Answer: The favorite food of the Mourningcloak butterfly larvae just happens to be willow leaves which is probably why your tree is being defoliated on a regular basis. The adult butterflies prefer sap, which may also be present on your property - perhaps some of your other trees have been attacked by sapsuckers. Once they punch a hole through the bark, sap will continue to drip, which will attract adult Mourningcloaks. These particular butterflies are abundant, especially in your area, and are not protected by law. Because of this, it is legal (although not always desireable) to control their populations. In the balance of nature, when populations of one type insect pest becomes large, predatory insects usually arrive to reduce the populations. That's why you'll see a surge in the cycles of plant defoliating larvae. If possible, wait for predatory insects to arrive and take care of the Mourningcloak larvae problem for you. Or, you can have your tree professionally sprayed with Bt, a natural bacterium which acts as a stomach poison. The larvae will continue to feed for a few days after being sprayed with Bt. but repeat sprayings will eventually control the population.
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