Damaging bean podborer (Maruca vitrata) populations of up to 100+ larvae/m2 have been observed in flowering and podding mungbeans in the South Burnett, Dawson Callide and Central Highlands. Although podborer is not an uncommon pest in these regions during wet summers, very high populations can inflict devastating damage with zero pod set observed in some crops where the pest is uncontrolled. In wetter seasons, the pest has also been reported on the Darling Downs, albeit usually in lower numbers. Other crops at risk from podborer attack are adzuki beans, navy beans and pigeon pea.
Typical bean podborer damage to flowering mungbeans. Note the webbing surrounding the damaged buds and flowers.
Early warning signs are large numbers of the distinctive moths flying in the crop. As the moths are flighty and often difficult to see when at rest, consider using a sweep net to catch them and to confirm the species. Flowers and buds that are webbed together are often the first visible sign of larval damage. However flowers infested with small larvae (<5 mm) may show no visible signs of damage until they are cut open. After initially feeding inside the flowers, larvae move to adjacent pods. Larvae are a pale translucent cream with rows of distinctive black spots.
Bean podborer moth Maruca vitrata in typical pose with body raised at front and wings outstretched. (25 mm wingspan).
Monitoring for podborer
2) Then multiply the mean number of infested racemes per plant by the number of plants per square metre, assuming one larva per infested raceme.
Many severely infested crops found by Entomologist Hugh Brier have populations up to 50 times the current nominal threshold.
Bean podborer can be controlled with the registered pesticide methomyl®, but where there is sustained podborer pressure, repeated sprays are often necessary. Podborers are also (co-incidentally) controlled by indoxacarb (Steward®) sprays targeting helicoverpa. However, recent DEEDI trials and in-crop inspections show the synthetic pyrethroids (deltamethrin* and alpha-cypermethrin* do not control bean podborer (*registered in mungbeans against green vegetable bug (GVB), and GVB and small helicoverpa respectively). This is despite previous trials (in the 1990’s) showing good podborer control with synthetic pyrethroids.