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in the sun on a hot day could quickly heat to lethal temperatures. The downside also shelters an egg from rain drops--giant wrecking balls in comparison to tiny eggs. And eggs under leaves are much less visible to visual predators like birds.
Given these clear advantages of being on the underside, I assumed the same was true for newly-hatched caterpillars. So, imagine my astonishment when I checked a milkweed with just-hatched eggs, and discovered that most of the caterpillars were on the top of leaves.
Why move topside?
I first suspected that trichomes were the reason. These are the tiny hairs that protect the milkweed leaf from herbivores. They create a tangle--a physical barrier--that a caterpillar has to remove before it can feed on leaf tissue. Trichomes make the underside of the leaf look whitish. The topside looks greener, because there are fewer trichomes on top. Could tiny caterpillars be moving to the topside to avoid the densest trichomes?
I checked Copilot AI--very helpful for questions like this. Here are reasons caterpillars move topside after hatching:
There are fewer trichomes on the topside. It takes less energy and time to shave them before a caterpillar can begin to feed.
I thought caterpillars on the topside would be more visible to predators, but this isn't the whole story. With warning coloration, caterpillars on the topside are protected from many predators, even if seen. Also, there may be small predators lurking on the underside of the leaf where they hide from birds. The moist underside of the leaf can also harbor threats like fungi and bacteria.
Here was my biggest surprise: "The topside of the leaf receives more sunlight, which helps warm the caterpillar and speed its digestion. The sunlight can raise their temperature above ambient by 3-8 Celsius. You can see them basking broadside to the sun. Warmer temperatures can promote faster growth, shortening the larval stage by up to 50%." By growing faster, the caterpillar spends less time in the vulnerable caterpillar stage.
More changes in behavior as caterpillars grow
The caterpillar gains 2,000 times in weight by the time it forms the "J." That's like a human baby growing to the size of an elephant in two weeks.
"Increasing size enables dramatic shifts in behavior as the caterpillars grow through their five instars. A tiny leaf-nibbler evolves into a voracious, strategic leaf-devourer."
Instar 1, ~2–6 mm
First, it eats the eggshell, then moves to the topside where it feeds in a small circular pattern that limits the inflow of latex. Mandibles are tiny, so they nibble soft tissue. Movement is minimal, because survival depends on staying hidden and fed.
Instar 2, ~6–9 mm
Still relatively slow-moving and cautious but begins to explore more of the leaf. Stripes start to appear. Slightly stronger jaws and growing confidence.
Instar 3, ~10–14 mm
Actively moves between leaves, feeding more aggressively. May rest on stems or leaf undersides. Needs more food to fuel rapid growth. Begins to show preference for fresher, less damaged leaves.
Instar 4, ~14–22 mm
Less vulnerable to predators, so covers more territory, often seen crawling across the plant. Feeding is intense and frequent. Preparing for final growth spurt.
Instar 5, ~25–45 mm
The caterpillar may notch the stem of the leaf, causing the leaf to sag to a vertical position, hiding the caterpillar. An alternate explanation for notching is that can reduce the latex flow.
"Eventually it stops feeding, wanders away from the milkweed to find a safe pupation site—often several feet away. Time to transform. Instinct drives them to seek sheltered, stable surfaces for chrysalis formation."
At various stages," they often choose sheltered spots during molting or extreme weather, since wind can dislodge them or make feeding difficult."
"Caterpillars may switch leaves or even plants to optimize cardenolide intake while avoiding overly tough or senescent foliage."
"Caterpillars may shift to leaf undersides or stems if they detect predator activity. Their warning coloration (bright stripes) helps deter some threats, but not all."
"Milkweed latex—a sticky, defensive secretion—flows more readily in humid conditions. This can jam their mandibles and cause feeding interruption. Caterpillars often “sabotage” latex flow by trenching or vein-cutting before feeding, and humidity may influence how effective these strategies are."
"Caterpillars may shift to drier parts of the plant—like leaf undersides or stems—during high humidity periods. Some studies suggest caterpillars may feed more actively during periods of lower humidity (e.g., midday) and rest or relocate during early morning dew or post-rain conditions."
"Monarch caterpillars begin to experience heat stress above ~38°C (100°F). High humidity can exacerbate this by reducing evaporative cooling, making it harder for caterpillars to regulate body temperature."
In summary: "Their movement patterns aren’t just about food—they reflect changes with increasing size--in sensory perception, risk tolerance, environmental conditions, and energy needs."
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Notes: I sourced many of the facts above from Copilot AI. I edited heavily. Where editing was lighter, I put the text in quotes. I did detect two errors made by Copilot.
My observation of newly hatched caterpillars moving to the top would have been impossible without my use of the leaf diagram. One could argue that the caterpillars were on top because the eggs had been laid on top (as often happens with eggs laid in captivity). But my leaf diagram proved most eggs had been laid on the bottom of this leaf.