Large bean podborer larva (16 mm) in pod
QDPI entomologists have received several recent reports of poor control after spraying bean podborer (Maruca vitrata) in Central Queensland mungbean crops.
A major pest of mungbeans in tropical and subtropical production areas, bean podborer is typically more problematic in coastal cropping regions and can be more difficult to manage than co-occurring Helicoverpa sp. populations.
Infestation pattern and nature of damage
Bean podborer usually occur in mungbean crops from budding onwards, laying eggs on floral buds and flowers, and occasionally on young leaves and growing tips. Freshly laid eggs are oval, flattened, and translucent, making them difficult to detect compared to helicoverpa eggs, which are spherical and white.
Bean podborer eggs (left), compared with newly laid helicoverpa egg (right).
Hatched larvae find their way inside buds and flowers where they begin feeding secretively, making them very difficult to detect with the usual sampling methods (and also control with insecticides). As the larvae develop, they create a webbed mass by weaving buds, flowers, and pods together with their silk.
Webbed racemes indicating entrenched bean podborer caterpillar(s).
Bean podborer larvae are relatively easy to distinguish from other caterpillar pests of mungbean. Early instars are slightly yellowish that changes to a dirty green (sometimes tinged with pink) by 3rd instar. Visible from the 2nd instar stage are rows of black spots running along their body that become pale in later instars, as they approach pupation.
Small bean podborer larva feeding inside a flower and a late instar larva feeding inside a pod.
The speed of egg and caterpillar development depends on temperature. Eggs take 3 days to hatch at 25°C but will hatch in 2 days at 30°C. Larval development (neonate to pupation) takes approximately 15 days at 25°C, but can be completed in 8-10 days under warmer conditions.
Effective sampling
Bean podborer moth (25 mm wingspan)
Often one of the first signs of bean podborer is the presence of moths in the crop. They can be spotted during the day sitting on leaves in a very characteristic ‘wings outstretched’ pose.
Closely inspect mungbean racemes for eggs, which are quite difficult to detect. Eggs develop a yellowish tinge and the larval head capsule becomes visible prior to hatching.
Manually inspect buds and flowers for larvae as beat sheeting will significantly underestimate the larvae present. Collect several racemes from different sections of the crop, pry open flowers and buds and count the number of larvae. To roughly estimate the bean podborer population, divide the number of caterpillars detected by the number of racemes sampled, and multiply that value by the estimated number of racemes per square meter to give an estimation of bean podborer larvae per m2.
Larvae/m2 = caterpillars X racemes/m2
racemes
The suggested break-even economic threshold is 5-7 larvae per m2.
Chemical control
Bean podborer are effectively controlled with chlorantraniliprole (e.g. Vantacor®), as the translaminar activity can kill caterpillars feeding inside flowers. However, it is important to spray bean podborer infestations before the caterpillars form their webbed shelters because the chemical is less likely to reach the larvae inside these webs (and larvae become more tolerant to insecticide as they increase in size).
Chlorantraniliprole is the preferred caterpillar insecticide for many agronomists due to its translaminar activity, low impact on natural enemies, and long residual control. QDPI trials in chickpea have demonstrated up to 3 weeks of residual control for helicoverpa larvae (at the same rate of product), however there is no such data for reproductive mungbean, and growth dilution (from setting new buds/flowers and pod formation) is likely to influence residual effectiveness at these crop stages.
It is also important to note that caterpillars may take several days to die after a chlorantraniliprole spray. However, larvae do stop feeding once they have received an adequate dose of insecticide.
Other products registered for bean podborer management in mungbeans include Warlock® (emamectin benzoate), Skope® (an emamectin benzoate and acetamiprid mixture), and methomyl. Methomyl does not provide residual control and both methomyl and Skope® are very hard on beneficials. Synthetic pyrethroids (deltamethrin, e.g. Ballistic) are not registered for bean podborer in mungbean, and will not provide incidental control if applied to manage other pests as they only give limited control of bean podborer (and they are also very hard on beneficials).
Steward® (indoxacarb; PER87650) may also be used to manage bean podborer in mungbean crops, but it should be used only in the permitted use window on the Helicoverpa armigera Insecticide Resistance Management Strategy.
While there have been reports of bean podborer resistance to older chemistries overseas, the potential for this pest to develop resistance to insecticides in Australian cropping situations is currently unknown.
In case of spray failures
While it is difficult to know what has caused the unexplained control failures of chlorantraniliprole used to manage bean podborer in this season’s mungbean crops, we urge agronomists to:
- Ensure thorough sampling of crops from budding onwards. Keep in mind the pest’s cryptic nature and fast development under warm conditions.
- Follow label instructions for chlorantraniliprole application (40mL/ha plus a non-ionic surfactant @ 125 gai/100L; a minimum of 30L/ha for plane application or 100L/ha for ground rig; and a medium droplet size).
- Re-check crops post-spray to assess for control success/failure. Continue monitoring for new infestations remaining aware of the potential of growth dilution.
For more information contact Trevor Volp (0429 641 912) or Melina Miles (0407 113 306)
Article by Trevor Volp and Melina Miles