Chemistry Reference
In-Depth Information
used for control of several insect pests of white potatoes [8,9], including early to mid-season
infestations of both CPB and PLH. These insecticides are toxic to both of these pests [10],
have excellent systemic uptake and translocation in plants, are used at low application rates,
and present reduced environmental hazards [11,12]. Although most neonicotinoids can be
applied as either a seed treatment, a soil treatment or as a post-planting foliar treatment,
many commercial growers prefer a soil application at planting time. Applied to the soil,
these materials are absorbed by the roots and translocated acropetally (xylem movement)
within the whole plant [12,13]. However, the effectiveness of soil-applied insecticides are
influenced by soil and climatic conditions, including soil moisture, clay and organic content,
rainfall, soil temperature, plant size, sorption, adsorption, and physical properties of the
insecticide, such as stability to chemical and microbial degradation [14,15,16]. Thus, when
used as a soil-applied insecticide, the length of effective protection can vary and additional
foliar insecticide sprays may be needed to control midsummer populations of both the CPB
and PLH.
The solubility of an insecticide and its activity in soil and uptake by the roots is known to be
important, but this relationship has not been well documented [17]. In general, increased
solubility is positively related to increased uptake of a systemic chemical. A compound's
relative affinity between soil and water phases is measured by log Koc (organic carbon
referenced sorption coefficients). With sufficient soil moisture, materials with higher log Koc
bind more tightly to soil colloids and release at a slower rate, affecting the availability for
root uptake; materials with a higher solubility enter the plant roots more quickly and thus
move through a plant faster than materials with a lower solubility. The uptake of
dinotefuran, which is 80 times more soluble than imidacloprid, in both yellow sage and
poinsettia plants was more rapid and resulted in quicker and higher percentage mortality of
whitefly nymphs compared with imidacloprid [18]. However, materials with a lower log
Koc are also more easily leached out of the root zone, which can reduce effectiveness. For
example, thiamethoxam has a strong potential to leach under heavy rainfall conditions [19],
and imidacloprid has a potential to leach to ground water [20]. Leaching of the insecticide
would impact insect pest control because less insecticide would be available for uptake by
the plant, resulting in reduced efficacy or in reduced longevity of efficacy. Although much
research has been conducted on the bioefficacy of the neonicotinoids against CPB and PLH
as soil-applied insecticides, little published information is available on the effect of rainfall
on these materials after application to the soil.
Therefore, laboratory and field studies were conducted to examine the effect of two levels of
simulated rainfall on three neonicotinoids with different solubilities for control of CPB and
PLH on white potato.
2. Methods and materials
Laboratory Trials.
Two “Superior” cv. potato seed pieces were each planted into 22.9 cm
(9”) diam pots half-filled with soil in the field on 23 Apr. The pots were then aligned in the
field in a straight row, each pot touching the next, to approximate a grower-practiced seed
