Commercial Horticulture > Vegetable > Vegetable Insect Pests > Insect Pests of Beans

Insect Pest of Beans

 Name Thumbnails Damage symptoms Plant injury Sampling method & economic threshold
 Aphids (many species, many colors)    
  • Any plant part may be attacked, but aphids prefer tender parts of plants like new seedling leaves, soft seedling stems, and developing pods (late-season attack).
  • Feeding on leaves may cause distorted or rolled leaves but plants can tolerate high populations.
  • Feeding on pods is a major problem.  Pod development is slowed down due to loss of sap and nonuniform cell growth may causes abnormal shapes.
  • Stunting of plants and delayed maturity can occur in extreme cases.
  • Growth of black fungus or sooty mold is very characteristic.  Run your finger along the black mold and it will be sticky and may rub off on your hands and clothes during scouting.
  • Aphids are attracted to yellow sticky traps available from many commercial suppliers.  They are cheap but the traps need to be deployed in high numbers and very timely, otherwise their effectiveness reduces drastically.
  • You may see winged aphids attracted to yellow backyard light. Winged aphids could indicate early migration to crops.  Check with your extension agent for correct ID before you panic!
  • Thrips may damage seedling as well as flowers.  They can disrupt pollination in late stages. Net result is pood pod set.
  • If pods are set and thrips are seen late in season, then damage is cosmetic. Continue to monitor low populations.
  • Field obseravtions in South Carolina indicate that event three thripsd per flower can disrupt normal pollination and fruit set.
  • During flowering season, put a Ziploc bag over several flowers and shake the flowers. You may use a little mosquito repellent DEET to make insects come out of the flower (remove the flowers after sampling).  Thrips will drop off into the bag and can be easily counted.
 Lesser cornstalk borer


  • Occasional pest of beans, damage could be severe in hot dry years. This is a polyphagous pest with many host plants so it readily migrates from field to field looking for host plants (especially moves from corn to vegetables).
  • Plants may be stunted and wilt during hot days because larvae attack roots and stalks near the soil line.  The larvae make silken tunnels attached to the entrance hole or feeding area.
  • This pest can cause major stand losses.  Look for wilting plants and check the soil around plant bases for purple and blue larvae.
  • Search for silken feeding tubes attached to the stalks.
  • Don't forget to scout for this pest during dry years.  Early detection of infestation is crucial to prevent stand losses.
 Cabbage looper    
  • Leaves are voraciously eaten by larvae (underside, along leaf margins).  High insect pressure is indicated if only leaf veins remain uneaten.
  • Look at the underside of lower leaves (feeding between viens).  Larvae have two pairs of prolegs on abdominal segments.  Larvae may feign death when disturbed and drop to the ground.
  • Indirect method - look for frass at the base of the plant.
  • Pheromone traps can be used to monitor moths (traps cannot predict actual feeding damage)
 Mexican bean beetle


  • This is a major defoliator of most bean crops, but also feeds on alfalfa and clover (herbaceous nitrogen rich crops with lush growth).
  • Larvae can be seen step-ladder like feeding symptoms especially on the undersurface of leaves but leave the upper leaf surface intact.  Extensive feeding looks like silvery shine on skeletonized leaves in a large patch.
  • Examine the underside of leaves for orange clusters of eggs, spiny orange larvae and/or adults with eight spots on wings.
  • Examine crop residue to look for overwintering adult beetles.
  •  Bean leaf beetle    
    • Adult is a major defoliator, especially young plants.
    • Reduce plant vigor and transmit some diseases.
    • Larvae live in soil but injury is not threatening in most cases.
    • Look for adults with four large black spots on wings feeding underside of leaves. 
    • Feeding damage is intervenal and seen as uniform holes on leaves.
     Corn earworm


    • This is a polyphagous pest with many host plants.  It is a major defoliator (early instar larvae) and a major pod feeder in later stages.
    • Larvae can be identified by searching for broad longitudinal lines on the side, above and below the spriracles.
    • Avoid planting beans close to corn.
    • Pheromone trap and black light trapping can be done. If you find over 5 moths/pheromone trap OR 10 moths per light trap, then be on alert and scout intensively for rising population.  Don't wait for pod damage to occur.
    • If pod damage is visible, apply treatments before pod damage exceeds 5%.
    • Keep an eye on the natural enemy populations like lady beetles, lacewings, and tachinid flies.
     Tarnished plant bug



    • There is increasing menace of pod feeders with piercing-sucking mouthparts and tarnished plant bug is one of the menacing pests.
    • This insect injects toxins into the plant which destroys plant tissue and leaves a scar on the feeding spot on pods.
    • Damage is most extensive during blooming period because they have the habit of feeding at the petiole of flowers which causes flower abortion.
    • Watch for aborted flower in between the planted rows.
    • Adult and nymphs like to feed while concealed.  Adults will readily fly if you approach noisily and then settle down quickly. Watch for this behavior in field while scouting.
    • Cover the flowers or branchaes with Ziploc bag, spray some DEET on the plant part in bag (you decide whether you want to do this or not), and shake the plant part.  Plant bugs along with other insects should drop into you bag making counting easy to do.
    • Sweep netting is also a good option for scouting for many insects, including plant bugs.  Take ten sweeps (180 degree movement of net) and drop all plant material into a Ziploc bag.  Sort and count insects as per needs.
    • During pod formation and seed development stages, open the pods and look for feeding scars on the seed.  Early scouting and action can prevent a lot of late season damage to flowers and pods.
    • Economic threshold from South Carolina is one bug per six feet of row.
     Stink bugs


    • This is another rising menace in several southern states of the U.S.  This insect prefers to insert it's piercing-sucking mouthparts into tender shoots and fruits.  Flowers may drop off the plant due to feeding on petioles.
    • Pod feeding causes brown or black spots. Such feeding may deteriorate seed quality and poor pod-fill.
    • May transmit diseases in certain crops (e.g., bacterial spot in tomatoes)
    • Scouting protocol is similar to tarnished plant bug (see above).
    • Economic threshold recommendations from University of Florida for the southern green stink bug in soybean is 36 bugs per 100 swings of sweep net.

    Image sources:  Cornell University, University of Florida, Clemson University,, Iowa State University, University of Minnesota, CropPest Ontario (Canada), Mississippi State University, University of California IPM Project