Bagworms are a common insect pest that can cause significant damage to ornamental trees and shrubs in a garden. These pests are named for the protective cases, or bags, that they construct from silk and bits of foliage to conceal themselves. Bagworms feed on the leaves and needles of evergreens, and when left uncontrolled, can defoliate entire plants.
The life cycle of bagworms starts in the spring when adult females lay their eggs in the protective bags. The eggs hatch into larvae, which emerge from the bags and begin to feed on the foliage of the host plant. As the larvae feed and grow, they construct larger bags to protect themselves and pupate within the bags. In the fall, adult males emerge from their bags, mate with the females, and lay their eggs for the next generation.
Symptoms
It’s important to note that early detection and treatment of bagworm infestations can prevent significant damage to garden plants.
- Browning or yellowing of leaves and needles due to the larvae feeding on the plant’s foliage
- Defoliation of plants as the larvae consume more and more leaves and needles
- Presence of small, cone-shaped bags attached to branches and twigs, which serve as the protective cases for the larvae
- Webbing on foliage or branches, as the larvae spin silk to hold the bits of foliage together and secure their bags to the branches
- Damaged or bare branches, as heavy infestations can result in the death of the affected plant or section of the plant
- Sticky honeydew or sooty mold on foliage, as bagworms can attract other insects that feed on the honeydew they produce.
What is a Bagworm
It is important to be able to identify bagworms in order to effectively control them and prevent damage to garden plants.
- Cone-shaped bag, approximately 1 to 2 inches long, made of silk and bits of foliage from the host plant
- The bag is often green or brown, blending in with the surrounding foliage, and can be found attached to branches and twigs
- The larvae are plump, worm-like insects that are green or brown in color, with black heads and legs
- As the larvae mature, they pupate within their protective bags
- Adult bagworms are moths, with a wingspan of approximately 1 inch
- The wings of adult bagworms are brown or gray and covered with small, black dots, making them difficult to see when at rest.
Treating a Bagworm Infestation
Bagworm infestations can be challenging to control, as the protective bags make it difficult to apply insecticides directly to the larvae. However, early detection and treatment can prevent significant damage to garden plants. Cultural practices, such as maintaining plant health and removing infested plants, can also help to reduce the severity of bagworm infestations.
It may be necessary to use multiple methods to effectively control bagworm infestations in a garden. For example, hand picking and pruning can be used to physically remove the bags and larvae from plants, while the application of insecticides can be used to kill the remaining larvae.
Physical Control Methods
- Hand picking: This method involves manually removing the bags from infested plants and disposing of them in soapy water to kill the larvae. It is most effective when the infestation is limited to a small number of plants. To hand pick, simply grasp the bag and gently pull it from the branch, being careful not to break the branch. Place the bags in a container of soapy water to kill the larvae. Repeat this process regularly to remove as many bags as possible.
- Pruning: This method involves cutting out infested branches and removing them from the garden to reduce the population of bagworms. To prune, use sharp, clean pruning shears to cut the infested branches as close to the main stem as possible. Dispose of the cut material in a sealed plastic bag or burn it to kill the larvae and adult moths. Repeat this process as needed to control the infestation.
- Burning: This method involves burning infested plant material to kill the larvae and adult moths. This is a less commonly used method due to the potential fire hazard, but can be effective in small, controlled settings. To burn, gather infested plant material into a pile and set it on fire, being careful to follow all local fire regulations and guidelines.
Cultural Control Methods
- Plant health: Maintaining the overall health of garden plants through proper watering, fertilization, and cultural practices can reduce the likelihood of bagworm infestations and increase the plants’ ability to withstand damage. This includes providing adequate moisture and nutrients, avoiding stress from over-pruning or improper pruning techniques, and avoiding damage from physical activities such as mowing or string trimming.
- Sanitation: Removing and disposing of infested plant material from the garden can reduce the number of bagworms and their eggs. This includes removing and disposing of fallen leaves, needles, and twigs from the ground, as well as removing and disposing of infested plant material from pruning or other cultural practices.
- Natural enemies: Encouraging the presence of natural enemies, such as birds, parasitic wasps, and predatory insects, in the garden can help control bagworm populations. This can be achieved through the use of bird feeders and bird baths, as well as by planting a diverse array of flowering plants to provide food and habitat for beneficial insects.
Chemical Control Methods
- Insecticides: Applying insecticides, such as Bacillus thuringiensis (Bt) or spinosad, directly to the bags or foliage of infested plants can kill the larvae. These insecticides are available in liquid or granular form and should be applied according to the label instructions, taking care to avoid contact with desirable plants. Repeat applications may be necessary to achieve complete control.
- Systemic insecticides: Applying systemic insecticides, such as imidacloprid, to the soil around infested plants can be an effective way to control bagworms. These insecticides are taken up by the roots and distributed throughout the plant, killing the larvae as they feed. Systemic insecticides should be applied according to the label instructions, taking care to avoid contact with desirable plants.