Wednesday, March 23, 2016

Arctostaphylos viscida

Arctostaphylos viscida
Arctostaphylos viscida Parry (Sticky whiteleaf manzanita) is seeing its best bloom in the past five years here in Southerstern Oregon. Manzanitas are in the family Ericaceae, which includes related genera Arbutus (madrone, strawberry tree), Calluna (heather), Erica (heath), Kalmiopsis, Pieris, Rhododendron (includes azaleas), Vaccinium (blueberries, cranberries, etc.), and over a hundred more genera, few of which known in cultivation. A. viscida is native from Southwestern Oregon south through most of California (to Kern County, possibly further south). It is a shrub or small tree, sometimes reaching a height of fifteen feet in optimal conditions, shorter further south. Small panicle-like clusters of urn-shaped flowers appear in Spring. The entire inflorescence is sticky, covered in glandular hairs, with glaucus leaves, thus the common name.

Arctostaphylos viscida, a variant with whitish florets
Sticky whiteleaf manzanita grows in association with oak (Quercus), madrone (Arbutus menziesii), various conifers (JuniperusPinusPseudotsuga), and Ceanothus among others. With many of these, it shares a variety of endo- and ectomycorrhizae, which are types of fungi which survive on or within the roots of a host and trade valuable nutrients (as well as increasing the surface area of the roots) for carbon and other exudates (root secretions). Dozens of species of mycorrhiza have been discovered in association with Arctostaphylos and associated plant species. Essentially, mycorrhizal fungi are symbiants which infect their hosts, but they benefit the plants in multiple ways and are necessary for the establishment and survival of countless taxa in the plant kingdom.

Arctostaphylos viscida is a pioneer shrub, entering the landscape in succession following grasses and other herbaceous plants. A. viscida paves the way for trees such as Douglas-fir (Pseudotsuga menziesii), and contributes to its establishment by hosting the necessary mycorrhizal fungi that Douglas-fir requires to survive. The same group of mycorrhiza also aids in the establishment of Arbutus menziesii, a relationship which may explain why madrone is so difficult to keep alive in cultivation (lacking the correct mycorrhizal symbiosis outside its native range).

Flowers of Arctostaphylos viscida
The flowers of Arctostaphylos viscida are similar to many (but not all) ericaceous plants such as ArbutusPieris, and Vaccinium by having fused petals forming an urn or cup, a stigma nearly twice the length of the stamen, and a ring of stamen surrounding the ovary and nectaries. When a pollinator enters the flower to imbibe nectar (assuming they enter the flower through the opening rather than biting a hole in the side), they first encounter the stigma before brushing past the anthers in an attempt to reach the nectar. While visiting the next flower, pollen is deposited onto the stigma before inadvertently collecting pollen from that flower. This ensures that the pollen is coming from separate flowers, which is good for overall fitness of the species.

Arctostaphylos viscida bark
The bark is similar to that of madrone (Arbutus menziesii): cinnamony, thin, and smooth once the outer layer of bark peels off. It has the unique characteristic of repelling water. This feature may contribute to the plants ability to survive in severe drought, though if there is inadequate rain or sun they do not flower well. Arctostaphylos viscida shrubs that grow on the north side of structures or under dense canopies of trees do not seem to flower reliably. The excessive shade from a northern exposure or tall trees promotes fungal infections that result in aborted buds and black spots on the leaves. A. viscida prefers open sunny habitats, and is more fit in chaparral than in dense woodlands, though it does perfectly well at forest edges, clearings, and slopes which in some respects have a similar ecology to true chaparral.

From left: A. viscida berries, crushed dried berries, and seed
Seeds of Arctostaphylos are produced in the form of berries. In A. viscida the berries are sometimes sticky, but this seems to go away when the fruit is mature. There is speculation that the berries (and seeds) are distributed by animals, probably birds. Laboratory tests of seed procured from coyote feces (what a fun job that must have been) found low germ rates. Birds are more likely, as the berries would likely pass through quickly (acid treatment in a lab helped break seed dormancy). Fire is another treatment that helps scarify the seed and break dormancy, and has the added benefit of reducing competition (especially tree saplings) which would otherwise shade out the slow growing manzanita. Suppressing fire is probably hindering the development of new colonies of manzanita. By briefly examining many of the stands in my area, it is difficult to find anything but mature plants 6' or taller. I sowed the seeds pictured above last Autumn, but have seen no signs of growth yet. It may take another year or two, and I may build a small fire over the pot to stimulate germination.

A honeybee (Apis mellifera), robbing nectar from a hole in the side of the flower.
Pollinators of many types visit the flowers. The plant exhibits a generalist pollination strategy, and is visited by a wide variety of pollinators. However, due to the shape of the flower (having a constricted opening), smaller insects (besides insects that are small enough to enter the opening) and short-tongued pollinators are excluded from performing as true pollinators and are instead more often seen stealing nectar by entering holes in the sides of the flowers. This likely reduces the chance that the defiled flower will produce fruit since the reproductive structures are bypassed, and the loss of nectar reduces the likelihood that the correct pollinators will be attracted to that particular flower, assuming the reproductive structures weren't damaged when the thief bit threw the side.

Just based on my direct observations, I have seen the following types of pollinators visit the flowers [though there are undoubtedly many more pollinator types throughout the entire range of A. viscida]:
  • Bees: Andrena sp., Apis melliferaBombus sp., Apis mellifera, and bees from the tribes Anthophorini and Eucerini.
  • Flies: Families Bombyliidae (Bombylius major), Empididae, and Scathophagidae.
  • Ants: Family Formicidae
  • Beetles: Family Coccinellidae
  • Hummingbirds: Selasphorus rufus
The smaller bees (Andrena, Apis, Nomada, etc.) will likely not be effective pollinators of Arctostaphylos. Flies in the families Bombyliidae and Empididae are likely effective pollinators due to the length of their probosces. Ants and beetles are unlikely to be effective, the former due to pollen damage from formic acid and the latter due to their herbivorous (flower and pollen eating) nature. Hummingbirds are likely to be effective pollinators, though they can damage floral parts in search of nectar and possibly thrips inside the flowers. Hummingbirds visit the flowers relatively infrequently and are probably second importance to the larger Bombus sp. and anthephorine bees (Anthophora or Habropoda). Other bees seen on the flowers of various species of Arctostaphylos (not limited to Oregon) include bees in the genera Osmia, Augochlorella, Lasioglossum, Halictus, and Eucera; wasps; flies (Eristalis and Volucella, Syrphidae); various Lepidoptera; thrips (Thysanoptera); and Anna’s hummingbirds (Calypte anna, speculated as feeding on thrips as well as some nectar). (See: Richardson et al. 2012, Eliyahu et al. 2015.)

A bumblebee (Bombus), collecting nectar legitimately.
Bumblebee queens are the largest and earliest Bombus to visit flowers. Though they are in the same family as honeybees (Apidae), bumblebees form annual colonies in which new queens endure Winter while the rest perish. Bombus tend to be large robust bees with long probosces and are able to fly in cool and cloudy weather, conditions which are unsuitable for honeybees. Their long tongues enable them to reach the nectar of most flowers quite easily. However, they have strong mandibles and are known to be nectar robbers in many plant species, namely by biting holes in the sides of flowers. While I did not witness nectar thievery by any bumblebees on the Arctostaphylos, there are some pollinators which will gather nectar legitimately (without defiling the flower) while other individuals of the same species exhibit thievery/robbing, so there is still a possibility that bumblebees are the hole-biters. However, the flower walls of A. viscida are weak and it is possible that small bees (Andrena, perhaps) are in fact the ones biting holes in the flowers. Close observation is needed to determine the truth.

Aside from suspected thievery, Bombus and anthophorine bees are known to vibrate their flight muscles to A.) warm their bodies in the absence of sunlight or warmth and B.) dislodge pollen from the anthers of the flowers they visit (buzz pollination), which is by far more efficient than brushing on the anthers with their bodies as most bees do. Add to this the minute electric charge possessed by both flowers (grounded, negative charge) and bees (positive charge, a trait shared with all insects). Pollen jumps from the anthers to the bee once it is vibrated off the anthers. Bees which pollinate utilizing buzz pollination, I deduce, are probably the most significant pollinators for Arctostaphylos, perhaps for all species.

Andrena sp., robbing nectar.
It's pretty clear from this image that this bee is robbing nectar from a hole in the flower. Most likely, the hole was already there and the bee was taking advantage of it, but further observation is needed to confirm that claim. Andrena are small bees, and though the flowers of Arctostaphylos aren't particularly protected, it is still unlikely that these small bees are the hole-biters. I would be interested in your observations if you are in a position to observe at all.

Andrena sp., robbing nectar.
Another consideration, the smaller bee genera (Andrena, Nomada, Apis, etc.), as compared to Bombus and anthophorine bees, appear morphologically unable to access the nectar legitimately due to the very constricted opening of the flowers of Arctostaphylos viscida. This, then, results in the small bees to seek out the holes to access the nectar rather than attempt to find flowers with wider openings. This may seem detrimental to the plant, but it could be that the loss of nectar due to robbing in many of the flowers actually incites legitimate nectar collectors to move between flowers more often, which potentially results in more pollen being transferred between more flowers.

A dance fly (Empididae) and Andrena.
Dance flies (or dagger flies, Empididae) are very frequent visitors of Arctostaphylos viscida, and move between flower clusters quite frequently. Their long proboscis is used both to eat prey (typically smaller flies) as well as for foraging on flowers. They do probably contribute to pollination since they visit so many of the flowers, though they are hardly likely to be as efficient as bees in this regard as little pollen is likely to adhere to the proboscis rather than the hairy bodies of bees.

A bee fly (Bombylius major, Bombyliidae) feeding on nectar
Bee flies (Bombylius, Bombyliidae) are frequent visitors of the flowers and move between flowers quite frequently. They hover over (or in this case under) flowers to feed on nectar. I'm not convinced they contribute to pollination very efficiently despite their frequency, though they may contribute to the efficiency of bees by drinking nectar and encouraging bees to visit more flowers in search of nectar. Bombylius major is a widespread species. While adults visit flowers for nectar, the larvae are parasitoids of ground nesting bees (and maybe wasps). Eggs are dispersed near the entrance of a ground-nesting bees nest (Andrena, for example), and the larvae enter the nest and feed on the bee larvae, and perhaps the stored pollen. Even though they harmful to ground nesting bees, their presence can signify a healthy host population.

Winged ant ♂
Winged ants, superficially similar to tiny wasps (like the tiny Chalcid wasps), are common visitors to flowers. They are distinguished from wasps by their elbowed antennae (I apologize for the photo), wasp antennae do not have 90° bends. They are defferentiated from termites by the constricted waste, similar to a wasp. It is unlikely they are of any consequence as pollinators, though I doubt their presence is harmful in any way. Flying ants are the reproductive members of an ant colony, sent off to mate and go on a doomed quest to start a new colony, although their chances for success are slim since they will probably die of starvation, dehydration, or as food for birds or other insects. In this regard their presence is good, there is now a food source for another insect or bird (hummingbirds, perhaps) which in turn is beneficial for everyone.

Ants are fairly common on the flowers of Arctostaphylos, but due to their small size in this case they are unlikely to contribute to pollination. As with other plant species (such as Iris chrysophylla) ants seem to coalesce on select inflorescences or clusters of flowers, while they may be absent from others. They are usually nectar thieves, though some have been found to contribute to pollination in some plants (and are the primary pollinators of a handful of orchids). The presence of antimicrobial secretions of varying potency in most ant species argues that pollen viability may be compromised. Consider, also, that they are (mostly) wingless and do not travel much between flowers, and are mostly hairless so pollen does not adhere well to their bodies. In some cases they may even fight off other insects that attempt to visit the flowers they are foraging upon, or most likely their presence deters pollinators from attempting to forage there at all, something I've observed directly.

Studying my photo, it seems viable that if the flowers did not have holes in the sides the ants may just contact the anthers when accessing the nectar, and upon entering another flower they may just contact the stigma since the entrance is so constricted. If I were to test this I would use some type of mesh that would have holes large enough for the ants to pass through, yet small enough to exclude larger pollinators. The mesh would be used to bag multiple flower clusters (the more the better) with flowers that do not have holes in the sides. I would assign numbers to the clusters, and observe them for the presence of ants, and proceed to observe which (if any) produced fruit. Testing viability of any seed produced would also be important. I would be happy to hear of anyone else's observations of ants on the flowers of any species of Arctostaphylos, I will add them here.

A predatory wasp


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  1. The anthers of Vaccinium and Arctostaphylus are poricidal. The flowers require buzz pollination. Honey bees cannot perform buzz pollination.


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