Western corn rootworm
The corn rootworm is a small beetle of the Chrysomelidae family, native to America. This pest is present in Alsace and Rhône-Alpes, where populations remain low.[1]
Symptoms
The larvae cause the most damage.
- attacks by foci or spots in the plots,
- Coronal roots eaten away,
- typical vegetative lodging, with gooseneck symptoms,
- lacerated ears, which are often the result of water stress caused by the absence of roots.
- But adults can also cause damage
- before flowering, the adults feed on the cuticle of the leaves. Later, they feed on the silks, pollen and even the grains at the top of the ear. Discoloured bands of varying width can be seen on the leaf blades, as well as severed bristles and hollowed grains.
Do not confuse with other pests :
- Rhizoctonia : Worms and atrophied but necrotic roots
- Nematode : Roots atrophied but present
- Scutigérelle : Attack restricted to certain plots
Biology
Adult
A small beetle, 5 to 7 mm long (female, male), whose elytra tend to be single-coloured and intensely black in the case of the male, with alternating black and yellow stripes in the case of the female.
Larva
The larvae are soft, creamy white and have a cylindrical body. They have a brown head capsule at the front and a dark brown anal plate at the rear, and vary in size from 2-3 mm to 18 mm at the third larval stage.
The female corn rootworm lays eggs up to 20 cm deep in the soil and more than 1,000 eggs. Overwintering takes place in egg form. In spring, as temperatures rise, the larvae hatch. Peak emergence occurs in May-June. The cycle continues with the development of 3 larval stages and pupation, which lasts 2 to 3 weeks in the soil. The adult stage corresponds to the flight period, from late June to mid-September. The insect only has one cycle per year. Adults can travel between 10 and 50 km from their place of birth. Adults live for around 45 days.
Risk situations
The greatest damage occurs when corn rootworm populations are abundant after several successive years of maize cultivation. Damage to roots increases the risk of water deficiency in the plant.
Evolution, impact on yield
In the absence of preventive and curative measures, yield losses can amount to several dozen quintals. Currently, in European countries where this pest has been present for several years, a minimum of larvicide and adulticide treatments, as well as the occasional cutting of monocultures, can keep beetle populations below the harmful threshold.
Control methods
Preventive solutions
- The absence of maize - even for a single year - greatly limits the population of corn rootworm present in the plot.
- Insecticide protection applied at sowing may be useful in the case of a relatively small population.
- Controlling the adults may only be worthwhile in exceptional situations (where the population is particularly abundant).
Curative solutions
There are no curative solutions for controlling soil-borne larvae. Only adult populations can be reduced by applying insecticide to vegetation.
Consult the leaflet Lutte contre les adventices, les ravageurs et maladies published each year by ARVALIS.
Annexes
S'attaque aux cultures
