Bugs for Birds
As you’re snuggled warm in your bed… do you ever wonder what happens to our bugs in the winter? Okay, so it isn’t the most burning question on your mind. But come spring, birds (especially migrants) rely on bugs “reappearing”. So how do they survive and where do they go?
Bugs are exothermic – which means they don’t generate their own heat like mammals and birds. Some just leave and spend the winter in a warmer climate. Our best known migrants are dragonflies and butterflies, but this is only a small percentage of our bugs. I’ll cover that approach in a future blog. Those bugs that stay need a plan. Overwintering strategies vary significantly by species. Usually it’s the egg, larva, or pupa stages that ride out the winter while the adults of those species die-off by the first frost. This includes many bees, crickets, grasshoppers, katydids, aphids, and ticks.
First, the survival plan needs a way to deal with the coldest temperature. If ice forms in the cells of a living organism, it destroys the cells and organism. To overcome this, most overwintering bugs undergo biological changes that enable “supercooling” so water in their cells remain a liquid (not ice) at temperatures below 32°F, just like antifreeze does in a car. That effectively changes the lowest temperature they can survive (their SCP supercooling point). There are two approaches bugs use regarding freezing – avoidance and tolerance.
Freeze Avoidance means a bug must spend the season somewhere that will remain above their SCP temperature. It’s the predominant survival approach for our winter bugs.
Bugs search out protective sites that are warmer than air temperature like in leaf piles or under rocks, logs, and man-made structures. Tree bark protects bugs, so some female species lay eggs into bark crevices. The dead stems of herbaceous plants hold bugs including some of our native bees. This is why you should “leave your leaves” and not clean up your planting beds until temperatures are warm enough for bugs to have emerged from diapause (more about that later).
Soil is particularly protective. Cicada nymphs dig deep underground below the frost line. Many beetles survive as grubs (larvae) in the soil. Adult queen bumblebees overwinter in holes underground while solitary mining bee eggs overwinter in individual chambers. Some butterflies, moths, and grasshoppers lay their eggs directly in the soil for overwintering. Flies can overwinter in the soil as pupae.
Then there are the numerous bugs that have evolved with aquatic egg and larvae stages to overwinter under water in ponds below the ice. These include dragonflies, stoneflies, mayflies, and caddisflies.
Freeze Tolerant bugs have a more complex process of supercooling that allows a portion of their body fluids to actually turn to ice! Our best local example is the Woolly Bear caterpillar. No need to worry about them roaming around in December. One study demonstrated survival to a temperature of -94°F. The adult Mourning Cloak butterfly overwinters in tree crevices with partial freezing. It can emerge on warmer days to feed on tree sap, even while snow remains on the ground. Other freeze tolerant examples are midges and cockroaches.
Survival challenges – But there’s more to survival than just dealing with the lowest cold temperature! Food, water, and overall weather conditions are critical.
Most bugs enter diapause, a state that suspends bodily functions including growth and maturation (similar to hibernation), although some bugs remain active. This results from a hormonal change that can be triggered by length of day or reduced evening temperatures.
Sufficient food reserves are critical for survival, even with diapause. Adults and larvae fatten up to increase body weight and store nutrients before winter. Once the bug emerges from diapause, it must find sufficient food before its remaining reserves run out. Most bugs must forage for food, so matching spring emergence with its food source (plants, pollen, prey) are crucial. Pupae are limited to a fixed supply of food until they emerge. Female mining bees leave pollen balls to feed their larvae as the eggs hatch in the spring before they emerge as adults.
Weather matters – The best chances for bug survival are stable temperatures without frequent freeze-thaw cycles. Moisture levels need to be sufficient as bugs can dehydrate, but too much and fungal pathogens will kill them.
Climate change and weather extremes are placing many species at risk for long term survival. Cold springs can cause bugs to run out of food reserves before they emerge. Warm spells can cause a bug to emerge too early before food is available. Or they may not be able to re-enter their supercooled state in time for the return of cold temperatures.
Our rapidly changing weather also causes the co-evolved strategies within an ecosystem to become out of phase. The result can mean host plants aren’t available as food sources at the right time, pollinators miss the timing of flowers preventing fruiting, or predator and prey miss each other. Without co-evolved prey, a species may overpopulate and wreak other havoc. Then there’s the introduced species, but that’s for another time…
Our bugs are truly some amazing creatures!
Karen Campbell, a member of Lehigh Valley Audubon and FocusOnNatives.com provided the content and images for this article.