Life cycle and description

• Adult stage: Adult moths are white to straw colored and are about 20mm in length. Moths have a large mouthpart similar to a snout. Moths are nocturnal, hiding in vegetation during daylight. Females have a 5 day lifespan and will oviposit between 100 and 400 eggs in a lifetime.
• Egg stage: Fresh eggs are cream in color and oval in shape (3mm in length). Eggs are deposited on most parts of the plant, but most frequently on the upper leaf surface in clusters of 2 to 5. Second generation eggs are commonly deposited near the ear. As eggs mature they develop three parallel, reddish-orange rows known as the ‘red bar stage’. Eggs hatch in 3 to 5 days.
• Larval stage: When larvae first emerge they are reddish in color, but change to dull white with prominent brown and black spots on each body segment. Immature larvae feed in the whorl while mature larvae bore into the stem. Larvae can be up to 40mm in length. Larvae in the diapause stage are not plain white and do not have spots. Larvae of southwestern maize borer can be distinguished from those of other borers by the hooks on the bottom of the prolegs that are a complete circle rather than only partially circular.
• Pupal stage: Pupae are dark brown in color and about 25mm in length. Pupae generally occur at the last site of feeding, typically in the lower third of the stem. Adults emerge in 7 to 10 days.
• There are at least two, sometimes three generations of this borer in a year.
• Typically, girdling and lodging damage are most serious in the second and third generations.
• Larvae overwinter in the stem below ground level in the diapause stage.

Confirmation

Characteristic three parallel red stripes on mature eggs and completely circular hooks on the bottom of the prolegs can be used to distinguish this species from other maize borers and closely related species, including the Neotropical maize borer and European maize borer.

Problems with similar symptoms

Several species cause similar symptoms (dead heart and feeding in the whorl) and tunneling damage including the neotropical maize borers (Diatraea lineolata), sugarcane borer (D. saccharalis),  southern maize stalk borer (D. crambidiosella) and European maize borer (Ostrinia nubilalis). The neotropical maize borer, however, can be distinguished by girdling of the maize plant at the base of the stem which is uncommon amongst the other species.

Why and where it occurs

Reduced tillage agriculture enables the larvae to overwinter until the subsequent season. Female moths are attracted to weedy fields for oviposition.

Host range

Maize, sorghum, rice, sugarcane.

Geographic distribution

Southern USA, Central America and the Caribbean.

Damage

• Mechanism of damage: Destruction of the growing point in the whorl leads to dead heart and yield loss. Tunneling and girdling of the stem near ground level interferes with the translocation of nutrients and water and makes the plant prone to lodging. Boring in the ears can result in ear rots that produce various mycotoxins.
• When damage is important: Second and third generations are generally most damaging. Damage is severe if high infestation results in severe crop lodging. In addition to yield losses, boring in the ears can result in ear rots that produce various mycotoxins.
• Economic importance: Extensive yield loss can be experienced following severe infestations, which lead to widespread crop lodging.

Management principles

Monitoring 

• Moth traps can be used to detect the presence of adult moths.
• Monitoring of leaves for characteristic eggs or larvae in the whorl can be used to confirm pest incidence.
• Once the species is spotted in the area, fields should be monitored once a week.
• Control measures should be implemented while larvae of the first generation are still feeding in the whorl. Once larvae burrow into the stalks, control is less effective and not economical. The economic threshold for control is when larvae are observed on 35% of plants.

Cultural control

• Early planting can help avoid periods of heavy pest infestation, particularly the second and third generations.
• Planting early maturing varieties also reduces the number of pest generations per crop.
• Harvesting early can reduce yield loss due to lodging.
• Management of infected stalks left in the ground can reduce the number of larvae that overwinter to subsequent seasons.

Chemical control

• Chemicals can be applied either as a spray or in granular form in the whorl. If applied as spray, chemicals should be applied to coincide with egg hatching or when larvae are still exposed in the whorl. Treatment of the first generation results in the most effective control. Treatment of second and third generations is often not effective or economical. 

Biological control

• There are several natural parasitoids of southwestern maize borer, including various Trichogramma wasp species that parasitize eggs. The most effective biological control agent is the late season wasp Trichogramma minutum. Apanteles diatraea is another important control agent.

Host resistance

• Various hybrids with resistance to southwestern maize borer have been developed and should be deployed in areas where the pest is a major problem.
• Bacillus thuringiensis (Bt) maize hybrids have also been shown to provide effect control of southwestern maize borer. However, use of Bt maize hybrids involves following various guidelines and is dependent upon country regulations.

References

Boyd, M.L. and W.C. Bailey. 2006. Southwestern Maize Borer Management in Missouri. University of Missouri Extension. http://muextension.missouri.edu/explore/agguides/pests/g07111.htm (15 November 2006).

CAB International. 2002. Crop Protection Compendium. Wallingford, UK: CAB International.

Daves, C.A., W.P. Williams and F.M. Davis. 2007. Impact of plant resistance on southwestern corn borer (Lepidoptera: Crambidae) biology and plant damage. Journal of Economic Entomology 100:969‑75.

Davidson, R.H. and W.F. Lyon. 1987. Insect Pests of Farm, Garden and Orchard. Hoboken, NJ: John Wiley & Sons.

King, A.B. and J.L. Saunders. 1984. The Invertebrate Pests of Annual Food Crops in Central America. London: Overseas Development Admin.

Ortega A. 1987. Insect pests of Maize: A Guide for Field Identification. Mexico, D.F.: CIMMYT.

Sloderbeck, P.E., R.A. Higgins and L.L. Buschman. 1996. Southwestern Corn Borer. Entomology Corn Insects. Kansas State University Cooperative Extension Service. http://www.oznet.ksu.edu/library/entml1/SWCB.pdf (15 November 2006).

Contributors: Gabrielle Turner and Biswanath Das