Trees sometimes feature web-like structures that can cause concern for property owners. While they might resemble spider webs, these silken creations are typically the work of certain insects. Understanding these webs helps to clarify their presence and informs appropriate responses.
Unmasking the Mystery
The “spider web looking things” commonly seen in trees are not spun by spiders. These structures are silken nests created by specific types of caterpillars. A fundamental difference exists between these insect webs and true spider webs. Spider webs are generally fine, often nearly invisible, single strands used primarily to ensnare prey. In contrast, caterpillar webs are dense, communal shelters made of silken sheets, designed to protect the larvae from predators and harsh weather conditions while they feed.
The Architects of the Webs
Two common types of caterpillars are responsible for these prominent webs: the Eastern Tent Caterpillar and the Fall Webworm.
Eastern Tent Caterpillars ( _Malacosoma americanum_ ) are native to the eastern United States. They build thick, cottony tents in the crotches or forks of tree branches during spring and early summer. These caterpillars are dark-colored, hairy, and can grow up to 2 inches long, often displaying a distinct white stripe down their backs with blue and orange spots. They hatch in early spring as tree leaves emerge, feeding on foliage by day and returning to their communal tents for protection at night or during cold, wet weather.
Fall Webworms ( _Hyphantria cunea_ ) construct their webs later in the season, from summer into fall. They build nests at the ends of tree branches, enclosing leaves as they expand their nests. These caterpillars are pale yellow to greenish with black or red heads and have long, silky hairs emanating from black or reddish bumps along their bodies. They feed within their silken enclosures, enlarging the web to encompass more leaves as they grow. Fall Webworms can affect a wide range of deciduous trees, including fruit and shade trees.
Impact on Trees
The presence of these web-building caterpillars often leads to concern about the health of affected trees. In most cases, the damage caused by these insects is primarily aesthetic, making trees appear unsightly with their visible webbing and defoliated branches. Established, healthy trees can withstand infestations without long-term harm, as they regrow their leaves the following season.
However, the impact can be more significant under certain circumstances. Young or newly planted trees, as well as those already stressed by factors like disease or drought, may be more susceptible to severe defoliation. Repeated, heavy infestations over multiple years can weaken a tree, making it more vulnerable to other pests or diseases, or leading to mortality in smaller trees.
Managing Web-Building Insects
Managing web-building insects like Eastern Tent Caterpillars and Fall Webworms involves a combination of approaches. For many situations, especially with established trees, non-intervention is a viable option. Natural predators and environmental factors often keep populations in check, and the damage is frequently cosmetic, allowing trees to recover on their own.
For active management:
- Physical removal is a direct method, particularly for accessible webs. This can involve pruning out infested branches or using a stick or pole to tear open or remove the webs, exposing caterpillars to natural predators like birds and wasps.
- For smaller trees, egg masses can also be scraped off twigs during winter months before hatching.
- Encouraging biological controls, such as attracting birds and beneficial insects, helps maintain natural checks on caterpillar populations.
- If chemical control is considered for severe infestations, it is recommended as a last resort and should be applied when caterpillars are young and most vulnerable. Products containing Bacillus thuringiensis (Bt) are preferred due to their targeted action and minimal harm to other organisms.