(Diabrotica virgifera virgifera)
In America it is known as the “billion dollar bug”. It appears the western corn rootworm is doing its best to live up to that reputation in Europe as well. Imported from the USA in the early 1990s, the beetle has already caused billions of dollars worth of damage in European maize crops as well. Until 2014, insecticides in the neonicotinoid group were used to keep its spread in check. Following the ban of these pesticides because they are harmful to bees, the problem has intensified out of all proportion.
Stark figures on the corn rootworm blight: • Maize is the world’s most important arable crop in economic terms • Around 7 million hectares of maize fields in Europe are already infested by the corn rootworm (approx. 36% of the total area of maize) • Between 20% and 80% yield loss
Exponential growth makes the problem far worse Females are ready to mate shortly after hatching. Their eggs do not need to be fertilised right away. Sperm is retained in the spermatheca for some time before it is used to continuously fertilise the eggs. Within a few weeks, a female lays an average of 400 eggs, of which 6-11% survive. This means that every beetle there are another 40 beetles the following year. It is this exponential growth that shows how strongly and rapidly the pest spreads if it is not contained.
How Diabrotica v. v. damages the maize plant The greatest damage is caused by the larvae of the corn rootworm, which eat the roots. This severely limits nutrient and water uptake, which reduces the plant’s strength. A maize plant that has been attacked by the pest cannot develop normally (goose neck symptom) and may even fall over. The beetles cause damage to the plumes (feeding on pollen), the grain threads, the cobs (feeding on the grain), and the leaves (strip feeding). Damage to the grain threads impairs fertilisation and causes the plant to produce hardly any corn grains on the cob. Yield loss in infested areas ranges from 20 to 80 % and can mean losing billions if the problem remains unchecked.
Crop rotation only solves part of the problem In many places, crop rotation has been recommended to control the pest. While this makes a lot of sense, it is also required by law in severely affected regions. However, this does not solve the problem over the long term. This is because the corn rootworm also lays its eggs in other crops such as squash, soybeans and wheat. The beetle also feeds on cereal grasses and its larvae have been observed to survive the winter for up to two years.
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