Turning up the heat: strange (and stinky) skunk cabbage

It may be hard to imagine finding flowers and pollinators in February, but the first flowers of the spring can already be seen blooming all across New England. Poking up from the frozen ground of swamps and stream banks, skunk cabbage (Symplocarpus foetidus) doesn’t exactly look like your average flower. It produces multiple small flowers on a central spike, or spadix, surrounded by a purplish hood known as a spathe. Few other flowers in northeastern North America share this odd floral structure – a characteristic of plants in the largely tropical Arum family, which includes popular houseplants like Spathiphyllum (peace lily), Philodendron, and Monstera.

Skunk cabbage flowering along a small stream in Massachusetts in late February. Skunk cabbage spathes are easy to overlook, but can be quite common in wetlands in late winter and early spring.

Aside from its bizarre appearance, skunk cabbage stands out from other native New England plants with another unique feature: its flowers have the ability to produce significant amounts of heat! Fueled by energy stored in the plant’s modified underground stem (called a rhizome), skunk cabbage can maintain temperatures of over 50 degrees within the spathe even as external air temperatures drop below freezing. Skunk cabbage flowers produce varying amounts of heat depending on environmental conditions as well as their age. Like some other related plants in the Arum family, skunk cabbage flowers are all female when the spathe first opens. These later become pollen-producing male flowers, with flowers at the top of the spadix transitioning first. Heat production peaks during the female phase, declining as the flowers age and begin to produce pollen.

Skunk cabbage spathe. This structure surrounds the plant’s central flower spike. Its mottled purple color may function to attract carrion-feeding flies that could act as pollinators.

Why does skunk cabbage produce heat? Despite several studies of this phenomenon, the answer is unclear. Production of heat may be necessary to allow skunk cabbage to grow and flower in a rather inhospitable environment, preventing the buildup of snow and ice around the spathe. By flowering well before most other plants, skunk cabbage may be able to take advantage of any insects that are active at this time of year, with heating performing the additional function of attracting insect pollinators. The purplish color of skunk cabbage spathes, in addition to their unpleasant smell, suggests that they may be pollinated by flies. Other primarily fly-pollinated plants share similar traits, tricking flies into visiting their flowers when searching for sites to lay eggs. By heating its spathes, skunk cabbage could provide an extra incentive for these early-season insects to visit its flowers.

Although scavenging flies hardly seem like likely pollinators, they can be quite important to the pollination success of some groups of plants that specialize in attracting them.

While flies have been documented at skunk cabbage flowers, frequency of successful pollination events seems to be quite low. Interestingly, some of the insects most often seen at skunk cabbage flowers are honey bees, which visit male skunk cabbage flowers as an early-spring source of pollen. However, it’s unlikely that honey bees act as effective pollinators, since they tend not to visit skunk cabbage’s nectarless female flowers and may not even make contact with the spadix itself, instead collecting pollen that has fallen to the base of the spathe.

Skunk cabbage provides an excellent example of just how much remains unknown about the ecology of some of our most ubiquitous (and fascinating!) native plant species. As you walk through the woods in late winter and early spring, keep an eye out for this strange plant in any area with wet soil and appreciate the incredible adaptations skunk cabbage has evolved that allow it to thrive at a time when few other flowers dare to bloom.

PC: Max McCarthy

Solitary wasps are fierce, fascinating, and totally harmless

In mid to late summer in the northeastern US, several species of large solitary wasp (belonging to the families Sphecidae and Crabronidae) frequent gardens, parks, and other open spaces. Despite their threatening appearance, solitary wasps are totally harmless. They are more interested in hunting other invertebrates–like spiders, flies, and bees–than they are in you. Solitary wasps are carnivores that capture and paralyze insects or spiders to feed their young, with many species specializing on particular types of prey. Unlike hornets, yellowjackets, and other social wasps, solitary wasp females build and provision nests independently of one another. Nesting locations differ among species and may include a variety of cavities both above and below ground.

Great Black Wasp (Sphex pensylvanicus) nectaring on Hairy Mountain Mint (Pycnanthemum verticillatum var. pilosum)

Digger wasps in the genus Sphex nest in the ground. In the northeast, the Great Golden Digger Wasp (Sphex ichnumoneus) and Great Black Wasp (Sphex pensylvanicus) are two particularly common species that can be seen drinking nectar from milkweeds, mountain mints, and other flowers. Females hunt katydids, stinging and paralyzing their prey before dragging it back to the nest. Although they are solitary, digger wasps sometimes aggregate, with many females constructing nests in close proximity. Each nest consists of a main tunnel with a number of side tunnels, each of which ends in a brood cell in which an egg is laid after the cell is provisioned with several katydids. When bringing paralyzed prey back to the nest, female Sphex leave the prey item outside the nest entrance while investigating the nest interior before dragging the prey down. If the prey item is moved slightly, the wasp will retrieve it and inspect the nest yet again. Sphex’s automatic nest-checking routine has captured the attention of several philosophers interested in the contrasting ideas of instinct and free will, inspiring the coining of the word “sphexish” (used to describe actions that appear thought-out and deliberate but are instead actually quite mindless).

Golden Digger Wasp (Sphex ichnumoneus) nectaring on Hairy Mountain Mint (Pycnanthemum verticillatum var. pilosum)

Isodontia grass-carrying wasps are a common sight around houses, gathering dry blades of grass and stuffing them into a crevice to furnish a nest. Grass-carrying wasps are predators of katydids and tree crickets and, like the digger wasps, leave their prey alive, but paralyzed, for their larvae to feed on.

Grass-carrying Wasp (Isodontia sp.) nectaring on Hairy Mountain Mint (Pycnanthemum verticillatum var. pilosum)

The giant cicada killer wasps (Sphecius sp.) are hard not to notice. Reaching lengths of an inch or more, these are among the largest wasps in North America. Even so, these formidable-looking insects are typically harmless. Females are not aggressive and although males may behave aggressively, they are unable to sting. Cicada killers sometimes form nesting aggregations, with many females utilizing the same patch of bare soil while males hover about looking for opportunities to mate. As their common name suggests, cicada killers hunt cicadas, paralyzing them and then flying back to their nest while carrying a prey item heavier than themselves. The wasp larva consumes the cicada and emerges as an adult the following summer.

Cicada Killer (Sphecius speciosus)

Other solitary wasps hunt soft-bodied prey. The thread-waisted wasps in the genus Ammophila are a group of impossibly-skinny caterpillar predators. They can often be seen flying with a caterpillar slung underneath their body, toting their paralyzed prey back to an underground nest. Interestingly, after completing their nests and filling the tunnel with sand, some thread-waisted wasps have been observed using a small stone held between their jaws to tamp down soil at the former nest entrance, a behavior sometimes considered to be an example of tool use!

Thread-waisted Wasp (Ammophila sp.) with caterpillar prey

Though they may lack the charisma of butterflies, bees, and other favorite garden insects, solitary wasps are a diverse group that play an essential part in regulating numbers of herbivorous insects. By leaving patches of bare soil for nesting and planting milkweeds (Asclepias sp.), mountain mints (Pycnanthemum sp.), joe-pye weeds (Eupatorium sp.) , and other favorite nectar plants, you can encourage the presence of these beneficial insects in your yard and enjoy their pest-control services and enthralling behaviors.

Photo Credits: Max McCarthy