"In my experience, the best time to find monarch larvae, especially 5th instars, is during the first hour of light on a morning in the 60s following an evening in the 80s or 70s. In this region (eastern Kansas), monarch larvae frequently leave the plants during the day and "hide" in the vegetation near the base of the milkweeds. The larvae could be leaving the plants to avoid heat stress but they may also be avoiding insects such as paper-wasps, yellow jackets, tachinid flies and other insects that search vegetation for soft-bodied insects. On warm evenings the larvae resume feeding at the tops of the plants but as the temperature drops into the 60s toward morning the larvae become sluggish and, in effect, get stuck at the top of the plants where they are quite visible. Fortunately, for the larvae, the predatory and parasitic insects are also too cold to be active under these conditions. As the sun rises, the larvae quickly heat up and move down the plants. The trick to easy pickings is to get to the milkweed patch before this happens. The early bird gets the monarchs so to speak."
I was up a bit early this morning, not as early as Chip, before 7 AM. I actually saw a third instar at the top of the swamp milkweed chowing down. When I returned at 9 AM, it had disappear. The caterpillar probably had relocated to avoid predators.