Forest Tent Caterpillars

Forest Tent Caterpillars

Forest Tent Caterpillars

Every 10 – 12 years the forest tent caterpillars’ population explodes. Outbreaks last for 2 – 3 years so based on last year’s influx of caterpillars it looks like the spring of 2017 will also see a large number of these pests throughout the Prairies. The forest tent caterpillars love trembling aspen (poplar), oak, ash, maple and white birch. The caterpillars eat the young leaves and consume large quantities completely defoliating entire trees. Tent caterpillars actively feed during the night and during the day you will see them relaxing on the tree trunks and limbs.

Defoliated trees typically refoliate after caterpillar attacks and after one attack experience no lasting damage but the new leaves will be smaller. If the trees are defoliated several years in a row it may result in the severe reduction in the radial growth of the trees. Trees can be weakened by repeated defoliation which over time can leave it susceptible to insects and other diseases.

The disappearance of foliage is a nuisance and can be unsightly but it can be avoided. If you are averse to using harmful insecticides there is an organic option. It is called Bacillus thuringiensis (Bt) and it is a naturally occurring bacterium, common in some soils, that causes disease in certain insects most notably leaf and needle feeding caterpillars. Its active ingredient is a crystal protein which paralyzes the digestive system of the insect. The infected insect stops feeding and starves to death. New strains of Bt have been developed to attack certain flies and mosquitoes. Bt is becoming an essential weapon in the battle against the West Nile Virus.

Using Bt helps reduce the amount of chemical insecticides in our environment and is harmless when eaten by beneficial insects and animals.

Don’t expect the caterpillars to die immediately like they do when they are sprayed with chemical insecticides that attack the insect’s central nervous system. After ingestion of sufficient product feeding stops immediately and death will follow in 2 – 5 days.

Bt degrades in the sunlight so it is best to spray in the early morning or evening. Bt remains on the leaf for about a week, shorter if it rains. For best results spray when the caterpillars first appear and are actively feeding.






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