Spruce budworms are some of the most geographically widespread and most destructive insects in North America. Several species of conifers are prone to moderate to severe defoliation from them. If you've hiked or driven through forests with large brown, evergreen trees, you've likely seen their effects. Since spruce budworms are native insects, they also play an important role in the ecosystem. Dead white spruce and fir trees that the worms leave behind are particularly suitable habitats for cave-nesting birds, rabbits, weasels, and other wildlife, according to the Michigan Department of Natural Resources.
The control or control of this pest is usually carried out by the local forest service over a large area. If you find trees on your property that are affected, you can use the following information to better understand and troubleshoot this problem on a smaller scale.
Good products for combating spruce budworms:
Spruce Budworm overview
The spruce budworm is a common pest of conifers. Source: Paul Williams
There are two types of spruce budworm native to North America, the eastern spruce budworm (Choristoneura fumiferana) and the western spruce budworm (Choristineura orae). Both species grow into small adult moths about 1.5 cm long with orange and brown wings. Their larvae have a shiny dark head, a red-brown body with white or yellow spots. Both eastern and western spruce budworms are univoltine moths, which means there is a new generation each year.
Spruce budworm life cycle
Like many other univoltine species, the adult moths will measure their life cycle against that of their main food source. Adults will emerge from their student phase from mid-June to mid-July. They are active in the late afternoon and evening. Mature female moths mate and lay a mass of 25-40 eggs on the underside of the host tree's foliage. Larvae appear about 10 days later. Typically, these caterpillars do not feed immediately after hatching. Instead, they spin silky threads on the bark of trees to create a cocoon-like shelter or hibernacula to hibernate. The larvae will emerge the following April to attack the foliage and new buds of their host trees. Examine your trees for small larvae from April to early May and start treatments before they reach their most voracious time in June. The 5th and 6th instar of this insect will do the most damage to trees in preparation for their pupal stage.
Common living spaces
The budworms of the eastern and western spruce have different geographical distributions in North America. As the name suggests, the eastern spruce budworm is found east of the Rocky Mountains, particularly in the Acadian forest in Maine and in the northern boreal forests near the Great Lakes. Eastern spruce budworm populations can severely defoliate balsam firs and white, red, and black spruce trees. They can also occasionally damage tamaracks, pines, and hemlocks.
The western spruce budworm is widespread in the western United States and Canada and can be found as south as New Mexico and as north as Alaska. In contrast to the budworm of the eastern spruce, the populations of the budworm of the western spruce prefer Douglas fir, large fir, silver fir, cork bark fir, blue fir, Engelmann spruce and white spruce. Ornamental conifers such as spruce and Scots pine used in landscaping are also vulnerable.
Depending on the length of their life cycle, the spruce budworm can be found in different parts of the host trees. Women lay eggs near tree buds. Newly hatched larvae find a more protected spot on the tree growth, e.g. B. in the vicinity of staminated (male) flowers, in cracks in the bark or under bud scales. When they emerge in spring, they will eat the buds and then migrate to the end of the branch. Their protected location can make it difficult to control and manage.
What do spruce budworms eat?
Spruce budworm larvae feed on the buds and new growth of the host trees. They bored into needles, buds, staminated flowers, and cones. They can also feed on older needles during severe infestation. The first signs of damage to the spruce budworm can be seen on the outer branches of the upper crown of a tree. When the larvae feed, they separate the needles at the base and dangle them along spun threads, eventually turning the branches brown and causing the needles to fall.
While it is uncommon for spruce budworms to kill a healthy tree in one season, the loss of foliage and thinning will slow the growth of healthy trees over time. Repeated infestation can eventually kill even a previously healthy tree.
The budworm has an outbreak cycle of 30 to 40 years. The last time there was a severe outbreak of the eastern spruce budworm in Quebec occurred in 2006, which led to the defoliation of 3,000 hectares of trees. Outbreaks can last for many years and are a major concern for the forest service. Scientists fear that the budworm threat could be exacerbated by warmer, more favorable climates and an earlier start to the feeding season.
How to get rid of spruce budworms
A spruce budworm on fallen leaves. Source: BC Gov.
Because both types of budworm are endemic to North America, they are well adapted to the environment and have natural control factors to keep their populations in check. In particular, budworms are often parasitized by several species of wasps and flies. Their number also depends heavily on the weather over a number of years. For severe outbreaks, you may want to use a combination of chemical and environmental controls to reduce their spread.
Organic or chemical control
Bacillus thuringiensis var. Kurstaki (Btk) is an effective biological control for temperate populations of spruce budworms and caterpillars when they emerge from their eggs. Once this bacterium is ingested by the budworm larvae, it attacks its digestive system and eventually kills it. Btk is available online or at local gardening stores and can be used to treat several pests. For forest landowners and homeowners with multiple affected trees such as columnar conifers, using Btk may be an appropriate solution. According to the US Department of Agriculture, this method was ineffective on large populations of this pest to significantly reduce their numbers.
Applications of horticultural oil can smother eggs that it successfully coats. Because it is difficult to identify where the eggs are on a particular tree, it is not a viable option for large-scale management.
Different states have different regulations for the use of chemical insecticides, especially in endangered water catchment areas and in environmentally sensitive areas. In addition to Btk, other registered insecticides include carbaryl, trichlorfon, acephate, malathion and fenitrothion. The insecticide needs to be used carefully and selectively as the spruce budworm's webbing can protect the larvae from contact with the chemicals. Check with your state and local forest service for the proper handling of the insecticides, and contact licensed professionals who can perform these treatments.
Environmental control
Deforestation of the Douglas fir by the budworm of the western spruce. Source: WA Department of Natural Resources
The forest service uses various environmental management techniques to control budworm outbreaks. For example, a young balsam fir growing under a mature balsam fir or white spruce is more prone to infestation if larvae fall from the canopy. In this case, it may be best to remove overhistory trees and replace them with non-host trees. Another method is to thin out or reduce the host tree density to obtain sturdy trees of similar age and height. Usually, spruce and fir trees are the healthiest under 60. Felling and thinning trees to reduce stand size and monitoring large trees require special equipment and training. It is therefore best to contact your forest service or the expansion offices.
Prevent spruce budworms
The best way to prevent this budworm is to closely monitor the health of your trees and give them the best growing conditions. In late summer or early autumn, assess the damage done by the spruce budworm of the current year. If there are only a few needles left in the tree canopy, this tree may be heavily used.
For homeowners who have some white spruce or Douglas fir trees, you can start fertilizing their trees from spring through summer. Feeding the trees improves their vitality and strengthens their natural defense system and can reduce the risk of defoliation. Many pests, in addition to spruce budworms, attack weaker plants as easy targets. If you keep your trees healthy, you can offer them joy and even ecotherapy for years to come!
frequently asked Questions
The pupal stage shown here is located between the larvae and the fully grown moth. Source: Markus Kolb
Q: How does the spruce budworm spread?
A: Adult spruce budworms are moths that can travel some distances to lay eggs. They can also be transported up to ten miles through storms and winds.
Q: How much damage does the spruce budworm cause annually?
A: The spruce budworm is endemic to the forests of North America. In normal years, they cause some damage to trees but are not devastating. During outbreaks that occur every 30-40 years, they can severely defoliate trees and kill many over a number of years.
Q: What does the spruce budworm eat?
A: Spruce budworms are a normal part of the forest ecosystem. Both their larvae and the adult moth provide food sources for parasitic wasps, parasitic flies, spiders, and birds
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