Bag worms are a nasty little pest, which can quickly defoliate over one hundred species of trees, including cypress, juniper, pine, spruce, apple, elm and willow.
So, what are they?
Bag worms, or bag moths, are a family of the Lepidoptera order of insects, making them closely related to moths and butterflies.
The family is fairly small compared to some others, comprising about 1350 different species, three of which are found in North America. These are the Evergreen, Snailcase and Grass bagworms. The Snailcase bagworm reproduces without the assistance of any males, and thus each egg is a perfect clone of its parent! They are often mistakenly referred to as Case worms, but the two species are very different. Case worm bags are quite flimsy and made primarily of silk, while bag worms use plant material from their environment to strengthen theirs.
Where are they?
They are found all over the world, and some of them have even managed to naturally colonize continents they were not formally native to. They thrive in the eastern United States as far west as Nebraska, north to New England, and south throughout Texas. Use of insecticides in urban areas has decreased populations of their predators, (mostly small wasps and hornets), and this is where the largest populations of bag moths are usually found. Woodpeckers, sparrows and white footed mice can also feed on the eggs inside the bags, but the eggs are so hard they pass straight through, starting a new infestation wherever the animal chooses to drop them.
Most bagworms are completely harmless, and you wouldn’t even know they were there, were it not for their little “sleeping bags” all round your garden. In fact, one Madagascan species locally named “Fangalabola”, is encouraged to breed on Acacia, and its pupae collected as protein rich animal food.
A few species, however, can become serious pests, and have caused a great deal of damage to Acacia in South Africa, and oranges (Citrus x-sinensis) in Florida.
A circular argument
Bag moths have the same life cycle as your average moth, larva to pupa to adult. It starts with a clutch of 300 to 800 eggs inside the bag. These were laid by the female who has since died. Sometimes she falls out of the bag, while in some species the eggs hatch inside the dead parent.
The eggs hatch into larvae, which climb out of their bags and head for the top of whatever plant they happen to be on at the time. They spin a fine silken thread up to three feet long, which is caught by the wind and off they float to colonize a new host.
When they find a suitable one, they attach themselves to a branch with silk, and start building their bag. They start with silk, and build on top of that with material from their surroundings. They poke their heads out of the bags and munch on your tree’s leaves and buds, and they can cause major devastation in a very short time. When they get to about an inch long, (although some species can grow to 5 inches!) they close up their head hole and pupate.
After attaining adulthood, the males fly off in search of a mate, while the females just wait for a likely suitor to turn up. Both sexes of adult bag moth have only vestigial mouth parts and legs, and the females have only vestigial eyes and no wings at all, as they never leave their bag. This is why both sexes of adult live only a short time – they literally cannot eat.
After mating, which the male accomplishes through the bottom end of his lady’s bag, both will die within hours and the whole cycle starts again.
So what’s the downside?
The larval stage of this insect can wreak absolute havoc with many species of ornamental and fruit trees, and very major damage can be done in only a matter of days to both home gardens and agricultural crops.
Please! Make them stop!
There are several methods you can use to control this pest, and which one you choose will depend upon a variety of factors. There is not much you can do to prevent these pests from visiting your garden, as their mode of transport is generally airborne.
Not using broad spectrum pesticides is always a good idea, you can retain many beneficial insects this way, such as the wasps that feed on bagworms. Adding flowering plants to the landscape has been shown to increase the numbers of beneficial predatory wasps. If only people would breed ladybugs instead of spraying aphids this world would be a better place.
What should I do?
The best and gentlest way to control bagworms is to go round your garden, remove the bags, and dispose of them by placing in a bucket of soapy water or a sealed container. If you have young children you can make a “Let’s get the bag ladies” game out of it – many hands make light work!
There are several species of bacteria (esp. Bacillus thuringiensis var. kurstaki) which have been shown to be effective in controlling bagworms. These are best applied when the worms are in their early larval stages. Bacillus thuringiensis is available at nurseries and garden centers as Dipel or Thuricide.
When should I do it?
Caterpillars generally emerge at the start of June, so removing the bags before this is the most effective manual control method. The timing could be slightly earlier or later depending on your specific climate zone, so if you are unsure ask your local nursery person for advice. The bagworms will feed voraciously and grow until around late August, when they close up their bags and cease feeding.