Arctostaphylos viscida, a variant with whitish florets
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|
|Arctostaphylos viscida bark|
|From left: A. viscida berries, crushed dried berries, and seed|
|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 mellifera, Bombus 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.|
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.|
|Andrena sp., robbing nectar.|
|A dance fly (Empididae) and Andrena.|
|A bee fly (Bombylius major, Bombyliidae) feeding on nectar|
|Winged ant ♂|
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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