Oak trees infested by caterpillars delay their leaf budding by three days the following spring, starving the insects and reducing damage. Researchers used satellite data to uncover this defense mechanism in German forests. The finding suggests trees actively adapt to herbivore attacks.
In northern Bavaria, Germany, oak trees hit hard by gypsy moth caterpillars in 2019 showed a striking response. The following spring, their buds opened three days later than those on unaffected trees. This timing mismatch left the hatching caterpillars without tender leaves to eat, causing many to die and halving leaf damage to 55 percent of the previous year's levels, according to Soumen Mallick at the University of Würzburg and his team. They analyzed Sentinel-1 radar satellite images covering 27,500 pixels—each roughly the size of one tree crown—in a 2400-square-kilometre area from 2017 to 2021, focusing on pedunculate oak (Quercus robur) and sessile oak (Quercus petraea) forests. The pedunculate oak is also known as the English oak. Mallick noted that this delay outperforms other oak defenses, such as tougher leaves or aromatic compounds that draw predators. He believes it reflects an evolutionary adaptation seen across multiple tree populations, not just individual stress from leaf loss. Experts praised the discovery but called for more evidence. James Cahill at the University of Alberta called it 'very plausible' yet correlational, urging data from additional outbreaks to confirm causality. James Blande at the University of Eastern Finland highlighted the need to probe underlying mechanisms. The study also sheds light on why forests sometimes green up later than climate models predict, emphasizing factors beyond warming temperatures.
