Oak Trees Delay Bud Opening After Caterpillar Attacks, Cutting Damage by 55 Percent

Oak trees heavily infested by gypsy moth caterpillars in 2019 delayed their leaf emergence by three days the following spring, slashing damage from subsequent feeding by 55 percent compared to the previous year. @NewScientist reported that this delay left hatching caterpillars without immediate access to young leaves, leading many to die.

The finding emerged from an analysis of Sentinel-1 radar satellite images covering a 2400-square-kilometre area in northern Bavaria, Germany, between 2017 and 2021.

Soumen Mallick at the University of Würzburg in Germany and his colleagues examined 27,500 pixels from the satellite images, each representing a 10 by 10 metre area roughly the size of a tree crown. The forests in this region are dominated by pedunculate oaks (Quercus robur) and sessile oaks (Quercus petraea).

In 2019, a massive outbreak of gypsy moths (Lymantria dispar) stripped leaves from many trees, with their hairy caterpillars feeding extensively on foliage.

The satellite data revealed how affected trees responded, showing that heavily infested oaks opened buds later than unaffected ones. Caterpillars hatch at the same time each spring, regardless of tree leaf emergence timing, according to Mallick. This mismatch reduced herbivory damage by 55 percent in the following year for those delayed trees.

Oak trees employ other defenses, such as producing tougher leaves or aromatic compounds that attract predators to the caterpillars. 'The delay in bud opening seems to be more efficient than all these other defence mechanisms,' said Mallick. He noted that the delay was observed across dozens of tree populations and was strongest in forests where it most effectively reduced herbivory.

James Cahill at the University of Alberta in Canada described the delay as a correlation, with evidence of causality not yet established. He suggested it could result from decreased plant vigor due to prior leaf loss or physiological constraints like resource depletion. Cahill added that data from more than one outbreak would help clarify the mechanism.

Mallick acknowledged that physiological constraints such as resource depletion could explain the delay but emphasized its adaptive potential given its consistency across populations. 1038/s41559-026-03071-9, highlights how such delays might explain why forests sometimes green later in spring than temperature-based models predict, especially amid warming climates.