Pollinator Post 7/29/26 (2)
I spend the rest of the morning exploring the native garden at Crab Cove. An American Sand Wasp, Bembix americana (family Crabronidae) is taking nectar on an inflorescence of Red-flowered Buckwheat, Eriogonum grande var. rubescens.
Sand wasps in the genus Bembix are familiar and common throughout North America, digging their burrows in dunes, on beaches, and other habitats with loose, deep sand. The female rapidly kicks out large quantities of sand using a “tarsal rake” of spines on each front leg. The burrow is excavated before the wasp goes hunting. Bembix are generalist, opportunistic hunters. True flies in the order Diptera are the usual prey. A victim is paralyzed or killed by the wasp’s sting, and is then flown back to the nest. Most species will lay an egg on the first victim, while some species lay an egg in the empty cell before starting to hunt. Once the egg hatches, mama wasp brings flies to her larva as needed. This “progressive provisioning” is rare in the insect world. When the larva reaches maturity, mama wasp closes the cell. Inside, the larva spins an oblong cocoon, weaving sand grains into the structure and resulting in a hardened capsule. Overwintering takes place as a prepupa inside this cocoon, but there are usually two generations a year.
Male sand wasps often engage in an elaborate flight ritual called “sun dances”. Males emerge before females, and fly erratically at dizzying speed one or two inches above the ground attempting to detect virgin females about to emerge from their underground nests. Both sexes are often seen taking nectar at flowers, especially from Asteraceae family. Bembix wasps are often victims of other insects such as cuckoo wasps (Chrysididae), velvet ants (Mutillidae), satellite flies (Sarcophagidae), bee flies (Bombyliidae), and thick-headed flies (Conopidae).
On a rose-tinted inflorescence of Red-flowered Buckwheat, an Inchworm caterpillar of Pug Moth, Eupithecia sp. (family Geometridae) has taken on similar delicate hues. Remarkable camouflage!
Inchworms are also called loopers and measuring worms. The majority of the inchworms are the larvae of moths in the family Geometridae. The name comes from the Greek “geo” for earth and “metro” from measure, because the caterpillars seem to be measuring the surface on which they are walking. The “looping” locomotion is mandated by the absence of prolegs in the middle section of the caterpillar’s body.
Eupithecia is the largest genus of moths of the family Geometridae. Occurring worldwide except for Australasia, species in the genus are commonly known as pugs. The caterpillars move in a looping manner and are commonly called inchworms. Adults are typically small, 12 – 35 mm, with muted colors. Most species rest with forewings held flat at right angles to the body, while the hindwing are largely covered by the forewings. They are generally nocturnal. Larvae mostly feed from the flowers and seeds of their food plants rather than the foliage. Many species have a very specific food plant. Many are commonly found on Asteraceae flowers.
Many species of Pug Moth caterpillars are specialized flower feeders and will actually absorb the pigments from the flowers and pollen they eat. Because of this dietary habit, their bodies take on the exact hues of their host plants to camouflage themselves from predators.
Uh oh, is that a caterpillar of the Genista Broom Moth on the leaves of a Coastal Bush Lupine, Lupinus arboreus? I saw a few of these caterpillars here in late March, but have not seen them again since.
As I look closely at the Coastal Bush Lupines, I find signs of a major outbreak of the Genista Broom Moth.
The Genista Broom Moth, Uresiphita reversals is a moth in the family Crambidae. The species is probably native to Mexico before spreading north into the U.S. and the Caribbean, establishing in the San Francisco Bay Area by 1980. The adult moths have a wingspan of 27-34 mm. They are on wing year round in multiple generations per year. The larvae feed on Acacia, Lonicera, Baptisia, Genista, Lupinus species, as well as “other pea family shrubs”. Genista is an Old World genus for “broom” in the pea family Fabaceae.
The bright colors of older Genista Broom Moth caterpillars are aposematic, or warning, coloration that signals to predators that they are toxic and unpalatable. These caterpillars accumulate toxic alkaloids from their host plants, such as lupines and brooms, and their vibrant yellow and black markings serve as a clear warning to deter birds, lizards, and other potential predators from attacking them.
While these caterpillars might be considered pests in gardens, they are useful biological pest control for a number of invasive plants in the wild that belong to the pea family, notably the brooms. They are helping to check the invasion of French and Scotch brooms that cover extensive hillsides and displace the native chaparral vegetation of California.
A large Genista Broom Moth caterpillar is attaching silk from its silk gland in the mouth to the terminal leaves of a Coastal Bush Lupine, while on an adjacent branch tip, much smaller caterpillars are already feeding within their communal web.
Several Genista Broom Moth caterpillars are feeding on lupine leaves on a branch tip within a communal silk web that they have constructed.
Many tiny Genista caterpillars have set up shop on the terminal leaves of a lupine, having woven a communal web in which to feed in relative safety from predators and parasites. The web is speckled with frass (insect poop) produced in active feeding.
A couple of the lupine pea pods have been chewed open. A tiny caterpillar is peeking out from one.
Apparently the Genista caterpillars also feed on the pea pods, in addition to the foliage of Lupine.
Genista caterpillars of various sizes are laying waste to the Lupine leaves within a common web they have constructed. Some of the damaged leaves have already turned brown.
This abandoned webbed tip has completely turned brown. Having exhausted their food source, the Genista caterpillars have apparently moved on to greener pastures, leaving behind only their wastes.
From what I read, these caterpillars can heavily defoliate branches or small shrubs during large outbreaks, turning leaves brown or black, but rarely cause permanent harm to healthy, established plants. Managing a Genista Broom caterpillar infestation involves hand-picking pests and pruning webbed tips.
Two European Paper Wasps are flying low over the Lupine, and one has landed on a webbed branch tip, trying hard to get through the silk web to the caterpillars. Last year in a home garden, I actually observed a European Paper Wasp capture a Genista caterpillar, chew it into a pulp and fly away with the minced caterpillar rolled up into a ball under it. Let’s hope there are enough of these Vespid wasps (including Yellowjackets) to provide natural pest control for the lupines in this garden.
Native to Europe, the European Paper Wasp, Polistes dominula (family Vespidae) is a social insect that produces an annual colony in a paper nest. Individual colonies are established anew each spring. The overwintering stage are mated females (queens). The overwintered queens emerge from sheltered spots in spring and search out sites to establish a new colony. Nests are constructed of paper, produced from chewed wood fibers of weathered fences, porch decks and other similar sources. Larvae are fed crushed insects, usually caterpillars. As the population increases, the original queen increasingly remains in the nest as new workers take over colony activities. A few of the wasps produced later in summer are males and increasing numbers of the females become sexually mature. Mating occurs and the mated females are the surviving overwintering stage. Males and non-reproductive females do not survive winter and the nest is abandoned by late fall. European Paper Wasps will sometimes feed on sweet materials, including honeydew produced by aphids. They may also feed on damaged ripe fruits. They also search for foods high in protein such as insects and fish. These are chewed and conditioned in preparation for larval consumption. The larvae secrete a sugary substance that is eaten by the adults. The cyclical exchange of food between adults and larvae is called trophallaxis. Because of their habit of hunting caterpillars, the wasps have become one of the most important natural controls of garden pests.
Just a few yards further, I come to a patch of California Bush Poppy that is still in bloom. The insect on that flower looks a lot like a Yellowjacket or European Paper Wasp. But its behavior tells me it’s an imposter. It is swinging its wings from side to side in a way that a true wasp never does. It is the Interrupted Hornet Fly, Spilomyia interrupta (family Syrphidae), one of my favorite hover flies! I quickly turn on the video on my camera to record the odd behavior.
In classic Batesian mimicry, many Syrphid flies mimic Hymenoptera (bees and wasps) species. The mimicry provides a measure of protection from predators for these harmless flies. Spilomyia is a remarkable mimic of Vespid species including Yellowjacket wasps. They mimic Yellowjackets both physically as well as behaviorally. The yellow-and-black outfit is only a start. The anterior portion of the forewings of Spilomyia are a darker brown than the rest of the wings, reminiscent of the Yellowjackets that fold their wings longitudinally at rest. The fly has a habit of resting on their back four legs and moving their two front black legs above their head, mimicking the way common wasps move their antennae. When threatened, the fly is able to produce a buzz in a frequency similar to some hymenopteran species. What’s more, Spilomyia is active during the same time of year as its model wasp, and frequently visits the same plants.
The Interrupted Hornet Fly, Spilomyia interrupta is an uncommon hover fly species native to western North America, particularly the California coast. Adults are often seen on flowers seeking nectar and pollen during the warmer months, June to October. Larvae are described as ’short-tailed’ and adapted for aquatic or semi-aquatic conditions in tree rot holes where they feed on decaying organic matter.
Wow, what strange eyes! The yellow eyes have black vertical stripes and black speckling. The genus name Spilomyia is from the Greek for ’spot/fleck’, and probably refers to the pigmentation of the eyes.
The specific epithet interrupta is Latin for ‘broken apart’, and probably refers to the central breaks in the first, third and fifth yellow abdominal bands of the Interrupted Hornet Fly, Spilomyia interrupta.