Chemistry Reference
In-Depth Information
longer available in the foliage when the CPB were placed on the leaves in the petri dish, as
the bioassay was conducted from 37 to 70 days after the in-furrow treatment applications. It
is also possible that CPB mortality within all treatments declined over time because the
laboratory test larvae were field-harvested from an untreated potato nursery, and only large
larvae (3
rd
and 4
th
instars) were available in late June whereas only small larvae (1
st
and 2
nd
instars) were available in late May and early June, and only adults were available in July.
Large CPB larvae and adults are generally more difficult to intoxicate than small larvae
[24,25,26]. Further, mortality of CPB on leaves from plants treated with thiamethoxam was
significantly (P=0.05) higher than that of all other treatments, including imidacloprid, under
the low rainfall regimen on 28 Jun and 5 Jul, the last two test dates, suggesting that
thiamethoxam remains more active in the plant over a longer period of time. Plants from
soybean seeds that had been treated with with thiamethoxam resulted in significantly fewer
aphid numbers than did imidacloprid-treated plants after 3 weeks, likely due to faster
imidacloprid metabolism in the plant, which would result in reduced insecticide activity
over a shorter time [27]. However, the difference between imidacloprid and thiamethoxam
observed in the laboratory trials was not observed in the field trial, possibly because the
final CPB larval field count was conducted on 25 June. Also, the effect of high amounts of
simulated rainfall may be more pronounced in the greenhouse pots than under actual field
production.
Similarly, the percentage leaf tissue consumed by larvae was significantly (P<0.05) higher
with dinotefuran and the untreated than with imidacloprid or thiamethoxam on all dates
recorded. However, dinotefuran resulted in significantly less leaf tissue consumption than
was observed with plants that received no insecticide in late May and early June (Fig. 2)
only under the low rainfall regimen; there were no significant differences between
dinotefuran and plants that received no insecticide under the high rainfall regimen, again
indicating that rainfall level impacted the effectivess of dinotefuran more so than that of
imidacloprid or thiamethoxam.
Data from the mortality and leaf-feeding laboratory bioassays showed significant (P<0.05)
insecticide effects in that both imidacloprid and thiamethoxam, applied at planting with the
seed piece, were significantly (P<0.05) more effective in reducing foliage consumption by
CPB larvae from planting (23 Apr) through 7 Jul under both the low and high rainfall
regimens compared with plants that were treated with dinotefuran or plants that received
no insecticide. The interaction showed that plants treated with dinotefuran, under the low
rainfall regimen, were significantly more effective in reducing leaf-feeding damage than
plants that received no insecticide only from planting through 7 Jun; dinotefuran was
ineffective in reducing foliage consumption under the high rainfall regimen for all dates
observed as compared with plants that received no insecticide. Similarly, leaves from plants
treated with either imidacloprid or thiamethoxam at planting resulted in significantly
higher CPB mortality from May through mid-July under the low rainfall regimen and
through mid-June under the high rainfall regimen as compared with CPB mortality on
plants treated with dinotefuran or on plants that received no insecticide. As observed with
the leaf-feeding bioassay, CPB mortality was significantly higher on plants treated with
