From ecoRI News (ecori.org)

Pitch pine forests are at greater risk of attack from the Southern Pine Beetle than forests with a mix of tree species, according to recent research by Dartmouth College. The study shows that the composition of forests is more important than other factors when predicting where the destructive pest will strike next.

The research, published in Forest Ecology and Management, adds to understanding of the Southern Pine Beetle and confirms previous research from the beetle’s southern habitat on the importance of characteristics that increase forest susceptibility to the pest.

The research finding has important implications for forest managers who need to predict and prevent infestation by a pest that is already responsible for significant forest damage and that is continuing its climate-induced move northward, according to researchers.

“Knowing which tree stands may be most susceptible to this beetle is extremely important information for managers working to protect our forests,” said Carissa Aoki, a post-doctoral research associate at Dartmouth and lead author of the study. “This research not only tells us that preventative treatment such as thinning can be effective, but also helps prioritize tree stands for treatment based on structural characteristics."

Southern Pine Beetles have a hard reddish brown to black exoskeletons and measure about 0.12 inches, about the size of a grain of rice.

For the study, researchers focused on southern pine beetle infestations in the New Jersey Pinelands, a forested area that spans the southern and central portions of the state. The  average coldest night of the winter in this region has warmed by about 7 degrees Fahrenheit in the past 50 years, creating favorable conditions for the Southern Pine Beetle to increase its range.

The last documented outbreak of Southern Pine Beetle in New Jersey occurred more than 80 years ago, but the mid-Atlantic states may see more regular outbreaks in the future. The northern New Jersey Pinelands, and pitch pine stands in the New England states with similar structural characteristics, are particularly at risk of infestation as the  beetle continues to move northward, according to researchers.

As of 2014, a new outbreak was detected on New York’s Long Island, and scattered throughout Connecticut the following year. These beetles have additionally been trapped as far north as Rhode Island and Massachusetts, though large-scale tree mortality hasn’t yet occurred in these southern New England states.

“The northward movement of the southern pine beetle is just one example of how climate warming is permitting rapid range expansions,” said Matthew Ayres, a professor of biological sciences at Dartmouth. “We can expect many more cases because the warming continues. This might mean you can grow a cherry tree where you couldn’t before, but you and your plants can also expect a growing battery of pests that weren’t there before.”

The researchers found that Southern Pine Beetle infestations in both wetland and upland areas were far more likely to occur in pure conifer stands than mixed stands of oak and pine. While wetland conifer areas were especially affected, wetland mixed sites had fewer spots than expected.

While the study confirms some of what was previously known about the species, the finding in the Pinelands — that a high percentage of pine trees in a stand is a more important factor than moisture levels — is in contrast to previous research that indicated strong evidence for the connection between high moisture and stand susceptibility.

Researchers also found that stands of intermediate age — about 25 to 75 years for pitch pine — were disproportionately infested. Forest stands comprised of older, larger trees tended not to be very susceptible, while young trees are known not to be susceptible and weren’t sampled. The volume of trees in a stand and the percentage of each tree that is green were also found to contribute to stand susceptibility.

The results indicate that the same tactics that have been effective at limiting beetle impacts in the South could also be effective in newly occupied northern ranges. Those tactics include monitoring to detect population increases, rapid suppression of spots when they are still rare, and thinning of trees for prevention.

Southern Pine Beetle activity occurs in extremes — either very rare or through infestations that involve millions of pests. During episodic outbreaks, the beetles readily kill even the healthiest pines through synchronized attacks that overwhelm tree defenses.

“The Southern Pine Beetle is one of the most aggressive tree-killing insects in the world. Outbreaks tend to be self-sustaining because the more beetles there are, the better they succeed,” said Ayres, a co-author of the report and with 25 years of experience studying the beetle species.