Agriculture Reference
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D
ISCUSSION
There was no significant reduction in pod number or total seed weight as a result of
D.
texanus
infestation in either year; rather, infested plants tended to have slightly larger stem
diameters and higher seed weights than uninfested plants in 2005, the year in which plants
were significantly smaller due to higher plant density. This need not imply a positive effect of
larval boring on plant productivity, but could reflect a tendency for females to oviposit
preferentially in larger plants, especially when average plant size is small, as inferred by
others (Richardson 1975; Andrews and Williams 1988). Similarly, Michaud et al. (2007b)
observed a difference of about 10% in
D. texanus
infestation rate between irrigated and
unirrigated portions of the same soybean field that they attributed to a larger average plant
size in the irrigated portion.
These data suggest that
D. texanus
larval boring may have no impact on soybean yield, at
least under the growing conditions encountered in the present study. The same was concluded
by Michaud et al. (2007a) with respect to the impact of larval boring by
D. texanus
on
sunflower yields. Larval boring is confined to the central pith of the stalk and does not
damage conductive tissues or reduce photosynthetic area, apart from the loss of a single leaf
early in the season due to petiole boring by the first instar. However, yield is complex
function of positive and negative factors influencing the plant and these factors may interact
in complex ways. Thus it is possible that other plant stress factors or growing conditions not
encountered in this study could interact with stalk boring to produce yield losses under other
circumstances. Nevertheless, we are inclined to consider
D. texanus
a non-economic pest of
soybeans unless harvest is delayed beyond the point where larvae girdle the plants.
Soybean represents a relatively recent and dramatic host shift for
D. texanus
whose
ancestral host plants are all composite species native to North America (Hatchett et al. 1975).
Soybean plants still represent a host of marginal suitability for
D. texanus
; adult body size
averages 50% smaller and fitness is substantially reduced when larvae mature on soybean
compared to cultivated sunflower (Michaud and Grant 2005; Niide et al. 2006). In contrast to
sunflower that hosts a wide spectrum of boring insects, including competing cerambycid
species (Michaud and Grant 2005), soybean is completely devoid of stem boring insects other
than
D. texanus
in the New World and is therefore completely free of competitors for this
niche. All indications suggest that intraspecific competition among
D. texanus
larvae can be
intense within ancestral host plants - hence the girdling behavior thought to represent a
defensive strategy for protecting the overwintering site. Thus, for genotypes of
D. texanus
that are able to survive in this plant, a field of soybeans likely represents a vast, untapped
resource with reduced competition.
In the early 1970's, Richardson (1975) examined more than 10,000 soybean stalks of
multiple cultivars and maturity groups infested by
D. texanus
in North Carolina and found
significant effects of stem diameter - larger diameter stems were infested at higher rates and
yielded superior larval survival than small diameter stems. In our study, we found larvae of
D.
texanus
infesting all stem diameters and size classes of plants with approximately equal
frequency (Figure 1). Although the average stem diameter of infested plants appeared slightly
greater than that of uninfested plants in 2005 (Figure 1a), the difference was non-significant
(Table 1). There was also no apparent shift toward larger stem diameters for infested plants in
2006 (Figure 1b) when plants were larger, but still not as large as many obtained by Andrews
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