STEM BORING OF SOYBEAN BY DECTES TEXANUS (COLEOPTERA: CERAMBYCIDAE) AND THE NATURE OF ITS IMPACT ON YIELD - Soybean and Wheat Crops: Growth, Fertilization, and Yield - page 116

I NTRODUCTION
The longhorned beetle Dectes texanus LeConte, a.k.a. the soybean stem borer, has been a
pest of soybeans throughout many eastern and midwestern states for more than forty years
(Falter 1969; Patrick, 1973; Laster et al. 1981). Cultivated sunflowers are a preferred host
plant (Michaud and Grant 2005) but D. texanus populations likely move between the two
crops, depending on their availability. These beetles are monovoltine, overwintering as late
instar larvae and pupating in spring. Adult females lay their eggs in leaf petioles in early
summer and the eclosing larvae tunnel down into the main stalk of the plant where they feed
on the pith core (Hatchett et al. 1975). As the plant matures and dries down, larvae cease
feeding and prepare overwintering chambers in the base of plants at, or just above, soil level.
Once a cavity is hollowed out, the larva girdles the stalk above the chamber and then seals
itself off below by plugging the entrance with shredded fibers. It is this final act of cutting off
the plant that causes economic losses as girdled plants break off at the slightest pressure and
will often lodge under the force of wind alone. When harvesting girdled plants, farmers must
reduce the speed of the combine substantially to avoid additional yield losses.
This pest has become increasingly problematic with the advent of no-till soybean
production that likely favors the survival of overwintering larvae compared to regimes of
tillage (Campbell and van Duyn 1977). Although yield losses due to the lodging of girdled
plants are obvious, it is less clear whether plant productivity may be negatively impacted by
larval boring during the course of the growing season. It is important to note that larval boring
kills only the leaf attached to the petiole on which oviposition occurs; plants are not killed and
stand establishment is never impacted, making it possible to measure yield impact at the level
of individual plants. Various workers have suggested or inferred that larval boring may
reduce yields by around 10% (Richardson 1975; Campbell 1980; Patrick et al. undated).
However, trials that demonstrate yield increases in response to foliar or soil drench insecticide
treatments directed at D. texanus (e.g. Buschmann and Sloderbeck 2005) are not necessarily
reliable indicators of direct impact because collateral control of other potentially yield-
limiting insects could be responsible for any portion of the observed effects. For example,
Michaud et al. (2007a) found no impact of D. texanus larval boring in sunflower when they
quantified yield at the level of individual plants in two successive growing seasons.
Nevertheless, they observed a significant yield increase in one year in response to both foliar
and soil drench insecticides that they attributed to control of other foliage- and root-feeding
insects. Laster et al. (1981) managed to reduce D. texanus infestation of soybeans in
Mississippi with eight treatments of methyl parathion, but treated plots did not yield more
than untreated plots in either of two locations. Andrews and Williams (1988) conducted
perhaps the most detailed study of D. texanus impact on soybean to date and found that
infested plants yielded more than uninfested plants, ostensibly due to adult females avoiding
plants in smaller size classes. In this study we examined the yield of soybean plants both with
and without D. texanus larval infestation in two successive years in west-central Kansas.