Pod Piercing Pests are Present-Stink Bugs and Tarnished Plant Bugs

Scouting for aphids may gear down once the soybeans reach the R6 stage but that does not mean that the fields are insect pest free from then until harvest. The pod piercing insects can move in and cause problems too. Stink bugs and tarnished plant bugs spend most of the summer months in other crops including wheat, canola, corn, various fruits and vegetables but are attracted to the maturing soybean and edible bean crop in August and September. And we are starting to see a few fields with populations high enough to cause concern. They do not feed on the surface of the pods but instead insert their piercing-sucking mouthparts through the pod into the bean seed and suck on the juices. Beans damaged by these insects take on an off-color in the area of feeding activity. Injured seeds either shrivel or develop a blemish known as a "dimple" or "sting" resulting in a lower quality seed (Figure 1). Edible beans may be heavily discounted as a result of these blemishes.

Figure 1. Dimples on soybean seed caused by stink bugs piercing into the soybean pod and sucking on the seed.
Photo Credit: Art Schaafsma, U of G, Ridgetown Campus.

There are two types of stink bugs that can injury beans; Green Stink Bugs and Brown Stink Bugs. Adult stink bugs are shaped like a shield (Figure 2) but the nymphs (juveniles) can look very different from their adult stage, having very short, stubby wing pads (Figure 3) and are often a different colour than the adults. In particular, green stink bug nymphs have a flashy display of black, green, orange and yellow.

Figure 2. Adult Green Stink Bug (left) and Adult Brown Stink Bug (right)
Photo Credit: Marlin Rice, Iowa State University

Figure 3. Green Stink Bug Nymph (left) and Brown Stink Bug Nymph (right).
Photo Credit: Marlin Rice, Iowa State University

The brown stink bug adult should not be confused with the beneficial Spined Soldier Bug which feeds on caterpillars and other insect pests. To tell these two apart, look at their feeding beak or needle-like mouthpart (Figure 4). The beak of the brown stink bug is slender to pierce through delicate plant tissue. The beak of the spined soldier bug is thicker so that it can harpoon into their insect prey. The soldier bug adult also has pointier ("spined") shoulders than the brown stink bug though this may be hard to determine unless you have them side by side to compare (Figure 5).

Figure 4: Beaks (needle like mouthparts) of a predatory (left) and Plant Pest (right) stink bug. Image Credit: University of Arkansas

Figure 5: Spined Soldier Bug, a beneficial insect that feeds on other insect pests.
Photo Credit: Marlin Rice, Iowa State U.

Control may be warranted in edible beans or in food grade, IP and seed soybeans if an average of one stink bug per one linear foot of row is found during the late R5-R6 stages.

Tarnished plant bug adults are much smaller than stink bugs (approx 5 mm in length), are yellowish to reddish brown in colour and have a small triangle shape on the back (Figure 6). They too can cause damage to edible bean seed (and most likely can impact food grade soybeans too). To scout for them, take 20 sweeps (180 degree arc = 1 sweep) in 5 areas of the field. Thresholds for edible beans developed by other US states is 1-2 tarnished plant bugs/sweep up to the later R6 stage. Control may be warranted in food grade soybeans if these same of numbers are found there.

Figure 6. Tarnished Plant Bug Adult
(Flax Council of Canada)