Early-season leaf feeding before the 3rd trifoliate can look serious because plants are small. However, soybeans can compensate for large amounts of leaf loss with little impact on yield. Soybeans will put on new leaves quickly at the top of the plant and unaffected leaves actually grow larger to compensate. Before the plants begin to flower, up to 100% of the leaves can be removed with almost no yield loss if the rest of the growing season is favourable. If the plant is cut off below the cotyledons it will not recover. Once the plant starts to flower (growth stage R1), leaf feeding becomes more critical because the plants requirements from photosynthesis increase. (Refer to Table 1.) There are two main spring leaf feeding culprits of soybeans in Ontario. These are slugs and bean leaf beetles.
Slug feeding can resemble hail damage, but damaged leaves have more of a “skeleton” appearance. Refer to Photo #1. Slugs feed on leaves, but they can also feed on germinating seeds, hollowing them out before they can emerge. There is one generation per year but two populations – one maturing as adults in the spring and one maturing as adults in the fall. Slugs are most active during cool and wet periods, and prefer environments with high humidity and cool temperatures. Crop residue or manure provides shelter from the sun.
Photo 1 – Slug feeding
|Percent Leaf Area Destroyed|
Tillage can be used against slugs, since the reduction of the crop residues on the surface exposes the slugs to dehydration. Plowing is not necessary. Vertical tillage (RTS, Turbo-till, etc.) in the spring can reduce slug numbers. Zone tillage or row sweepers also helps reduce spring feeding.
- Planting into conditions that help the crop to grow quickly can avoid heavy damage. Waiting to seed later in the spring can reduce damage if the field is known to have high slug numbers.
- There are no economical chemical methods available in field crops. Slug baits are available but are expensive and are only recommended for use in small problem areas. Experiments with 28% nitrogen/water mixtures or foliar potash applications have proven to be inconsistent.
Bean Leaf Beetles (BLB)
Adult feeding appears as small round holes between the veins of the leaves. Cotyledons and seedling plants can be clipped off by heavy populations. Late-season pod feeding can also be a problem. Bean leaf beetles feed on the surface of the pod, leaving only a thin film of tissue to protect the seeds within the pod. These pod lesions increase the pod’s susceptibility to secondary pod diseases such as alternaria. Pods may also be clipped off the plant, but this is not the primary cause of yield loss. The most important concern is that BLB is a vector of bean pod mottle virus. This virus causes the plant and seed to become wrinkled and mottled, reducing the quality of the seed.
The bean leaf beetle can be confused with other insects, such as the lady beetle. The way to distinguish a BLB is by the small black triangle behind the head. Bean leaf beetle adults vary in colour, but always have a small black triangle visible behind the head. They may or may not have four spots, and are about 5 mm (1/5 in.) in length. Refer to Photo #2.
Photo 2 – Bean Leaf beetle
- CruiserMaxx seed treatment does an excellent job to control early season feeding. Fields with a history of bean leaf beetles should be planted with treated seed.
- If early season numbers are high enough to clip off plants below the cotyledon, the field should be sprayed. If leaf feeding exceeds 30% before flowering and 15% during flowering, then a foliar pesticide application is warranted. Matador, Silencer, Lagon, and Cygon are registered for the control of bean leaf beetle
- For late-season feeding (pod fill to maturity), the action threshold is 25% leaf feeding unless pod feeding is observed. If 10% of the pods on the plants have feeding injury and the beetles are still active in the field, a spray is warranted. Days-to-harvest intervals should be considered before spraying.