This month’s Tree ID event at Schurmeier Woods was fun for all, with a lot of enthusiasm around learning about trees and the environment they live in. Several people had questions, such as, “Why don’t my Oak trees have acorns?” and “How do insects find their host plants?” Master Naturalist Pam Johnson, who co-led the workshop with forestry expert Ken Beach, wants to share the answers to the event’s FAQs.

Why haven’t I seen acorns on my oak for years?
Oaks are monoecious, which means they have male flowers and female flowers on the same tree. Some monoecious plants can self-pollenate but not oaks. They are wind pollenated. Because the female flowers mature after the male flowers have released their pollen, there must be another oak of the same species nearby which releases its pollen at the same time as the female flower matures to fertilize the female flower. Without that other tree, the oak will abort nut development. Another factor could be age. Most oak species don’t produce acorns for 20 years.

What is the reason behind good and bad mast years?
A heavy drop of acorns is called a mast. If we take oaks, masts usually occur within an oak group either white or red. Oaks usually mast asynchronously. Predator reduction is an advantage for oaks that mast. During mast years, oaks produce unlimited food for acorn predators. The year following a mast year is typically (but not always) a bust year for acorn production and many acorn predators perish. According to wildlife ecology expert Douglas Tallamy, this boom or bust approach to acorn production helps keep acorn predator numbers well below what would make oak reproduction iffy during most years. Weather extremes can also affect oaks. Since oaks are wind pollinated, rain can shut down pollination. A late freeze can kill the oak catkins (male flowers). Weather is not the dominate factor, however.

When is a good time to plant tree seeds (oaks, maples, elm)?
As a general rule the best time to plant the seeds is when the seeds drop. American Elms and Slippery Elms drop in spring. Red and Silver Maple samaras in early summer and oak acorns in the fall. The white oak group germinates within days of falling to the ground. The red oak group acorns require a dormancy period exposed to winter temperatures to germinate in the spring. So, the red oak group can be stored in the refrigerator until spring.

What are the effects of climate change on insect-plant interaction?
Is climate change decoupling insects from their host plants? This is a complex question. According to Doug Tallamy you might think so, but to date there is little evidence that insect herbivores have been unable to track developmental changes in their host plant.

He goes on the explain that “unlike migratory birds (and insects like the monarch) that time their migration primarily by changes in day length, the timing of insect life histories is governed by the same environmental cues that control the time of plant development. So if warmer winter temperatures trigger early leaf expansion in a plant, they will also trigger early emergence of insects that depend on that plant. Many insects track host plant phenology directly by inserting eggs into the vascular system of a plant. Egg hatch, then, is triggered by sap flow, directly linking the insect’s life history to seasonal changes in their host plants. Climate change is impacting insect herbivores in other ways, though. When droughts and floods kill host plants, the insects tied to those plants suffer and populations decline.” *

Further studies on individual insects and their hosts are needed as climate change accelerates.

How do insects find their host plant?
Insects that eat plants find the plant species host plant by smell. Insect bodies have thousands of chemoreceptors. Their antennae are packed with receptors capable of detecting minute quantities of plant odors.

*Tallamy, Douglas W. 2025. How Can I Help? Saving Nature with Your Yard.

Douglas Tallamy is a professor in the Department of Entomology and Wildlife Ecology at the University of Delaware.