 |
 |
 |
 |
 |
 |
 |
|
|
Zoo-Phytophagous Heteroptera
(Rob Wiedenmann, Christine Armer, Mike Irwin)
Orius insidiosus (Heteroptera: Anthocoridae) is an important
generalist predator in soybeans that also feeds on plants as a diet supplement
or when prey is unavailable. Because O. insidiosus feeds on plants,
changes in plant tissues, such as those due to plant stresses, may directly
affect the insect. Soybean mosaic virus (SMV) occurs wherever soybeans
are grown. Like many plant stresses, SMV can elevate amino acid levels
of plant tissues. Increased amino acid levels could benefit O. insidiosus
by making available scarce resources, thus increasing predator fecundity
and longevity and maintaining populations when prey is not present. Then
when prey are available again, the predator can rapidly suppress the pest
population.
This project focused on the interaction of O. insidiosus with
SMV. The objectives were:
1) determine the precise plant tissues where O. insidiosus feeds;
2) monitor fecundity and longevity of female O. insidiosus on
SMV-infected plants and uninfected plants with and without prey; and
3) compare amino acid content in those tissues for SMV-infected plants
and uninfected plants;
Objective 1.:
Specific plant tissues where feeding occurs were determined to assay the
amino acid uptake and verify effects on the predator. Experiments were
conducted to test whether Orius insidiosus ingested phloem, xylem
or mesophyll contents from soybean plants. Potential uptake of phloem
sap was examined by radiolabeling photosynthate with 14CO2 and then measuring
the accumulation of radiolabeled metabolites in feeding animals. Most
insects feeding on radiolabeled plants ingested no or very low levels
of label; only 3% ingested small amounts of label, indicating the experimental
insects fed very little, if at all, on the phloem. In contrast, well known
phloem feeding insects used as positive controls accumulated substantial
levels of labeled metabolites after feeding on known host plants. O.
insidiosus fed on xylem contents, as shown by ingestion of safranin-labeled
xylem fluid. A few of the insects showed signs of feeding on the mesophyll,
as indicated by the presence of chloroplasts in the gut. However, the
small diameter of the food canal may have limited passage of chloroplasts,
which would contribute to an underestimation of the frequency of mesophyll
feeding. Some radiolabeled metabolites remained in the mesophyll so those
insects that ingested low levels of radiolabel probably ingested label
from the mesophyll, which supports the notion that some level of mesophyll
feeding occurred. Feeding site determines the nutrients ingested during
phytophagy. These insects obtain water from the xylem, and may have ingested
small amounts of starches, sugars, and amino acids from the mesophyll.
The results suggested that facultative phytophagy by this heteropteran
predator primarily provides the predator with water, but also may provide
some nutrients that supplement a prey diet and help the predator survive
periods when prey are scarce.
Objective 2.:
Orius insidiosus feeds on thrips, aphids, and other small arthropods.
This predator also feeds on plant tissues, and can be affected by the
plant's physiological status, which is altered by age of the leaves or
diseases such as soybean mosaic virus. This study examined the effects
of soybean mosaic virus on longevity and fecundity of O. insidiosus,
due to feeding on soybean leaves of different ages infected with a mild
or a severe strain of the virus. Females provided with prey on uninfected
plants and on plants infected with a mild virus strain lived longer than
females on plants infected with a severe strain. When caged on leaves
without prey, females lived longer on older and younger leaves than on
middle-aged leaves. Fecundity and percentage of egg hatch did not differ
significantly between leaf ages and virus strains. Leaf hair density appeared
to have the greatest effect on longevity and fecundity; when the leaf
hairs were sparse enough for O. insidiosus to reach the leaf's
surface readily, other chemical factors, such as amino acid and starch
levels, may have influenced the longevity and fecundity of O. insidiosus.
This study shows that some plant stresses might negatively impact yield
both through direct reduction in plant health, and indirectly when the
plant stresses reduce the predator population. Because many heteropteran
generalist predators are also facultatively phytophagous, we need to consider
the dual impact of plant stresses on crop yields and on these important
biological control agents.
Objective 3.:
Soybean mosaic virus (SMV) is one of the most important viruses infecting
soybean plants world-wide. Seven strains of the virus cause a variety
of symptoms. The strains not only affect the appearance of the plant,
but also can affect the chemical makeup of the plant. Changes in leaf
chemistry can differentially affect insects feeding on the plants, making
plant viruses important not only for direct yield effects, but also for
their effects on plant damage due to insect feeding. This study compared
a mild (G1) and a severe (G5) isolate of SMV to determine effects on leaf
amino acid, starch, and water content, and on plant hair density of soybean
plants. Infection with a severe isolate of SMV tended to elevate overall
amino acid levels, and amino acids were significantly higher in young
leaves than in older leaves. Many individual amino acids were higher in
plants infected with a severe isolate of strain G5 than in plants infected
with a mild isolate of G1 or in uninfected control plants. Older leaves
had significantly more starch, and leaves of control plants and plants
infected with strain G5 had more starch than did leaves of plants infected
with strain G1. An interaction between leaf age and SMV infection indicated
older leaves infected with strain G5 had less water than did older control
and G1-infected leaves, but that younger G5-infected leaves had more water
than young control and G1-infected leaves. Plants infected with strain
G5 tended to have denser leaf hairs than did uninfected plants or those
infected with strain G1. The youngest leaves had significantly denser
leaf hairs than did older leaves. Distinct viral strains differentially
affect the chemistry and leaf hairs of leaves of different ages, and interactions
do exist between infecting strain and leaf age. Thus, a given virus cannot
be assumed to cause only one set of responses in a host plant; leaf age
and viral strain must also be considered.
Publications:
Armer, C. A., R. N. Wiedenmann & M. E. Irwin. 1999. Effects of soybean mosaic virus on the facultatively phytophagous predator Orius insidiosus (Heteroptera: Anthocoridae). Environ. Entomol. 28: 1036-1043.
Armer, C. A., R. N. Wiedenmann & D. R. Bush. 1998. Plant feeding site selection on soybean by the facultatively phytophagous predator Orius insidiosus. Entomologia Experimentalis et Applicata 86: 109-118.