Bean leaf beetle: Injury and damage

Generally, depending on life stage, bean leaf beetles exist as either root-feeding larvae (McConnell 1915, Leonard and Turner 1918), or leaf- and pod-feeding adults (e.g., Chittenden 1891, Smelser and Pedigo 1992a, Pedigo and Zeiss 1996, Lam and Pedigo 2000). Depending on the stage of the soybean plant the amount of feeding activity or injury can have a profound impact on soybean growth and development.

The relationship between differing levels of injury and the yield response is termed the damage curve (see below). A number of factors can change the relationship between injury and yield. One of these factors is the nature of the insect injury to the plant. The type of injury suffered by plants can be classified as indirect, direct, or quantal injury (Higley and Peterson 1996). Indirect injury is the result of an insect feeding on any non-yield-bearing tissue of a plant (e.g., leaves or roots). Conversely, direct injury is the result of an insect feeding on the yield-bearing tissue of a plant (e.g., the pods of a soybean plant). Quantal injury is where the quantity of injury is independent of yield (e.g., the transmission of disease agents). In terms of the damage curve, direct injury is often conceptualized within the region of linearity (see below). That is, for every unit of yield loss there is a unit of injury. However, indirect and quantal injury are more difficult to confidently describe.

The yield-injury relationship in plants: Components of the plant damage curve as hypothesized by Pedigo et al. (1986). Some plants may be able to sustain injury without any affect on yield (tolerance) while some, such as soybean, may even produce slightly greater yield with a small amount of injThe yield-injury relationship in plants: Components of the plant damage curve as hypothesized by Pedigo et al. (1986). Some plants may be able to sustain injury without any affect on yield (tolerance) while some, such as soybean, may even produce slightly greater yield with a small amount of injury (overcompensation). The dashed blue line through this curve denotes the point at which yield loss is detectable and attributed to injury (damage boundary). The remaining components of this curve all describe yield-loss functions (see Pedigo et al. [1986] for a complete discussion). Importantly the damage curve function can change depending on numerous factors including the stage of the pest or plant or the environment. Determining this function for a pest is a critical step in developing and refining an economic injury level.

Little is known about the damage curve as it pertains to bean leaf beetle larvae; however, there has been a long-held belief that bean leaf beetle larvae injure soybean. The only information regarding the yield-injury relationship in soybean from bean leaf beetle larvae is McConnell (1915) where he states:

"One larva is capable of destroying a considerable number of nodules [of cowpea, Vigna unguiculata]."

McConnell's article suggests that this kind of injury relationship would be similar in soybean. In short, there is no economic injury level for bean leaf beetle larvae because, in part, the injury-yield relationship is unknown for this life stage.

The relationship between adult injury of soybean and yield is better understood. The bean leaf beetle feeding period is about 21 days or less during which time they will consume 1 (Kogan 1983) to 0.384 cm2 (Smelser and Pedigo 1992a) of leaf tissue per day. This kind of injury is known as indirect injury because it does not directly affect the yield-bearing tissues of the plant. The previously-mentioned root feeding by larvae would also fit into this category.

Direct injury results from bean leaf beetles feeding on soybean pods. This direct injury occurs an average rate of 0.184 pods per bean leaf beetle feeding day (BLB-day). This means, on average, a bean leaf beetle will eat about two tenths of a pod per day.

Bean leaf beetles also cause yield loss through Quantal injury by the transmission of bean pod mottle virus or by the secondary invasion of fungi (e.g. Diaporthe spp. and Phomopsis spp.) into the pods.

Direct injury:
Pod injuryPod injury


Indirect injury:
Leaf injuryLeaf injury

Seedling injurySeedling injury

Root injuryRoot injury

Quantal injury:
Green stemGreen stem

Leaf mottlingLeaf mottling

Seed discolorationSeed discoloration