The Not So Darling Caterpillar

Rita Perwich

Consulting Rosarian, San Diego Rose Society

When my daughters were young, one of our favorite books to read together was Eric Carle’s The Very Hungry Caterpillar. We fondly read and reread how hungry that caterpillar was, how much he ate and how big and fat he got. We thought he was so cute and darling…but that was long ago in the days before I became a gardener and grew roses. As a gardener, I encounter way too many real-life hungry and gluttonous caterpillars that grow big and fat on my roses. Better not to tell you the things I think and say to them but I will tell you they definitely do not include the words ‘cute and darling.’

Know The Life Cycle The life cycle of a moth or butterfly which are members of the Lepidoptera order consists of four stages of development: egg, caterpillar or larva, pupa or chrysalis and adult. The process of change from one form to another during growth is called metamorphosis. The female butterfly or moth lays her eggs on or near the plant the caterpillar will feed on. Depending on the species, the eggs are laid singly, in groups or in a pattern. Eggs are usually laid in warm weather and many species have several generations in a growing season. After the caterpillar or minute larva cuts through its eggshell, it feeds upon the plant. As it grows, its skin becomes tight so it stops feeding and molts. It resumes feeding until it outgrows its skin and molts again. The stages between moltings are called instars and the mature larva may look different from the first instar. The pupa is the third stage in the life cycle. It is the resting stage where it finally loses interest in eating and wanders away from the plant. Many species protect themselves by forming a cocoon or chrysalis but some pupate in leaf litter or just below the soil surface. When the adult emerges it flies off in search for a mate and the life cycle begins again. Butterfly and moth adults have similar anatomy but generally butterflies hold their wings vertically when at rest while moths hold their wings flat across their bodies. Butterflies usually fly during the day and moths usually fly at night.

Getting to Know the Pest Caterpillars rank high up on my ‘Dreaded Pest List.’ Their only function is to continually eat and their chewing mouthparts can devour many times their weight. The worst of the bunch have gourmet tastes and burrow into the rose buds and riddle the blooms with holes. There are over 110,000 identified species of moths or butterflies. Caterpillars that feed on rose leaves, buds and blooms include the orange and rose tortrix, the tobacco and rose budworm, the tussock moth, the leafroller, the tent caterpillar and the omnivorous looper. Mating females lay eggs from late spring through to the fall on their preferred host plants. They are also drawn to the plants because of pheromones left by previous activity. Larvae are smooth or hairy and range in color from cream to tan to brown to green depending on the color of their host plant and also the species.

Look for Signs of Caterpillar Activity Daily

It is easy to spot holes in buds, damaged blooms and chewed and skeletonized leaves but the caterpillars can be hard to find. Sometimes you will see their black droppings (frass) deposited on foliage and this is a tipoff to search close by for actively feeding caterpillars. When l see a leaf folded over, two leaves ‘silked’ together or a leaf rolled up and tied with silk, I am always hopeful that I will find the hiding caterpillar. I have learned the hard way that caterpillars are very quick and nimble and slip easily away from gloved hands. So now my first step is to cut the folded, silked or rolled leaf off the plant. The second step is to open the leaf over a bucket so the wriggling caterpillar satisfyingly free-falls into the bucket. You can easily guess the third step. Cut out and dispose of damaged buds which may still harbor caterpillars. Look also for eggs which are usually laid on the underside of the leaves.

A Helping Hand for the Gardener

General predators of caterpillars include birds, assassin bugs, minute pirate bugs, damsel bugs, lacewings, predaceous ground beetles, parasitic wasps and spiders. We need to protect these beneficial predators as without them, caterpillar populations can explode. And remember they eat a lot!

Pesticides Bacillus thruringiensis (Bt) is a microbial insecticide which poses no threat to humans or pets. It also has the advantage of not multiplying or accumulating in the environment. There are different strains of Bt that attack specific kinds of insects. Choose the product that targets only caterpillars, Bacillus thuringiensis subspecies kurstaki (Btk), labeled Bt for Caterpillars and Worms. The highly alkaline environment of the caterpillar’s gut triggers the Btk bacterium to release a crystalline protein called an “endotoxin.” This poisons the caterpillar’s digestive system, dissolves holes in its gut and causes a general infection that kills the caterpillar. Btk is safe to use near bees and beneficial insects but to be effective caterpillars must feed on the treated leaves. Caterpillars that become ill or die after ingesting Btk are not dangerous to birds or other animals that feed on them. Btk is most effective on small, newly hatched caterpillars. Since it breaks down rapidly, repeat applications as specified on the label are critical. It will not get rid of caterpillars within the rose bud as Btk must be eaten by the insect to work. Fully-grown caterpillars may no longer be vulnerable to insecticide applications. The other non-chewing life stages of egg, pupa and adult are not affected by its application. Avoid the use of broad-spectrum insecticides to reduce the risk to beneficials. Spinosad is a microbial-based insecticide but it does have a negative impact on some beneficial insects and must only be used when there is no bee activity.

I love seeing monarch butterflies wafting through and landing gracefully on my roses so I purposely buy and grow milkweed for the monarch caterpillars. This is definitely biased and perhaps a little unfair since I snip all other caterpillars in half. What makes a monarch caterpillar better than any other caterpillar? Here is the distinction: monarch caterpillars are smart enough to stick to milkweed. They don’t chew up my roses’ leaves or feed on my rose buds and blooms. That is a very endearing and intelligent quality, and together with the caterpillars’ very attractive yellow and black stripes, it makes the caterpillars actually quite cute, and I will even venture to say, quite darling too.