Usually in the wild, you find just one monarch egg or caterpillar on a milkweed plant. But when raised at home or in the classroom, you may place a number on each milkweed stem provided for food.
Periodically, you need to transfer them to a fresh milkweed stem--and when you make the transfer, it's important to count caterpillars so you can spot disease or problems.
Imagine if a dairy farmer didn't know how many cows she had? It really helps if you know how many caterpillars there are when you make a transfer, so you don't leave any behind on the old stem and throw them away.
The leaf diagram shows all the leaves on a stem of milkweed. I developed the leaf diagram because I had a terrible time counting eggs or tiny caterpillars. I constantly got mixed up and had to recount!
When counting, it's essential to have a small but bright flashlight, holding it so the monarchs cast long shadows.
The far lefthand column represents the pairs (tiers) of leaves, with the lowest pair on the bottom. The heavy line down the center divides the diagram into left leaves and right leaves. Label the leaves with a sharpie marker: L1, or R1 for the lowest leaves, then L2 and R2 for the leaves the next level up. The second and third columns show the Up side and Down side of each leaf on the left side. Columns 4 and 5 show the Up and Down sides for the leaves on the right side. Don't forget to count any caterpillars on the vertical stem and add them somewhere. The last column on the right contains the totals for each pair of leaves, with a grand total at the bottom.
Using the diagram to find out what's going on
Hatching success: You can calculate the % of eggs that hatch simply by making a diagram of the stem before hatching, and then another one after hatching. Dividing the number of caterpillars found by the number of eggs provided gives % hatching success.
Example: Female #3 laid 37 eggs, according to diagram #1. Four days later they hatched. When I made diagram #2 of tiny caterpillars after all hatched, I found only 30 caterpillars. This gives a hatching success of 81%, or a loss of 19%. Careful search with the hand lens revealed two eggs apparently didn't hatch, 5%, and I saw two caterpillars that were larger than the rest; these may have cannibalized some of their siblings. A few might have escaped from the stem, because when I misted the stem, I saw two caterpillars rappel an inch down from a leaf. But then they climbed back up the silk again. I checked the countertop for any wandering caterpillars, but they are so tiny they would be hard to see. Another stem of eggs laid a few days later had much higher success.
Once you use the diagram to sharpen your observations, you can spot the caterpillars doing many fascinating things. Eggs are almost always laid on the underside of the leaves. But upon hatching, caterpillars move to the topside where the leaf is less protected by tiny hairs (trichomes), which make a bushy barrier. Caterpillars have to shave these away before they can eat, wasting time and energy.
Recently hatched caterpillars have moved to the upper surface of the leaf where there are fewer hairs that interfere with feeding.
A few days after hatching, the caterpillars stay more consistently on the undersurface, where it's darker and more protected. Since the caterpillars have grown, the greater density of hairs on the undersurface isn't as big a problem as before.
Although monarchs are "cold blooded," they do move about to find better temperatures. Experiment! You could keep your caterpillars in a cool room with an overhead light on. Expect them to move to the shady (down) side of the leaf. Then place a heat lamp above the stem. You might then see them move to the upper surface where they can warm up, speeding their growth and shortening their exposure to predators. This would be an example of "thermoregulation," like lizards basking on a warm rock.