Even as Insect Populations Decline, Insects Are Causing Unprecedented Levels of Plant Damage

In a study published in Environmental Entomology, the emerald ash borer completes its life cycle in North Carolina in one year instead of the sometimes two-year life cycle that occurs in northern areas of the United States.

According to the researchers, the findings will help them know when to release parasitoid wasps, a key strategy experts have used to try to manage the insect.

This pest originated in Michigan and spread from there, so most of what we know about it in the United States comes from a cooler climate, according to the study's lead author, Ryan Bohannon, a graduate student at North Carolina State University.

invasive insect life cycle could aid management in southeast

(Photo : Stephen Hocking/Unsplash)

The emerald ash borer, also known as Agrilus planipennis, was discovered in the United States in 2002.

It is now legal in 36 U.S. states and five Canadian provinces, as per ScienceDaily.

According to Kelly Oten, assistant professor and extension specialist in forestry and environmental resources at NC State, the insect has been documented in more than 60 of the state's 100 counties.

The emerald ash borer feeds on all ash tree species native to the United States, including the white ash, green ash, Carolina ash, and pumpkin ash in North Carolina.

Adult beetles lay their eggs on the bark of trees, and the larvae bore into the tree and feed on the tissue within.

Researchers have attempted to release tiny wasps, some as small as a gnat, that attack, and feed on the emerald ash borer in its natural habitat.

However, Oten claims that the wasps have yet to establish themselves in the warm, southern climate.

The timing of the life cycle of the emerald ash borer could be crucial.

Researchers in central North Carolina collected emerald ash borer specimens from under the bark of infested ash trees, brought them back to the lab, and measured them to determine their life stage.

This was done every two weeks for 26 months.

They also captured adult insects to learn more about when they are actively flying and laying eggs.

The researchers are also investigating whether allowing infested trees to re-sprout can extend the trees' lives while also giving the wasps more time to establish.

They've also been looking for trees with natural genetic resistance to the pest.

Also Read: The Emerald Ash Borer is 'Winning' Along East Coast

Damaged Caused by Emerald Ash Borer

While EAB adults feed on the foliage, the damage is insignificant.

The larval feeding of the cambium layer just beneath the bark causes primary damage, as per the Iowa State University.

This damage disrupts the flow of water and nutrients in the tree, resulting in progressive canopy thinning and dying branches, usually from the outer or upper parts of the canopy down.

The decline usually takes two to four years, but it can happen in as little as one year or as much as five years.

Epicormic sprouts (suckers) form low on the trunk and major branches, and there is usually noticeable flecking of bark caused by woodpeckers foraging for larvae.

Vertical fissures in the bark are occasionally observed, with galleries or tunnels visible beneath the split.

To manage trees infected or likely infected with EAB, there are two options.

You can either replace a failing/declining ash tree with another species or use preventive insecticide treatments to preserve and protect your ash tree.

Infected ash trees will die unless preventative systemic insecticides are applied. These preventative treatments are most effective on trees that do not have EAB damage and have a full canopy.

To be eligible for treatment, trees must have less than 50% canopy thinning or less than 30% of their branches die.

When the entire crown has visibly small leaves (thinning) and there are only large leaves in the center of the tree (on the suckers), the tree has already suffered too much damage for insecticides to 'save' it.

Other causes of ash tree decline should be removed rather than treated, as preventing EAB damage will not reverse the decline in trees impacted by other diseases or insects.