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Damage potential

The Western Corn Rootworm (Diabrotica virgifera virgifera LeConte) is economically the most important corn pest world wide. In the USA and in Canada the Western Corn Rootworm causes annual losses of about one billion USD due to yield loss and expenditure on pest control (Krysan & Miller, 1986; MetCalf, 1986; Chandler et al., 1995). Germany is affected by the Western Corn Rootworm. Its first appearance in Southern Germany in 2007 was followed by further infestations in additional locations in the following years. Since that time around 350,000 of the 2 million hectares under maize cultivation are at risk due to monocropping of maize (Baufeld, JKI plant health). The Western Corn Rootworm is a crop rotation pest. Damages emerge mainly in maize monocultures and in regions with a high percentage of maize fields. However, no damage occurs when maize is cultivated in a wide crop rotation and in regions with few maize fields.

The more Diabrotica spreads over Europe the higher the economic losses are. Today losses of 300 billion Euros are predicted, but long term prognoses estimate up to 500 billion Euros lost due to Diabrotica infestation. Long term potential losses are hard to predict since it is not known how Diabrotica populations will establish and spread in different European regions. The actual extent of damage also depends on the behaviour of the farmers. If they change crop rotations, damages by Diabrotica can be reduced or even be totally avoided.

Major damage is caused by soil-based Diabrotica larvae feeding on maize roots. Depending on the instar, the larvae feed on the root hairs of the maize plant and penetrate primary roots, which seriously affect water and nutrient uptake. Infested maize plants are easily uprooted since the root system is damaged. Plant stability is reduced and lodging of strongly infested plants can be observed. Damaged plants are able to compensate the damages by regenerating roots which allow them to grow again after lodging, giving the plants a typical gooseneck-like shape (Fig.1). However, this is only the case when water and nutrient supply is sufficient. Further problems caused by Western Corn Rootworm infestation are secondary infections on the roots, e.g. by Fusarium species.

 

gooseneck-like shape

Fig 1: gooseneck-like shape

(Source: Dr. M. Zellner, LfL Bayern)

 

The damage caused by adult beetles is by far less severe. Adults prefer to feed on maize leaves. Furthermore, they feed on pollen and silk (Fig. 2), which results in poor pollination and reduced grain production. Moreover, maize seeds often develop in different seed sizes (Fig. 3), which is negatively affecting commercial seed production. Whether silk feeding enhances fungal infections on the maize cob needs to be further investigated.

 

fresh fooddifferent corn size

Fig. 2: fresh food

(Source: Dr. P. Baufeld, JKI)

Fig. 3: Different corn sizes

(Source: Dr. U. Heimbach, JKI)

 

Damages are severe but vary highly depending on the local growth conditions and on maize varieties which may have good compensation abilities. Experience show those four to five years after the first Diabrotica occurrence yield losses may average 10 to 30% in maize monocropping. This dimension is already an economic loss for the farmer. In dry years even yield losses of 80% are possible. An economic damage threshold is an important measure to predict possible damages and to plan control measures. Economic thresholds should be adapted for each European region depending on local conditions.

 

Root damage by Diabrotica larvae is validated using the Node-Injury-Scale, which was developed at the University of Iowa.

 

Node-Injury-Skala (Oleson et al., 2005)

 

Value   Description

0.00No feeding damage (lowest rating that can be given)
1.00

One node (circle of roots), or the equivalent of an entire node, eaten back to within approximately 1½ inches of the stalk (soil line on the 7th node)

2.00Two complete nodes eaten
3.00Three or more nodes eaten (highest rating that can be given)

 

Check http://www.ent.iastate.edu/pest/rootworm/nodeinjury/nodeinjury.html for testing root damage.

 

testing root damage



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