Invasive Alien Species in Northern Ireland

Diabrotica virgifera virgifera, Western corn rootworm

Diabrotica virgifera virgifera LeConte, 1868
 
Introduction
The western corn rootworm is in fact a leaf beetle (Family Chrysomelidae) not a true worm. The name probably refers to the larvae which are grub-like and feed on corn (maize).
This is a major pest of maize in North America and, increasingly, in Europe. Since forage maize is now a major crop in Ireland note should be taken of the potential for introduction and spread of this pest. At present climatic conditions in Ireland are considered borderline for its establishment but an alarmingly rapid naturalisation and spread in southern England recently, means that complacency is inappropriate.
 
Description
The western corn rootworm is a relatively small and drab beetle. It superficially resembles the brown willow beetle, a common Irish leaf beetle. However, though variable, it generally has a more distinct and colourful pattern on the upper surface and is shiny rather than matt. The head is black, the pronotum dull reddish and the elytra yellowish, often with three distinct darker stripes, but in English examples, with a more diffuse dark colour. Brown willow beetles tend to be a uniform yellowish colour with perhaps one stripe on each side of the elytra, not three as in some forms of Diabrotica, but Diabrotica with a more diffuse elytral darkening could be difficult to distinguish from willow beetles. Of course, willow beetles do not normally occur on maize or other forage crops.
The larvae are pale whitish or buff and elongate, rather grub-like with a brown head capsule. They may be found burrowing into the roots of maize plants causing weakness in the roots and lodging.
 
Country of origin
The western corn rootworm is a native of North America. There are two races or subspecies of which the western one Diabrotica virgifera virgifera has established itself in Europe. In North America this race ranges from northern Mexico through the corn belt and into Canada.
 
Current distribution
It is unusual among crop pests in that spread does not appear to be dependent upon the movement of affected produce. It has appeared at widely separated localities in Europe for which the only common denominator appears to be that the sites are in the vicinity of an airport. How it travels is unknown but some association with commercial airline activity is suggested. Its establishment on five farms in the vicinity of Heathrow and Gatwick airports was reported in 2003 (Ostoja-Starzewski, 2005). In 2004 it was still present in these areas but had not spread. In 2005 no beetles were detected on an outbreak farm near Gatwick after crops on the farm were rotated. However, a significant increase in beetle numbers was recorded on farms near Heathrow. Monitoring of 20km buffer zones around the 2004 sites showed five new sites in 2005.
 
Location in Ireland
Not so far recorded in Ireland.
 
Life cycle
Both larvae and adults feed on the maize plant. The adults consume the pollen of flowering maize, the silks, leaves and developing seed heads. They will also feed on any other plants with pollen in areas where maize is absent.
Unlike the adults, the larvae are strictly tied to the maize plant. Eggs are laid after about a month of feeding, in spring, in soil at the base of the seedling maize plants. When the larvae hatch they burrow into the developing roots and base of maize stems. Damage to the roots can affect growth and cause lodging (flattening of the crop). The mature larva pupates in the soil and the adult emerges in late summer to feed on flowering maize.
 
Wildlife and habitat impacts
None known.
 
Human impacts
Economic damage to maize crops.
 
Key vectors
Probably spread by aviation but the specific means is unknown.
 
What you can do as an individual
Members of the farming public have a particular role to play here by reporting suspected occurrences to their local county advisor, to AFBI, Applied Plant Science Division at Newforge Lane (contact: Mr S. Clawson, APSD) or to CEDaR, Botany Department, Sciences Division, National Museums Northern Ireland, 153 Bangor Road, Cultra, Holywood, County Down BT18 0EU. Tel: 028 9039 5256, email cedar.info[at]nmni.com and the author of this article (roy.anderson[at]ntlworld.com).
 
Advice to specific groups relevant to this species
Apart from the reporting of any suspected occurrences to regulatory authorities, no further action is recommended. Suspect rootworms or their larvae should not be destroyed until seen by an expert.
 
Management measures
Western corn rootworm is best managed by crop rotation. Removal of the maize crop from an infected site is usually enough.
Maize crops in Northern Ireland are being monitored by pheromone-trapping by the Agri-food and Biosciences Institute on behalf of the Department of Agriculture and Rural Development.
 
Further information
Links
For anyone interested in reading more about this beetle:
Defra leaflet – http://www.defra.gov.uk/planth/pests.htm
EPPO leaflet – http://www.eppo.org/QUARANTINE/Diabrotica_virgifera/Diab-95.pdf
 
Literature
Ostoja-Starzewski, J.C. (2005). Diabrotica virgifera virgifera Leconte (Col., CHrysomelidae) in Britain: distribution, description and biology. Entomologists’ mon. Mag. 141: 175-182.
 
Text written by:
Dr Roy Anderson