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Yes! I see very few Harvesters and miss them entirely some years. Populations tend to be small and quite localized. The difficulty in detecting them lies in part with their feeding habits. In the butterfly stage, Harvesters shun flower nectar, as apparently the stubby proboscis is ineffective at reaching nectar, which is often warehoused deep in flowers. Instead, they tap minerals from damp gravel or other places such as my dirty Jeep window. When not feeding, the butterflies often rest on leaves in very shady haunts where it would be hard to notice them.
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This group of aphids is tightly clustered together as they often are, and actively exuding "honeydew" droplets. Two fresh drops can be seen on the far right side of the aphid colony. Harvester butterflies partake of aphid honeydew and are seldom found far from aphid colonies. But the aphids - and the Harvester uses a variety of aphid species that in turn rely on various plant hosts - are far more important to the Harvester's life cycle than just providing nutrient-rich honeydew for the butterflies. Unbelievably, their caterpillars eat them. An aphid-eating caterpillar and thus truly carnivorous, forsaking the vegan fare of most caterpillars. It's often said that the Harvester caterpillar is North America's only carnivorous caterpillar, but I don't think that's true. Butterfly caterpillar, maybe. But when the vastly more speciose moths are taken into account, there surely must be carnivorous moth cats out there. Some, such as the Planthopper Parasite Moth (Fulgoraecia exigua) come close by tapping body fluids from host animals.
I have yet to see a Harvester caterpillar and despite searching all of the little aphid colonies on these alders, came up dry. They are cool-looking larvae - I HAVE seen photos - and slowly eat their way through the aphid colonies. Young, or early instar caterpillars, are even said to use silk that they spin to pin down victims making it easier to nosh on them. Apparently, the caterpillars can be tough to spot as they often become coated with the white waxy deposits that bedeck the aphids. Sort of like wearing an aphid-mimicking ghillie suit that makes the cats harder to detect.
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Apparently, many aphids create sound via specialized organs, and this attracts ants. Why would they do so? Perhaps because ants can be voracious predators and might ward off would-be aphid attackers to protect their honeydew stash. Then how do the Harvester caterpillars infiltrate these warrior-like guards? At least one study shows that the caterpillars also create sounds, and this might confuse the ants. It's possible chemical trickery by the cats also lulls the ants into thinking the butterfly larvae are just strange-looking aphids. I'm not sure all of this aphid/ant/caterpillar business has been worked out, but even with what we know, it's a fascinating story.
I am left with a few Harvester photographic bucket list items. Foremost is finding a caterpillar. As this site is not very far away, and I am routinely in that area, there should be more chances this year. Harvesters produce 2-3 broods annually so there should be more larvae in the offing. Two, I want a photo of a Harvester tapping aphid nectar. That will take either much luck, or most likely a lot of patience and time. Three, I'd like to find a chrysalis and ideally an egg. Then I would have pretty much the entire Harvester life cycle on pixels. Such tasks can often take years to accomplish, though.