The beginning of the school year brings a new crop of high school students who will be working with me at the Civic Park pollinator garden in Walnut Creek. I usually get a lot of questions about the garden throughout the school year, but to my delight the first week was full of enthusiasm and queries while finding tiny discoveries. I say tiny because the specimens I am writing about here are no larger than 1/2-inch.
While deadheading Black Sage (Salvia mellifera), the students and I discovered tiny clumps of mud along the stem. These “mud pots” are nests created by the potter wasp (Euodynerus species)—aptly named since they resemble clay pots.
When people think of wasps they usually think of the aggressive yellow jacket. Yellow jackets are considered social wasps because they live in groups or colonies. They build papery nests in the ground and during the summer the colony can grow to as many as 50,000. With the exception of the queen, the working brood lives for just one season. As summer wanes and food sources become scarce, they become more aggressive and interfere with our outdoor activities.
But there are many species of solitary wasps — those who build individual nests and lay one egg per nest. The potter wasp is a solitary that often goes unnoticed because of its small size. The black- and ivory-colored adults are not aggressive toward people. They are considered beneficial to the garden ecosystem because they pollinate plants as they feed on flower nectar. They also kill pests such as aphids, cabbage worms and tomato hornworms.
The female creates her nests in the spring and produces several generations of wasps each season. These pots, or brood cells, are packed with parasitized caterpillars that provide an immediate food source for her young as they emerge and develop. After packing the cells, she deposits a single egg and seals the nest.
The red drops on the leaf vein of the Valley oak (Quercus lobata) are called galls. When the female Cynipid wasp (Andricus kingi) deposits her egg on the leaf, she also injects a chemical. The tree’s response is to create the gall, which keeps the wasp safe and with food until it reaches its adulthood and emerges from the gall.
According to Ron Russo, an expert on California galls, “There are over 150 species of galls associated with oak trees. Scientists suspect that these tiny wasps are able to manipulate genes in specific plant tissues, inducing the plant to create galls characteristic of their species. But the exact mechanism for this process remains a mystery”.
As for eradicating galls, there is no need since they do not harm the tree and the wasps are an important food source for birds who rely on insects for 97 percent of their diet.
The little bump at the base of the Cottonwood (Populus fremontii) leaf is formed by a small green aphid. Like the oak gall, the little bump is the tree’s reaction to a chemical released by the aphid. It does not harm the tree, however if you want to control aphids, an application of horticultural oil can be applied on the tree when it is dormant. The oil kills the overwintering aphids in the bark fissures on the tree.
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