Every spring, forests often turn green quickly when insects hatch. Caterpillars and other leaf-eating insects appear in concentration when young leaves have just grown. At this time, the leaves are soft, juicy and rich in nutrients, providing them with an ideal "spring meal." However, a new study published in "Nature Ecology and Evolution" shows that oak trees have evolved a set of "time tactics": after being severely gnawed by caterpillars in the previous year, they will actively delay leaf sprouting in the following spring, and use "a few days in late spring" to significantly reduce caterpillar survival rates and feeding damage.
Studies have shown that when oak trees are infested with a high density of caterpillars in one year, they will not germinate according to the "normal schedule" the next year, but will delay leaf germination by an average of about three days. For caterpillars that have just hatched from eggs and expect to find young leaves immediately, these few days' delay means "an empty table" - the leaves are still wrapped in the buds and cannot feed, causing a large number of larvae to die due to lack of food in a short period of time. Data show that this delay of only a few days is enough to reduce leaf damage caused by caterpillar feeding by about 55%, making it an extremely efficient physical-temporal defense strategy.
The research team comes from several scientific research institutions such as the University of Würzburg in Germany. They believe that instead of continuously increasing chemical defense substances such as bitter tannins in leaves, it is more cost-effective to adjust the germination time as a "low-cost" method. The first author of the paper, Dr. Soumen Mallick from the Biological Center of the University of Würzburg, pointed out that compared with consuming a lot of resources to synthesize defensive chemicals, "delaying germination" for a few days can significantly weaken the caterpillar population, which is energy-saving and efficient for trees. The researchers also emphasized that this discovery has fundamentally changed people's traditional understanding of forest spring phenology - tree germination does not just passively respond to climate factors such as temperature, but also proactively and flexibly adjusts according to biological stress.
To reveal this mechanism, scientists combine ecological field surveys with advanced remote sensing technology. In the past, understanding the response of trees to leaf-eating insects often required long-term and detailed tracking of individual trees. This time, the team used the European "Sentinel-1" radar satellite to conduct systematic monitoring of about 2,400 square kilometers of forest area in northern Bavaria, Germany. Radar can penetrate clouds and capture subtle changes in canopy structure and moisture content, allowing for a detailed portrait of differences in the timing of a forest's "return to green" in spring.
The research team analyzed a total of more than 130,000 satellite observation records from 2017 to 2021. Each observation pixel is 10 meters × 10 meters, roughly equivalent to the scale of a tree crown, covering a total of approximately 27,500 pixels in 60 forest plots. In 2019, a large-scale outbreak of gypsy moth (commonly known as the "American white moth", an important leaf-eating pest among other groups) broke out in the study area, providing a key "natural experiment" for research. Jörg Müller, professor of conservation biology and forest ecology at the University of Würzburg and co-corresponding author of the paper, said that the radar records clearly show which trees were gnawed "bare" that year, and how they were "deliberately late green" in the following spring.
The research also provides an answer to a phenomenon that has long puzzled ecologists: In some years, even when temperatures are high enough, forests overall turn green significantly more slowly. Previously, research usually attributed this phenomenon to climate fluctuations and frost risk, but the new results show that insect pressure can also drive trees to actively delay leaf unfolding, causing spring phenology to show a more complex change pattern between different years. Researchers pointed out that many current forest models mainly consider abiotic factors such as temperature and precipitation, but ignore the dynamic game between plants and insects. Therefore, they may underestimate or misjudge the response of forests in real situations.
In the context of climate warming, tree species such as oak trees are facing a "tug of war in time." On the one hand, global warming generally pushes trees to germinate earlier to take advantage of the growing season; on the other hand, the threat of high-intensity leaf-eating insects drives trees to delay budding after being severely eaten by larvae to avoid the intensive incubation period of larvae. Andreas Prinzing, co-corresponding author of the paper and professor at the University of Rennes in France, pointed out that this "trade-off between early and late" reflects the high adaptability and resilience of forests under the dual pressures of climate change and ecological interactions.
It is worth noting that this defense strategy of oak trees is "conditional" and "reversible" - only after experiencing a real high-intensity damage in the previous year, the tree will choose to delay budding in the next season. This means that it is difficult for insect communities to "hedge" this tactic in the long term, because the trees do not postpone fixedly every year, but dynamically adjust according to the actual risk, thereby maintaining a certain first-mover advantage in the evolutionary process. The research team said that in the future, they will further analyze the signaling pathways behind this mechanism through controlled experiments, such as how the tree body "remembers" the degree of damage in the previous year and regulates the sprouting time of buds after overwintering.
According to reports, the research was led by the University of Würzburg and was completed in conjunction with the University of Göttingen in Germany, the Thünen Institute in Braunschweig, the Adam Mickiewicz University in Poznan in Poland, the Technical University of Munich, the University of Lorraine in France, the Czech University of Life Sciences in Prague, the Julius Kuhn Institute in Germany, the Bavarian Forest National Park, the French National Center for Scientific Research, and the University of Rennes. The related paper is titled "Satellite data show trees delay budburst across landscapes to escape herbivores" (Satellite data show trees delay budburst across landscapes to escape herbivores), and was published in the journal Nature Ecology and Evolution in May 2026.