Malta Star Thistle is spreading throughout Tuolumne and Calaveras counties. Malta star thistle looks like yellow star thistle but has shorter spines. It is being reported from Columbia, Tuolumne and up into US Forest Service property. In the Italian Bar and Apple Colony Road areas, Malta star thistle is forming thick stands.
The Common Eumenid Wasp
When oak trees first leaf out, their leaves are vulnerable to attack by oak worms, the most common being the oak groundling, Telphusa sedulitella and here. These small moths emerge as adults just as the oak leaves open. These new leaves do not have protective chemicals in them yet, as the tree needs time to synthesize and accumulate sufficient toxins to defend their foliage.
The small green worms start eating the oak leaves. There is a race on now, as the caterpillars eat and grow, and the oak leaves synthesize their defensive chemicals. The time period between bud opening and defensive chemical buildup is all the time the oak worms really have to grow and pupate. Once the leaves have accumulated their toxins, the oak worms can no longer easily attack the trees.
It is in this time interval, between leaf bud opening and the leaves building up their defense, that we see the primary predator of oak worms, the eumenid wasps. Although they resemble yellow jackets, eumenids are not social insects; that is, there is no queen and no colony, and as such, they are not aggressive.
Eumenid wasps are beneficial, hunting oak worms and reducing defoliation. The wasp females search oak leaves for the caterpillars, paralyze them, and then carry them to their nests, where the paralyzed worms serve as food for the developing wasp larvae.
Here is where most people encounter eumenid wasps, as the wasps often nest in crevices under the eaves of houses. The male wasps tend to hover near nesting sites, waiting for females to return, and the opportunity to mate. This behavior can make people think there is a serious problem, because they see a lot of wasps and assume they present a risk.
But don’t worry. The wasps are policing your trees, removing defoliating insects, maintaining the health and vigor of your oaks, and are not aggressive in nest defense. In the Bay Area, oak worms actually do defoliate oaks. Thanks in part to the actions of eumenid wasps, this does not occur in the foothills.
The wasps disappear when the oakworms pupate, usually by the first or second week of June.
The best taxonomy work on these wasps is probably “A character analysis of the North American potter wasps (Hymenoptera: Vespidae; Eumeninae)” by James M. Carpenter and Jeffrey M. Cumming.
IMAGES: click on an image to enlarge it.
Here are abdomens of three individual eumenid wasps, to allow comparison of the abdominal striping patterns.
Eumenid wasp stinger details:
Details of thorax:
Details of face:
Click on an image to enlarge it.
The pine tree ant, liometopum luctuosum, is a common pest species in the mid-elevations areas, common in Groveland, Sonora and Columbia up through the Crystal Falls elevation and in Forest Meadows through the Arnold area. The species prefers to nest in large ponderosa pine trees, but will also build nests in houses.
The pine tree ant is similar to the odorous house ant in several ways and is sometimes confused with them. Both are uniformly darkish brown and both gives a noticeable odor when crushed. The body size of pine tree ants ranges from that of the odorous house ant to about twice that size, in the same colony. You need to look at a number of specimens to assess body size and you should be able to spot the size differences between individuals in a single colony. OHA have little noticeable size variation like pine tree ants do. And unlike the OHA, pine tree ants nest in ceilings, kick out piles of sawdust and can form very long networks of trails. Trails sometimes follow along telephone lines, leading ultimately to a branch of a remote pine tree.
Houses serve as nesting sites
The species becomes a noticeable pest when it sets up satellite colonies in houses, commonly in ceilings that use ceiling tiles like Celotex. The materials are similar from the ant’s point of view, I presume, to the rotting heartwood of a mature pine tree. The ants can easily excavate tunnels and chambers and store developing larvae there. The excavated material is often discarded by the ants and forms piles of sawdust under the areas of attack.
These ants appear to overwinter mainly in very large pine tress. In spring the ants start trailing between large pines and the houses they have selected for nest sites. I have tracked these ants and at peak trailing, it appears that there is no end to the inter-connectiveness of the trails. I once took some worker ants from a trail in Pine Mountain Lake to Columbia College and placed the ants in a trail there. The relocated workers instantly joined the trailing ants. Could it be that all pine tree ants form a sort of super-colony?
Also, following the trailing ants, every once in a great while you might get lucky to see nest commensals (other species of insects that live as nest parasites in the colonies) also walking in the trail, just like the workers.
One feature of pine tree ant infestations is the accumulation of piles of sawdust beneath nesting areas. These photos, by Jon Shattuck (28 August 2017, Sonora area) are characteristic. These piles can often be found inside homes, where the sawdust falls onto tables, furniture or floors. In this case, the debris fell outside, on the deck. See also Pine Tree ants (2)