Insects - maize

Contributors to this section: CIMMYT, Mexico (Etienne Duveiller, Monica Mezzalama, Eloise Phipps, Thomas Payne, Jesper Norgaard), Independent consultant (Jesse Dubin).

Khapra beetle

Importance

High economic and phytosanitary importance.

Significance

Trogoderma granarium is a general storage pest. It occurs mainly on cereals and cereal products, oilseeds (especially groundnuts and oilcakes), pulses and pulse products, and compound animal feeding stuffs. It is principally serious under hot dry conditions; complete destruction of grain and pulses may take place in a short time. In humid climates, it is not competitive with other storage pests with faster rates of increase.

Symptoms

Presence in stored grain. The stage most commonly seen during inspection is the larva, and the most usual evidence for infestation is cast larval skins.

Trogoderma granarium (photo: EPPO)

Hosts

Grain and stored product hosts.

The beetle is by nature an omnivorous protein scavenger and has been found in many locations that would not be obvious food sources.

Geographic distribution

Trogoderma granarium is established within an area roughly limited by the 35° parallel to the north, the Equator to the south, West Africa to the west and Myanmar to the east; i.e. the warm dry regions along the Suez route from the Indian subcontinent to Europe. T. granarium has been introduced into other areas with similar climatic conditions, especially the alternative route between India and Europe around Africa. Initially, these introductions caused severe damage but outbreaks have been local and most have been eradicated. T. granarium has also established in some areas with unfavorable climates, in protected storage environments only, for example in Western Europe and Japan.

EPPO distribution map: http://www.eppo.org/QUARANTINE/insects/Trogoderma_granarium/TROGGA_map.htm

Biology and transmission

The natural spread of this pest is limited. International spread is mainly by larvae in commodities, empty sacks, and in the structure of ships and dry cargo containers. Second-hand bags are also a means of spread.

Detection/indexing methods used at CIMMYT

• Physical inspection of seed.

Treatment/control

• Fumigation

Procedures followed in case of positive test used at CIMMYT

• The seed lot is destroyed (the insect is quarantined in Mexico).

EPPO protocols

EPPO Diagnostic Protocols for Regulated Pests PM 7/13(1): Trogoderma granarium. EPPO Bulletin 32: 241-243. [online] Available from URL: http://www.eppo.org/QUARANTINE/insects/Trogoderma_granarium/TROGGA_protocol.pdf. Date accessed 10 April 2010 

References and futher reading

CAB International. 2007. Datasheet: Trogoderma granarium. In Crop Protection Compendium, 2007 Edition. Published as CD-ROM and website. Wallingford, UK: CAB International.

EPPO. Data Sheets on Quarantine Pests: Trogoderma granarium. [online] Available from URL: http://www.eppo.org/QUARANTINE/insects/Trogoderma_granarium/TROGGA_ds.pdf. Date accessed 10 April 2010

Larger grain borer

Importance

High economic and phytosanitary importance.

Significance

Infestations in maize may start on the mature crop in the field, i.e. when moisture content is at or below 18%. Weight losses of up to 40% have been recorded in Nicaragua from maize cobs stored on the farm for 6 months. In Tanzania, up to 34% losses have been observed after 3 months storage on the farm, with an average loss of 8.7%. 

Symptoms

These insects infest both stored grain and maturing maize ears in the field. In a very short time, the adults produce large quantities of floury dust as they bore into and feed on the grains. Damaged grains can readily be identified since they are usually covered by a film of this dust.

The adult beetles are 3 to 5mm long, cylindrical in shape and reddish brown to dark brown. Their heads are turned down and appear to be covered by a hood. They are able to fly.

Prostephanus truncates

Hosts

Major hosts: maize, cassava (Manihot esculenta), dried stored products, wood.

Minor hosts: Dioscorea (yam), Sorghum bicolor (sorghum), triticale, wheat. 

Geographic distribution

P. truncatus is indigenous in Central America, tropical South America, and the extreme south of the USA as a major, but localized, pest of farm-stored maize. It was introduced into Tanzania, probably in the late 1970s, and has become a serious pest of stored maize and dried cassava in that part of East Africa; it has since spread into Kenya, Burundi, Rwanda, Malawi, Zambia, Mozambique, Namibia and South Africa, and is almost certainly present but unreported in several other countries in the region. It was first found in West Africa in Togo in 1984 and it has since spread to Benin, Nigeria, Ghana, Niger and Burkina Faso. A separate outbreak occurred in Guinea in Conakry.>/p>

Biology and transmission

P. truncatus may be attracted to maize grain and dried cassava over short distances. However, field studies in both Mexico and Togo suggest that there is no long-range attraction of adult P. truncatus to maize grain or cobs, or dried cassava; this is not surprising because wood is the major host of this beetle.

Adults frequently initiate their attack on stored maize cobs with intact sheaths by boring into the base of the maize cob core, eventually gaining access to the grain via the apex of the cob by crawling between the sheathing leaves. Adults bore into the maize grains, making neat round holes, and as they tunnel from grain to grain they generate large quantities of maize dust. Adult females lay eggs in chambers bored at right angles to the main tunnels. Egg-laying on stabilized grain, like that on the maize cob, is more productive than on loose-shelled grain as the oviposition period is longer, equal in length to the life of the female, and the eggs are laid at a greater rate.

Larvae hatch from the eggs after about three days at 27°C and seem to thrive on the dust produced by boring adults. For example, large numbers of larvae develop and pupate in dust at the base of dense laboratory cultures. Larvae may also crawl into and feed on slightly damaged kernels. Pupation takes place inside the kernels, and emerging adults then cut their way out of the kernels.

The success of this pest may be partly due to its ability to develop in grain in low moisture conditions, under which many other storage pests are unable to multiply.. For example, Sitophilus oryzae, a species occurring in a very similar ecological niche, needs a grain moisture content of at least 10.5% for development. Thus, in dry conditions, P. truncatus probably benefits from the absence of any significant competition from other storage pests.

Detection/indexing methods used at CIMMYT

• Physical inspection of seed.

Treatment/control

• Fumigation; resistance

Procedures following in case of positive test used at CIMMYT

• The seed lot is destroyed.

References and further reading

CAB International. 2007. Datasheet: Prostephanus truncatus. In Crop Protection Compendium, 2007 Edition. Published as CD-ROM and website. Wallingford, UK: CAB International.

CIMMYT. Maize Doctor information sheet: Grain borers. [online] Available from URL:http://maizedoctor.cimmyt.org/index.php?option=com_content&task=view&id=221. Extended information sheet: Grain borers. [online] Available from URL: http://maizedoctor.cimmyt.org/index.php?option=com_content&task=view&id=329. Date accessed 10 April 2010