mm.
Eggs are oval-shaped and may be red, orange, or yellow. They measure 0.80 mm by 0.35 mm. The mature larva measures approximately 10 mm in length; their bodies are beige, and dark at both ends. They have three (3) pairs of legs near the head and one fleshy “proleg” on the last abdominal segment.
Life cycle
Adult bean leaf beetles overwinter in grassy edges of fields or leaf debris. They emerge in May or June when the weather warms up. They also mate at this time and will lay 250 to 300 eggs at the base of soybean plants. The eggs hatch in about 10 days on average.
The overwintering bean leaf beetles produce one generation a year in Québec. The previous season’s beetles die in late June to make way for the next generation. The new larvae undergo three growth stages over the course of 18 to 24 days and transform into nymphs (pupa). The nymphs remain underground for seven (7) days and then emerge as adults.
Damage
Beginning in spring, the overwintering generation of bean leaf beetles feeds on the leaves of the soybean plant, leaving holes between the leaf veins. They also attack the cotyledons and new seedlings.In July and August, it is the turn of the new adults to damage the leaves, and the pods as well.
As a result, the seeds themselves will be affected in multiple ways, as the damaged pods will expose the seeds to damp or dry conditions, or to injury leaving them vulnerable to fungal diseases. Bean pod mottle, caused by a virus transmitted by the bean leaf beetle, can also affect seed quality.The larva can cause root damage.
Scouting adult beetles
- At the Emergence Stage (VE to V2) : Select five rows in the field at random and count the number of bean leaf beetles in 5 m of the row. Remember to check the undersides of the leaves and to move carefully in order not to alarm them;
- Beyond the Emergence stage (V2) and until the Full Pod stage (R4) : Determine the percentage of defoliation on five (5) plants in ten (10) locations selected in the field at random;
- Stage R5 and R6 : Select ten (10) sections in the field and assess the damage to ten (10) plants. Establish the percentage of defoliation on five (5) of the ten plants examined in each section.
Bean Leaf Beetle Economic Threshold
Ontario’s Economic Threshold for Bean Leaf Beetle Control is used to determine whether to treat the soybean plants. Along with this threshold, the recommendation is to consider the economic value of the crop as well as the yield potential in the field in order to make the right choice.
Control is necessary when the number of bean leaf beetles or the percentage of defoliation reaches the numbers listed below at the different stages.
- At the Emergence stage: 16 adult bean leaf beetles per 30 cm row;
- Vegetative growth stages: 30 % to 50 % defoliation;
- The stages from Beginning Bloom to Full Pod (R1 à R4) : 15 % to 35 % defoliation;
- From stage R4 to R6 : 25 % defoliation or 10 % damaged pods.
