1st, 2nd and 3rd instar Monarch larvae typically exhibit a type of feeding behavior known as trenching. They do not actually chew all the way through the leaf; rather, they feed on only a few of the layers:
4th instar larvae will chew out a circular area in milkweed leaves, this helps reduce the amount of sticky sap that flows as the larva feeds:
5th instar Monarch larvae are voracious and quickly consume entire leaves. In order to minimize the flow of the milkweed's sap, they will sometimes chew a notch in the leaf's petiole, causing the leaf to hang down. This behavior is known as flagging.
Milkweed Beetle - Feeding by this red and black beetle is characterized by a notching of the tips of the milkweed leaves.
Tussock Moth - Feeding by these hairy caterpillars is extreme. Large numbers of caterpillars are commonly found on a single plant which they completely defoliate.
Aphids - These tiny yellow insects are often found in large numbers on milkweeds. They tap into the plant and feed on its juices, so feeding damage is not obvious; however, they are a useful indicator of milkweed, especially when differentiating Blue Vine milkweed from Bind weed.
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