Caterpillars spitting or vomiting green fluid are normally reacting to poisons. Insecticides and pesticides often cause a caterpillar to spit or vomit bright green.
This green fluid is close to the same color as a caterpillar’s hemolymph (blood). It is easy to tell the difference. Hemolymph dries to a dark color, nearly black, in less than five minutes.
Other than poisons, the other primary cause of green spit is a defensive reaction to being disturbed. Some species automatically spit green fluid at the slightest touch. Others must be pestered quite a bit before they resort to spitting green fluid. Read more about this defense reaction at the bottom of this page.
The most common source of caterpillar poisoning is plants. Most wholesale nurseries treat their plants with pesticides/insecticides to kill plant pests. Most customers do not care whether the plant is safe for caterpillars to eat. They want a beautiful full plant. Some plant pests, such as thrips, are difficult to kill. Any insecticide that will kill thrips will affect and normally will kill caterpillars.
The retail nursery may have no idea that the wholesale nursery has treated the plant and when asked, will honestly say that they have not treated the plants. When asked, they’ll often say that the plant has not been treated. It won’t occur to them that the wholesale nursery may have treated the plants. To them, it isn’t that important. They haven’t had an interest in butterflies and, try as they might, can’t fully understand your concern. They aren’t uncaring. Many honestly and truly don’t understand. (On the other hand, some do understand and will be dishonest with you. Learn your nurseries to know which will be honest with you.)
Many of us grow our own plants and we know our plants have not been exposed to pesticides by our own actions. We should remember that our neighbors could have sprayed their yards while we were away from home. A rain after they sprayed, or irrigation cutting on after they sprayed, could wash the pesticide across the soil and into our yards, where roots of our host plants can draw the pesticide into our plants. Some of us live so far in the country we don’t have a neighbor close enough for any of their actions to affect our plants. We are the fortunate ones. Our only concern is the state spraying mosquito spray from airplanes. We’ve had that happen to us.
Flea/tick medication is deadly to caterpillars. Whether the medication is a pill given to a pet, a drop placed on the skin of the pet, or a collar placed around its neck, it is a pesticide that can kill caterpillars. Pills and drops are absorbed by the pet and is exuded in the pet’s skin oils. When the animal is petted, the insecticide transfers to the hands of whoever is petting the animal. This insecticide can last a month or more, deadly to caterpillars if hands are not washed after petting a cat or dog (and touching caterpillars or their food after petting a cat or dog). This alone is reason enough to insist that every visitor washes hands well before touching a caterpillar or their food.
If one picks host plant leaves to feed to caterpillars and only one or two caterpillars begin spitting green liquid, it could be that those caterpillars ate the part of the leaf that tick-medication-contaminated-fingers touched when the leaves were picked. The other caterpillars ate non-contaminated parts of the leaves.
YES! Do protect your pets. Simply wash your hands well before touching caterpillars or their food. As additional protection, when applying the medication to your pets, wear disposable gloves.
Poison can be introduced to your caterpillars though many other sources. A doorknob at the last store you left, the shopping cart handle, the money you were given in change … the last person to touch it may have just used strong insecticides and may not have washed his hands. Perhaps it was the mosquito repellent, routine termite treatments, lawn treatment, city/county mosquito spraying, shaking hands with someone who has handled pesticides recently, or one of many other sources. If you don’t believe that people will handle pesticides without gloves and go shopping without washing, think again. They do.
Monarch caterpillars won’t spit in defense until it has become fairly well irritated. It takes a good bit of disturbance before they begin to spit out green fluid. Overcrowding will cause Monarch caterpillars to spit out green fluid. If your caterpillars are spitting green fluid but you are positive they are not exposed to poisons and they do not act poisoned in any other manner, try reducing the number of caterpillars in one rearing container. Remember that overcrowding caterpillars refers more to the amount of food surface more than the size of the container. Two stems in a huge container is the same area for caterpillars as the same two stems in a smaller container simply because the caterpillars will be on the stems except when they are molting or pupating.
Species known to quickly spit when disturbed are skippers, Tawny and Hackberry Emperors, sulphurs, and a few other species.
Side note: a common misconception is that if a milkweed has aphids, it is safe for caterpillars. Not necessarily so! There are many pesticides that won’t affect aphids but are deadly to Monarch caterpillars.