Oak Trees Open Buds Three Days Later After Heavy Caterpillar Infestation, Satellite Data Shows

Oak trees heavily infested by caterpillars one year open their buds three days later the following spring, according to satellite data analysis. This delay slashed damage caused by feeding caterpillars by 55 per cent compared with the previous year. The findings come from a study of a 2400-square-kilometre area in the northern Bavaria region of Germany between 2017 and 2021.

Forests in the study area are dominated by pedunculate oak (Quercus robur) and sessile oak (Quercus petraea). Each satellite image pixel analysed represented an area of 10 by 10 metres, approximately the size of one tree crown. The team analysed 27,500 pixels in total using Sentinel-1 radar images to assess tree canopy condition.

In 2019 there was a massive outbreak of gypsy moths (Lymantria dispar) in the study area. Gypsy moth caterpillars feed on tree leaves and cause extensive damage when plentiful. Caterpillars hatch at the same time each year regardless of tree leaf emergence.

This wrong-foots the insects when they attack again, leading many to die before leaves emerge. @NewScientist reported that the delay in bud opening seems to be more efficient than all these other defence mechanisms.

Oak trees also produce tougher leaves and aromatic compounds that may attract predators of caterpillars. The observed delay is seen across dozens of tree populations and is strongest where it most effectively reduces herbivory. Soumen Mallick at the University of Würzburg in Germany led the research.

He thinks other deciduous plants may do it too. Mallick says the delay could be explained by physiological constraints such as resource depletion.

James Cahill at the University of Alberta in Canada called the finding plausible but noted it remains a correlation. “The delay could be caused by decreased plant vigour as a result of the leaf loss,” Cahill said.

He added that having data from more than one outbreak would help work out what is going on. ” James Blande at the University of Eastern Finland said the mechanisms are intriguing and are a key aspect requiring further research. Forests sometimes turn green later in spring than computer models predict they will based on temperatures, especially as the climate warms.

“This point that plants respond to much more than climate change is very important,” Cahill said. 1038/s41559-026-03071-9.