Wednesday, August 24, 2016

Croton setigerus

Croton setigerus Hook. (syn. Eremocarpus setigerus (Hook.) Benth.), alternatively known as turkey mullein or dove weed, is a small annual plant native to dry regions of the Western US. They grow between four and twelve inches tall, something that may vary regionally, and have various leaf sizes between individuals. The seeds are eaten and distributed by wild turkeys and doves, thus the common names. Male flowers are tiny, appearing in clusters at the ends of branches.

A single female flower

Female flowers are often singular or in small groups and grow out of the axils of the upper branches. The female flowers are basic, consisting of only a carpel which has a single exerted style. The entire plant is covered in dense unbranched hairs that appear soft but are mildly rough, and can be irritating to the skin.

Croton setigerus is a member of the Euphorbiaceae (subfamily Crotonoideae), or spurge family, and has similarly toxic attributes. Though not strongly toxic to humans, ingesting any part of the plant is not recommended. If eaten by livestock, the plants form indigestible masses in the gut called phytobezoars which can become life threatening and require surgical removal.

Diterpenes found in the plant, particularly the chemical eremone, are reputed to stun fish. For this reason the crushed plants were used to kill or stupefy fish in shallow water by some Native American tribes such as the Klamath and the Paiute tribes. The crushed leaves and stem were sometimes used in conjunction with Chlorogalum pomeridianum and Trichostema lanceolatum for this purpose.

There are native and nonnative pollinators that are more than willing to visit Croton setigerus, seemingly for both pollen and nectar. I'm not quite certain how the pollen is transferred to the stigma, there is little incentive for bees or other pollinators to visit the stripped down female flowers hidden beneath the top layer of foliage. Perhaps it simply falls down when pollinator activity dislodges it from the anthers, or maybe wind has a role to play.

Apis mellifera

An issue of the American Bee Journal from 1915 as well as a handful of other early 1900s publications note Croton setigerus as an important nectar source for fall buildup after the honey harvest, claimed to produce a "thick amber honey." I was ready to proclaim that honeybees, at least the ones here, do not visit Croton. Clearly this would have been in error as I have caught one in the act on a plant near one of our apiaries. It was, however, just a single honeybee in an area with a few thousand individual specimens of C. setigerus, so this would indicate that it is pretty low on their list of favorable flora. I would expect that there must be a dearth to push them towards visiting such a plant en masse.

Apis mellifera

Judging from the behavior of the bee and lack of pollen in her corbicula I would conclude she was collecting nectar. However, if visitation remains very low, then this will yield little influence as a honey plant. Over 20,000 foragers are required to produce a pound of honey, but keep in mind that some of that honey is consumed by those same bees, so the number is probably much higher.

Halictus ligatus
Halictus ligatus is one such native bee that I have watched visit Croton setigerus on more than a few occasions. The bees use their proboscis to probe the small male flowers and in the process of doing so coat themselves in pollen. Perhaps there is no nectar to speak of, and they are simply duped into thinking so and thus attempt many flowers, but I am sure there is something they are able to attain from these flowers such as an oil or something like nectar if not the real deal. Either way, pollen is an important component to the survival of most bees, being their sole source of protein.

Halictus ligatus
Halictus ligatus is a ground nesting species native over a wide range from Northern Mexico (and possibly further south) and through most of the US states, north into the Eastern Canadian provinces. They are active for most of the year, possibly raising multiple generations per growing season, and visit a wide variety of plants from many families. For these reasons they are probably a very economically important species and are likely to have a big hand in crop pollination where there is habitat and year long forage, probably less common or nonexistant where monocultivated crops are prevalent.

Philanthus crabroniformis
One of my favorite native hymenopterans, predatory wasps in the genus Philanthus, were foraging for nectar on one of my observations. These wasps are known as beewolves, named after their prey. While there are European species that prey specifically on honeybees, the American beewolves prey mostly on halictid bees which are numerous here, such as Halictus ligatus which was foraging nearby.

Philanthus crabroniformis
The beewolf is heavily armored and resistant to the defensive stings of the bees it is after, and swiftly stings the bee with its own stinger. The venom doesn't kill the bee, but paralyzes it. The bee is then brought to the beewolf nest, a solitary tunnel excavated in mildly sandy soil, and left in a brood chamber for a single egg. The larvae will feed on the bee until it is ready to pupate. Unlike bees, who attain protein for their larvae from pollen, many wasps (with the exclusion of gall wasps and other plant feeders) hunt or parasitize other insects as a protein source for their young.

Philanthus aren't the only wasps I've seen visit Croton setigerus. Thread-waisted wasps (Ammophila sp.) and spider wasps (Pompilidae) were also visiting the small male florets on one day. I did observe a general decrease in pollinator activity when it was hot, over eighty-five degrees Fahrenheit.

Melanoplus devastator
Melanoplus devastator, the devastating grasshopper (possibly named for its feeding preferences in agricultural settings, and thus economical impact), was common in this field where I did most of my observations. They were often found on Croton setigerus, leading my imagination to consider them as potential pollinators. Before you dismiss this possibility, however unlikely, consider that there is at least one known orchid which is proven to be pollinated specifically by crickets (same order as grasshoppers, Orthoptera), although this is far away on an island in the Indian Ocean. In my own garden, usually in the evening, I have observed katydid nymphs and adults (also in the order Orthoptera) visiting flowers to feed. Only one grain of pollen is required to produce a single viable seed, so is it impossible that a grasshopper could be a pollinator?

Avise, Joan. "Doveweed (turkey mullein), Croton setigerus." n.d.
Burrows, George E, and Ronald J Tyrl. "Toxic Plants of North America." 2nd ed. United States: Iowa State University Press, 2013. Print.
Cary, Russ. "Yosemite wildflowers: Turkey Mullein (Croton setigerus)". 2006.
“Croton (plant).” Wikipedia. N.p.: Wikimedia Foundation, 14 Apr. 2016.
HerbiGuide. “Doveweed.” n.d.
"Croton setigerus - dove weed." Discover life. 2010
Micheneau, Claire, et al. “Orthoptera, a New Order of Pollinator.” Annals of Botany (2010): 299.
Pellett, Frank Chapman. "American Honey Plants." 5th ed. N.p.: Dadant & Sons, 1979. Print.
Pleasants, J. E. "Southern California Bee Keeping as Seen by an Inspector." The Bee Keepers Review XXI.1 (1909): 298-301. Print.
Richter, M. C. "Honey Plants of California." Bulletin No. 217J (1911): 1004-005

Saturday, August 13, 2016

Sceliphron caementarium

Sceliphron caementarium ♀ perched on an unused beehive bottom board.
While going through empty honeybee hives I often find signs of other insects taking up residence within. The usual suspects are lesser wax moth larvae (Achroia grisella) and European earwigs (Forficula auricularia). On occasion, I have discovered small clay nests such as in the photos below and just yesterday I discovered the culprit. The black and yellow mud dauber, or Sceliphron caementarium, is a large yet docile solitary wasp in the Sphecidae or thread-waisted wasp family. This isn't the first time I've found wasps inside unused beekeeping equipment, I occasionally come across a nests of semi-social wasps Polistes aurifer and P. dominula. None of these species are of consequence to honeybees, and the paper wasps are probably beneficial to an apiary since there is a high likelihood they will prey on wax moth larvae which, unchecked, will destroy unused wax comb. Sceliphron caementarium, however, hunts spiders to provision its nest while the adults feed on nectar.

Sceliphron caementarium nest
Nests of the black and yellow mud dauber are composed entirely of mud, so this species is highly dependent on a nearby water source. Adult females find a muddy area to alight, then make a pea sized ball of bud with their front legs and mandibles before carrying it to a sheltered location to build. Empty bee boxes provide the perfect shelter as there is a small entrance protecting the nests from attack from mammals or other large predators. The species is found in nearly every state in the US, Mexico, Southern Canada, and even South America and Eurasia (among many exotic countries) as an invasive species. It is interesting to compare the colors of the nests as they represent different types of mineral constituents found in different regions.

Nearly completed mud nest on old honeybee comb.
As mentioned above, the adult females hunt spiders to provision their nests, but they don't kill them. Their powerful sting immobilizes the spider to keep it from harming the larvae or pupae while in the nest but keeps it alive so it doesn't go bad, fresh meat in other words. The wasp lays her egg on the first spider she packs in the nest, then continues to place up to 25 or even more spiders in the cell. Their favorite prey are often orb weavers (Araneidae), lynx spiders (Oxyopidae), crab spiders (Thomisidae), jumping spiders (Salticidae), and sac spiders (Anyphaenidae), though they seem to have a preference for spiders that build webs. The most fascinating evidence suggests that individuals have different preferences in the types of spiders they like to paralyze for their nests, even when in the same region.

Completed mud nests on old honeybee comb.
After provisioning and covering each cell entrance, the entire nest is coated in a thick layer of mud to protect the brood from parasites and perhaps also to moderate the temperate conditions inside. While the wasps themselves are docile and do not defend their nests, they are considered to be aesthetic pests due to their nests being perceived as unsightly, especially when attached to buildings and other man-made structures. There is even the possibility that fatal plane crashes have been the result of an unfortunate nest location.

Sceliphron caementarium pupal case
Inside the nest, at this time, pupal cases were present with larvae in the prepupal stage. There were few spiders present, though at one time earlier in the year the cells were likely full of spiders. The larvae, in preparation for pupation, devour first all the spider hemolymph (blood), then the spider carcasses in their entirety. The pupal case is very thin and malleable, made from what seems to be some sort of secreted resin, and held in place by thin strands. I speculate the case may be suspended from the walls to prevent excessive heat or cold from conducting through the mud and harming the pupae.

Sceliphron caementarium ♀ with the characteristic and very elongated pedicel (waist).
Although these are large wasps which surely evoke fear in the hearts and minds of the uninitiated, they are certainly not prepared biologically for defense and would be more likely to flee than sting. Unlike the social wasps, they do not spend the night in their nests but rather sleep in thickets of woody debris or other secluded locations (such as an empty beehive). As a result, or by circumstance, they do not aggressively defend their nests as in genera Dolichovespula, Vespula, Polistes, and a few others. Also unlike the social wasps and bees which sting as a form of defense, the venom of Sceliphron caementarium is solely formulated for the paralysis of spiders and lacks many of the pain-inducing compounds found in them. The quantity of venom is also significantly reduced. For instance, the European hornet (Vespa crabro) is estimated to have ten times the venom compared to S. caementarium. Stings by the black and yellow mud dauber, apparently, are most likely the result of rough handling.

Sceliphron caementarium
While the larvae feed on living yet immobilized spiders, adults feed on nectar. Like many wasps they do not have very long probosces so they are most often found on flowers with easily accessible nectar, such as plants in the Apiaceae, or carrot family. While they do not intentionally collect or eat pollen, it is inevitable to contribute to pollination when visiting many flowers of the same species.

Sceliphron caementarium ♀

Further reading:

Eaten, Eric R. "Black & Yellow Mud Dauber." Bug Eric. N.p., 9 Aug. 2016.
"Sceliphron Caementarium (Drury, 1773)." Featured Creatures. Ed. Jennifer L. Gillett-Kaufman. University of Florida, n.d. Web.
Tumilty, Hannah, and Megan Bain. "Sceliphron Caementarium." BioWeb. University of Wisconsin, n.d. Web.
"Species Sceliphron Caementarium - Black and Yellow Mud Dauber." BugGuide. Ed. Troy Bartlett et al. Iowa State University, n.d. Web.
Wikipedia contributors. "Black and yellow mud dauber." Wikipedia. 5 Apr. 2016. Web.