22 de julio de 2020

Snail Tale

https://www.inaturalist.org/observations/14020490

Snails estivate. If you want to watch them do it, late summer is a good time. Just head over to either of the two entrances to Mission Trails Regional Park located on Calle de Vida in Tierrasanta. At this time of year, the dry vegetation teems with the inactivity of estivating snails. Estivation, sometimes called summer sleep, is an adaptation that allows the snail to survive the hot dry summers of a Mediterranean climate until winter rains stimulate it to resume feeding, sliming and reproducing.


White garden snails (Theba pisana) invaded the park a couple of decades ago. Unlike most other snails, these Sicilian natives prefer to estivate high up. In MTRP they climb stalks of similarly non-native mustard, grasses, and thistles, but they also cling to Yerba Santa, Laurel Sumac, Chamise, California Sagebrush and other natives. On one visit to MTRP, I tired of counting snails, but estimated that there were over 500 on one sagebrush plant, and all around me the scene was repeated. Elsewhere, others have reported over 3,000 snails on one tree. The white garden snail is an agricultural pest, known to damage vegetable and fruit crops (especially citrus) and ornamental plants by feeding on foliage and leaving a thick slime trail that can inhibit pollination and lower the quality of harvested produce. At this time of year, even the native plants look stressed, so it is hard to determine if the snails are damaging them.

You can enliven your observations of Theba pisana by bringing along a water spray bottle. A few spritzes, and like magic, the snails come to life, albeit at a snail’s pace. I read that it would take 15 minutes or so. My snails were slower than that—30 minutes after I spritzed them, most were moving around and began to feed on the lettuce I provided. (I placed a stalk with 20 or so clinging snails in a terrarium for this experiment.) True to form, after feeding, the snails migrated to the lid of the terrarium, the highest point in their now-limited world of plastic.

I sprayed the clinging snails frequently to deter them from secreting mucus and affixing themselves to the terrarium. The mucus dries to a hard wall called an epiphragm that protects the snail from desiccation during dry periods. (An “epiphragm” or covering wall can be compared to a “diaphragm” or dividing wall such as the muscle that divides our chest and abdominal cavities.)

A white garden snail can be distinguished from the common brown garden snail (Cornu aspersum) by its whitish color, smaller size of adult snails, and the lack of a flange or ridge along the shell opening. As noted above, the white garden snail is likely to climb up structures to estivate, while the brown garden snail seeks out sheltered areas such as under logs or rocks. Milk snails (Otala lactea) are white with brown bands, are larger, do not frequent arid environments, and have a flange at the shell opening. Milk snails were introduced from Europe and are edible.

Like all snails, white garden snails are hermaphrodites. Each snail lays about 120 eggs at a time in the ground after winter rains begin. Hatching occurs after 20 days and the snails live an average of 2 to 3 years.

San Diego County has earned the distinction of being the only place in the United States where the white garden snail is established. Los Angeles County has reported the snail, but claims to have eradicated it. San Diego officials have also tried to eradicate the snail. In the early part of the 20th century, when the snail was first discovered in La Jolla, teams went out with hoes and axes to clear entire canyons of vegetation, followed by a crew that sprayed a petroleum mist that they then ignited. This scorched earth strategy was apparently successful in eliminating the snails from La Jolla for a time. In 2003, the Cedar Fire burned the vegetation along Calle de Vida, temporarily eradicating the snail from MTRP. But they are back and appear to be thriving. Take a walk along Calle de Vida. You can’t miss them. They are everywhere you look, estivating right in front of your eyes.

Ingresado el 22 de julio de 2020 por milliebasden milliebasden | 2 comentarios | Deja un comentario

25 de marzo de 2020

Hedypnois (Crete Weed)

Recently I've noticed observations in San Diego County of Cretanweed (Hedypnois rhagadioloides). The plants all looked a lot like Crete Weed (Hedypnois cretica) which I have observed many times. Currently there are 232 observations from San Diego of Hedypnois cretica and 45 observations of Hedypnois rhagadioloides. I checked Plants of the World Online and learned that Hedypnois rhagadioloides is an accepted species and that Hedypnois cretica is a synonym for Hedypnois rhagadioloides subsp. rhagadioloides. (This is the only accepted subspecies on POWO, but on iNat there are a few observations in Australia and Greece of a different subspecies: Hedypnois rhagadioloides subsp. tubaeformis. ) The Jepson eFlora lists Hedypnois rhagadioloides and it says Hedypnois cretica is a synonym for it (with no mention of subspecies) but they call it Crete Weed, not Cretanweed. I like the name Crete Weed better than Cretanweed--even though the spelling is different, the resemblance to "cretin" makes it sound like a dumb plant.

Ingresado el 25 de marzo de 2020 por milliebasden milliebasden | 0 comentarios | Deja un comentario

04 de noviembre de 2017

The Hunt for Trirhabda

On a late spring walk in Mission Trails Regional Park, I had my first encounter with a skeletonizing leaf beetle. While examining the leaves of a goldenbush plant (Isocoma menzieii), I found several dull greenish-yellow beetles that were cruising along the stems and leaves. I tend to notice goldenbush most often in the fall when it graces us with spots of color in the brown landscape. This flowerless plant was almost unrecognizable to me with its gray hairy leaves. An unexpected surprise, the beetles were a decorative surrogate for the flowers that were lacking at this time of year. As I continued along the trail, I found the same kind of beetles on other goldenbush plants.

Although the name “skeletonizing leaf beetle” may send shivers up the spine of any plant lover, it is apt since the larvae consume leaf blades, leaving behind the skeleton-like veins and midribs. The genus Trirhabda has 26 species in the U.S. and Canada, 16 of which have been reported in California. The Latin name probably refers to the three spots on the pronotum (tri=three, rhabdus=rod). Trirhabda are “stenophagous,” meaning they eat a limited variety of plants. Their preferred plants are in the Asteraceae and Hydrophyllaceae families. Different species of Trirhabda prefer specific plant species and have adapted to tolerate secondary compounds found in their host plants. Since each species may feed or rest on plants other than the host plant, you cannot identify the species of Trirhabda solely by the plant it is on. This is unfortunate, since the species look similar. I decided not to let that deter me. After reading about Trirhabda species and their preferred plants, including several species of plants which are common in MTRP, I decided to go on the hunt for Trirhabda.

Proving the idiom “seek and ye shall find” took only a few visits to the trails of MTRP. In addition to Isocoma menziesii, adult Trirhabda beetles were feasting on coastal sagebrush (Artemisia californica), California encelia (Encelia californica), and yerba santa (Eriodictyon crassifolium). How many different species of Trirhabda did I find? I'm not sure, although some morphological differences were apparent – the overall size, shape of spots on the pronotum, and color and hairiness of the elytra—suggesting 4 different species.

In spite of my efforts, I did not find Trirhabda beetles on coyote brush (Baccharis pilularis) – not too surprising since there is only one historical record of the species that prefers this plant in San Diego County. And now, the season has passed and there are no Trirhabda beetles to be found in MTRP. Adult Trirhabda beetles may spend their entire 90-day lifespan on one plant. Females lay multiple clutches of eggs at the base of the host plant, where the eggs spend the winter. Come February, the eggs will begin to hatch and the larvae will ascend the stem of the host plant and munch away for several months before returning to the soil at the base of the plant to pupate. After that, my window of opportunity: I will resume my hunt for Trirhabda on coyote brush in MTRP.

Sources:
http://www.sbnature.org/collections/invert/entom/cbphomepage.php
https://www.zin.ru/animalia/coleoptera/pdf/clark_ledoux_et_al_2004.pdf
http://bugguide.net/node/view/35037/tree

Ingresado el 04 de noviembre de 2017 por milliebasden milliebasden | 9 observaciones | 2 comentarios | Deja un comentario

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