Aphids are a widespread family of plant pests, whose abilities to suppress shoot
and root growth are well documented for many terrestrial plants. Only a few studies have
been conducted on conifer aphids of the genus Cinara. Cinara pseudotsugae are known
to attack Douglas-fir seedlings, an important crop in the Pacific Northwest. Douglas-fir
are most susceptible to aphid damage as seedlings, especially in nurseries where
conditions favor aphid outbreaks. A parasitoid wasp (Pauesia sp.) attacks C.
pseudotsugae, and may be useful as a biological control agent. Studies of its natural
history and host interactions are needed to assess its potential as a bio-control agent.
This study examined the effects of an experimental range of aphid densities on the
growth of total shoot and root volume and biomass and shoot morphology of Douglas-fir
seedlings. Eighteen-week tests explored short term effects of different aphid feeding
intensities in both the greenhouse and field. Long term effects were tested by exposing
greenhouse seedlings to 16 months of aphid feeding. The ability of the plants to recover
was tested by allowing one set of seedlings to grow aphid free for one year, after being
exposed to aphid feeding for 18 weeks. The success rate of parasitoids over an 18 week
period was compared to aphid density in both greenhouse and field tests.
Increasing aphid destiny was significantly related to decreasing root and shoot dry weights in greenhouse tests. Growth suppression increased slightly during the second year of testing, regardless of whether or not aphid feeding continued. The results for root and shoot volumes were highly variable. However, root tissue density was significantly reduced after the second year of testing. Few shoot characteristics showed consistently significant aphid effects among the trials. Stem diameter and height decreased and needle density of new buds increased significantly with aphid feeding in most tests. Root and shoot growth of field plants did not show any significant aphid effects.
Percentage of parasitoid success was independent of aphid density except at the lowest aphid densities. There was a block effect on parasitoid success in the field test, that may have been a result of varying environmental conditions.
These results indicate that even short term aphid feeding can have long lasting effects on plant growth and structure. The effect on shoot and root growth was small, but there were no signs of recovery. The long term effects of the reduced root tissue density on Douglas-fir is unknown. / Graduation date: 1998
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/33920 |
| Date | 03 July 1997 |
| Creators | Smith, Julia P. |
| Contributors | Schowalter, Timothy D. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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