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One has to feel sorry for plants. They are the primary producers. That means they are at the bottom of the food chain. They produce the food that all other organisms ultimately rely on. They are dinner for more organisms than any other creatures. And they seem so defenseless. If an animal starts dining on a plant, it can’t run away. It must stand there and take it.
Obviously, plants aren’t as defenseless as they seem. First, they can lose many, if not all of their leaves and survive just fine. Second, they make all kinds of chemicals that make them distasteful or downright poisonous to many animals. One of the best known plant defenses is that of the milkweed plant.
Milkweed plants contain a milky white latex. The latex is contained in tubes called laticifers that travel alongside the veins of the plant. In these laticifers, the latex is held under pressure. So if the laticifer is cut, the pressure forces the latex out of the wound where it hardens on contact with the air.
The latex contains a double whammy for any dining insect. First, the latex contains a number of poisonous chemicals including cardiac glycosides that are bitter tasting and toxic to many animals. This poison is also found in the leaves, but in a lesser concentration than in the latex. Second, the latex that is forced out of a wound quickly hardens and gums up the mouthparts of any dining insect.
One weakness of this system is that any cut in the laticifer releases the latex pressure on the outer end of the leaf. Exploiting this weakness allows a number of insects to feed on milkweed. This is evidenced by the chewed leaves I find on the milkweed plants in my back yard. Of course, the most well known of these insects is the monarch caterpillar. Every young budding naturalist knows that the monarch caterpillar can take in the poisons from the milkweed and use them for their own protection.
I have looked for the monarch caterpillars feeding on my milkweed, but have never found them. I assumed that they must be present because something was feeding on my milkweed and as far as I knew, milkweed was poisonous to all other insects. So imagine my surprise when I learned that there are a number of insects that can feed on milkweed. And like the monarch, many of these absorb the poison into their tissues and use them for defense. Still, there is the matter of the latex to deal with. Any insect, including these milkweed feeders, stand to have their mouths glued shut by excess latex. In fact, studies have shown that the number one cause of mortality in young monarch caterpillars is getting gummed up by the latex.
After a little research, I discovered how to tell which insect had been dining on my milkweed, even if the insect had left the table. Here are 4 of the more common milkweed feeders and the telltale marks they leave behind.
Before they start feeding on the milkweed, the young caterpillars must disrupt the flow of the latex. They do this by cutting a circular trench through the undersurface of the leaf. This trench severs the laticifers and gobs of latex are forced out of the wounds. Since all of the laticifers in the circle have been cut, the laticifers within the circle are no longer under pressure, and release very little latex when chewed upon. The young caterpillar can now safely feed on the leaf within this circle.
A small circular hole within the milkweed leaf indicates feeding by a young monarch caterpillar. If you turn the leaf over, you may see a ring of hardened latex on the rim of the circle.
The older monarch caterpillars are large enough to devour most of the leaf, so cutting dainty little holes in the leaf will not do for them. They must release the latex pressure from the whole leaf at once. So they chew a hole in the laticifer in the petiole or stem of the leaf - reducing the pressure of the whole leaf at one time. Since the petiole has been cut partway through, the leaf hangs down at an angle. This is called “flagging” So a flagging leaf indicates an older, larger monarch has been for dinner.
Milkweed Tussock Moth caterpillars
Unlike monarch caterpillars which are solitary feeders, milkweed tussock moth caterpillars feed in large groups, often ten or more feeding together. As young caterpillars, they feed in between the laticifers without cutting them. Since the laticifers follow the veins, small holes in the leaf that avoid the veins is evidence of the young milkweed tussock moth.
As the caterpillars grow in size, they spread out over the leaf until all that is left is a skeletonized leaf with the veins still intact. Eventually, they may devour all but the largest veins of the leaf.
Red Milkweed Beetle
Red Milkweed beetles like to feed at the tips of the milkweed leaves. Like the monarchs and the milkweed tussock moth caterpillars, the red milkweed beetle absorbs the milkweed poison into its tissues and is thus afforded the protection. Its bright red coloring serves as a warning to possible predators of its bad taste.
Before feeding, the beetle makes one or more cuts in the laticifer along the main vein. Once the laticifer is cut, it is safe for the beetle to feed on the leaf tip.
If you see a milkweed leaf that has the outer 1/3rd of the leaf missing, turn the leaf over and look for the hardened latex that has oozed out of the cut laticifer along the main vein. A red milkweed beetle has been the diner.
Milkweed Leaf Beetle
While the red milkweed beetle dines on the end of the leaf, the milkweed leaf beetle prefers to feed on the side of the leaf. Instead of severing the laticifer along the main vein, they cut one of the side veins.
Damage along the side of the leaf indicates a dining milkweed leaf beetle.
It might seem ingenious of these insects to disarm one of the main defenses of the milkweed plant. However, this is not a one sided battle. The milkweed does fight back. As mentioned above, cutting the laticifers is a risky proposition, especially for the smaller insects. They must deal with this initial oozing of the latex.
But once the table is set properly, they can then sit down to an appetizing meal of milkweed leaves - cardiac glycosides and all. But they had better not tarry at the table, because once the laticifers have been cut, other insects can join them at the table.