WMOD00409

• Contract/Permit #: 1-07-81-V0175
• Title: State of Montana Activities in the High Plains Cooperative Program: 1981-1983
• Date: 6/30/1983
• State: MT
• Weather Modification - Doc Type: Report

Watering took place every other day ~lith tap water to keep the soil near
field capacity. Sunlight was not supplemented by artificial light. Plants
were observed frequently and notes were made of their foliage color and th'2
attributes of conditions.

Greenbug infestations developed from a residual greenhouse population; no
attempt was made to poison them anywhere in the greenhouse during the
experiment. Greenbug counts were made on 7 March, 10 March, 12 April, and 21
April 1982. When counting together, each person counted all treatments of
particular replicates. Additional observations were then made of plant
condition and the proportions of young and winged greenbugs.

Plant osmotic potential was measured by pressure-bomb techniques
(Scholander et al., 1965) on one culm from each treatment of replicates 2 and 3
immediately after the 10 March greenbug count. Both of these replicates were
selected randomly.

Results and Discussion

Greenbug densities by treatment, replicate and date are listed in Table 1.
During the 7 and 10 March counts, greenbug density was negatively related to
plant density with slopes (r2), respectively, of -3.09 (0.83) and -3.69
(0.88). These significant relationships were not repeated a month later when,
apparently because of plant maturation, the greenbugs were nearly absent from
all treatments (Table 1).

The insects could have been responding to several different plant-density
controlled factors: lighting, temperature, plant osmotic pressure, and plant
nutrient levels (Windle and Franz, 1979). We reject the possibility of the
first two because variation in either factor would have been negligible
considering that the plants were short (all less than Idm) and the prevailing
greenhouse conditions were always moderate with air movement over the bench
(Wyatt and Brown, 1977).

Results of our investigation into wheat osmotic pressure-aphid density
levels are given in Table 2. Both replicates 2 and 3 showed significant linear
relationships of aphid density to osmotic pressure. As has been shown by
others (e.g., Dixon, 1975) these aphids were more abundant on plants of higher
water status. Our counts, though, showed no differences in proportion of
young, nonwinged/wingless forms due to treatment. The increased aphid density
with higher plant water status (lower density) is probably due to a combination
of factors, including water movement, photosynthetic rate, and solute
concentration, which are enhanced by an increased osmotic pressure. This
results in more food to the greenbugs. Because we observed greater levels of
chlorosis on more dense wheat plantings, indicative of nutrient deficiencies,
we are unable to exclude the possibility that the greenbugs were most dense on
the least dense wheat plantings due to a greater nutrient concentration in
those plants (Wearing and van Emden, 1967). The greenbugs may have caused some
of the wheat leaf yellowing (Osborne, 1972).
Regardless of the particular influence on the aphids, plant osmotic
pressure and nutrient concentration are positively correlated (Hsiao, 1973),
except when plants have been moderately water stressed and then rewet (Van

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