Measuring insect biodiversity on invasive Eucalyptus in California.
For those of us who have spent much time in the Bay Area, trees of the Eucalyptus genus are familiar and often dear to our hearts. Many are grown ornamentally, due to their beauty and fragrance. One species, however, has successfully escaped cultivation and become a serious issue. Eucalyptus globulus is an Australian tree introduced to California in around 1865 (US Forest Service 2013) and has become naturalized along the entire coast. The tree is notorious for displacing native flora and for being a potential fire hazard due to the large amounts of volatile oils the plant contains. What is not well understood, however, are its effects on the Californian insect biota.
Eucalyptus poses a direct threat to living organisms that it comes into contact with. Eucalyptus is a heavily defended plant, and essential oils distilled from the plant have fumigant effects strong enough to warrant commercial interest (Negahban and Moharramipour 2007). This toxicity makes it difficult for non-adapted hosts to feed on the plant, and could potentially deter insects from associating with the plant at all. These aromatic chemicals could also play a part in its alleged flammability and likely play a role in the allelopathic behavior of the tree.
Toxicity aside, Eucalyptus heavily suppresses understory plants by dropping a very steady rain of dead leaves and bark into the understory which smother plants life and leaches toxins in the soil (May & Ash, 1990). This could explain why only other invaders such as English ivy are able to prosper beneath these trees. Combined with the displacement of native flora patches of Eucalyptus could essentially be relatively barren of insect life due to their lack of food for herbivores and insect-repellant oils.
As Eucalyptus was once introduced, however, a whole host of herbivores have slowly been introduced with it. Whether these introductions were purposeful or not in the last decade several new species have begun to show up in California. Most notable of these are several species of psyllid, eucalyptus longhorn borers, the lemongum gall wasp, and eucalyptus leaf beetles. Of these beetles, Trachymela sloanei occurs all throughout California while the other only in the far south of the state. These insects can significantly damage and limit eucalyptus growth (UC IPM, 2013), as well as possibly providing some food for native predators. My expectation is that these imported eucalypt-lovers benefit native biodiversity in eucalypt-impacted Californian forests.
All this being said, Eucalyptus globulus has likely taken a toll on the insect fauna of California where it is present. Surprisingly few measurements of this have been made, however, and as such I have decided to do a few quick surveys myself. A simple sampling of E. globulus could reveal insect diversity on individuals of E. globulus, which is what this survey aims to assess. It seems likely to me that insect diversity and density on E. globulus will be significantly lower than on neighboring plants.
Methods
7 E. globulus trees were chosen around Davis, California and Marin County, CA. The parts of the tree that could be reached were extensively swept with a sweep net and any insects that resulted were then aspirated. This was repeated all around the tree, sweeping at least 4–5 times per sweep. On 3 of the trees with significant bark cover, bark was peeled back and any insects underneath were recorded.
Results
Only a few groups of insects were captured with the sweep net, and some were likely incidental.
Tree Families Taken
1: Chironomidae
2: Chironomidae
3: None
4: Chironomidae, Cicadellidae (1)
5: Chironomidae
6: Chironomidae, Psyllidae (1)
7: Chironomidae, Chalcididae (1), Chrysomelidae[Trachymela] (1)
As can be seen here, diversity on the eucalyptus was extremely limited. Of the bark peeled back, none showed many signs of insect life.
The general state of the tree was also observed. Heavy damage was observed on most of them, appearing as large chunks taken out of the leaf. The damage tended to be very similar in shape from tree to tree, and on both newer and younger leaves.
Discussion
Many of the insects found on E. globulus were incidental. Chironomids were found in by far the greatest number but not of any number significantly higher than on surrounding trees. Since they were found in primarily riparian areas it stands to reason that they were simply resting in the trees since they certainly were not feeding on them. The cicadellid that was found was the only one on all the trees and also could have been incidental. Chalcididae are primarily parasitoids of lepidoptera and diptera, with limited exceptions. This also indicates that the members found on the eucalyptus were incidental as well (Universal Chalcidoidea Database, 2013).
This leaves only two groups of insects as potentially using the Eucalyptus as a host. The psyllid could be incidental as well, but as stated above could also be a Eucalyptus feeder. The beetle T. sloanei, however, is the clearest example of an introduced eucalyptus feeder.
T. sloanei feeds on eucalyptus foliage as both an adult and a larvae, with larvae favoring younger foliage and adults feeding on older. It feeds primarily at night, hiding beneath bark and in the foliage of the plant during the day (UC IPM, 2013). The patterns of feeding that this beetle make are quite similar to the damage found on the eucalypts in this experiment. They appear to be present throughout Davis and are likely responsible for much of the damage to eucalypts. Their relative scarcity in the samples, however, is explained best by their feeding preferences. Since they feed at night sweeps of foliage would not be likely to expose many of them.
Neighboring oaks were also sampled throughout the duration of the experiment. All the families found on the eucalyptus trees were also found on the oaks with the exception of the Chrysomelids and Psyllids as well as a great number more wasps (Including Eurytomidae, Cynipidae, Pteromalidae, and Torymidae) of an unknown number of species, and variety of assorted flies and beetles. Overall, the diversity on the oak was not extensively surveyed enough to give a good estimate. The little data that was taken paints a very bleak picture due to the massive difference in diversity between the two trees.
My brief survey seems to demonstrate that E. globulus are much lower than surrounding diversity. If this is the case, single eucalyptus trees could have an impact on the diversity of their surroundings while thicker stands could be almost devoid of insect life, with the exception of the few introduced herbivores. This seems to imply that a mixed eucalyptus and native plant forest would have decreased overall diversity, and also that a stand of eucalyptus in native forest would negatively impact insect diversity.
Potential issues with this study are definitely caused by insufficient sample size. A larger-scale study is definitely needed to truly assess impacts of eucalyptus, particularly of the bark and roots. The more trees that can be included, the better. In addition, only low hanging branches were sampled. This could lead to bias. The time of the year and the time of day also play a factor and so a long-term study seems needed to truly understand the role this plant plays in Californian ecosystems.
Conclusions
Diversity was significantly lower on the eucalyptus than in the surrounding environment. This may indicate a repellant or fumigant property of eucalyptus and present eucalyptus as a danger to native insects and ecosystems. Many aspects remain unanswered, however. Whether the pests of eucalyptus can be consumed by native predators remains unknown. In addition, the actual prevalence of the various eucalyptus pests remains somewhat unknown, depending on the variety of pest. If at least some of these questions can be answered, an assessment can be made of the danger eucalyptus poses to California insect diversity.
Works Cited
“Chalcididae.” Universal Chalcidoidea Database. The Natural History Museum. Web. 06 June 2013.
“Eucalyptus Globulus.” Eucalyptus Globulus. US Forest Service. Web. 06 June 2013.
“Eucalyptus Tortoise Beetles.” UC IPM. Web. 06 June 2013.
May FE, Ash JE (1990) An Assessment of the Allelopathic Potential of Eucalyptus. Australian Journal of Botany 38 , 245–254.
Negahban, M. and Moharramipour, S. (2007), Fumigant toxicity of Eucalyptus intertexta, Eucalyptus sargentii and Eucalyptus camaldulensis against stored-product beetles. Journal of Applied Entomology, 131: 256–261. doi: 10.1111/j.1439–0418.2007.01152.x