Effect of spider mite herbivory on expression of defense genes in cotton, corn, and tomato.

<p>Fold induction was calculated relative to plants free of spider mites and not treated with the insecticides (Untreated). Ubiquitin gene was used as an internal standard. All treatments were replicated four times for each plant species. Means with different letters were significantly different at <i>P = </i>0.05 (Wilcoxon test). Spider mites induced expression of <i>CoA ligase</i> and <i>chitinase</i> in cotton (A), and elicited significant expression of all four genes in corn (B). <i>Trypsin PI</i> was the only defense gene induced by spider mites in tomato (C).</p

Concentrations of phytohormones (ng/g fresh weight) in cotton, corn, and tomato plants exposed to neonicotinoid insecticide.

<p>U: Untreated, N: Neonicotinoid insecticides thiamethoxam (cotton), clothianidin (corn), and imidacloprid (tomato). Four-week old plants were used in the experiment. Tomato plants were treated with soil applications of imidacloprid seven days prior to the experiment. Means were compared using ANOVA (<i>F</i> statistic) or non-parametric Kruskal-Wallis test (<i>X²</i> statistic).</p

Effect of the neonicotinoid insecticides on transcription of defense genes in cotton, corn, and tomato.

<p>Fold induction was calculated relative to plants free of spider mites and not treated with the insecticides (Untreated). Ubiquitin gene was used as an internal standard. All treatments were replicated four times for each plant species. Means with different letters were significantly different at <i>P = </i>0.05 (Wilcoxon test). In all three plants, the neonicotinoid applications altered transcription of the genes regulated by salicylic acid and jasmonic acid. Expression of <i>CoA ligase</i> and <i>chitinase</i> increased in cotton treated with thiamethoxam independently of spider mite herbivory (A). None of the genes were induced in clothianidin-treated corn, and spider mite herbivory did not elicit gene expression in these plants either (B). Expression profile of tomato plants exposed to imidacloprid was dominated by strong <i>chitinase</i> induction, which was independent of the spider mite presence (C). Expression of <i>trypsin PI,</i> a pivotal plant defense employed against the spider mites, was halted in the imidacloprid-treated plants exposed to <i>T. urticae.</i> Similarly, expression of <i>PAL</i> was suppressed in tomato plants treated with imidacloprid and exposed to the herbivore.</p

Changes in phytohormone concentrations in cotton, corn, and tomato plants treated with the neonicotinoid insecticides.

<p>Applications of thiamethoxam to cotton plants (<i>N = </i>8) significantly decreased levels of OPDA (A). Concentrations of this phytohormone were seven times lower in these plants than in untreated cotton. Similar effect on this phytohormone was noted in corn plants (<i>N = </i>8) exposed to clothianidin, where OPDA was reduced by 50% compared to untreated corn (B). Imidacloprid applied to tomato plants (<i>N = </i>8) also lowered quantities of OPDA (C). While the OPDA concentrations were reduced significantly in these plants, levels of total SA increased over three times in tomato plants treated with imidacloprid (D). Four-week old plants were used in the experiment. Tomato plants were treated with soil applications of imidacloprid seven days prior to the experiment. Values are means±one standard error. Asterisks mark means that are significantly different (<i>P</i><0.05; ANOVA, mixed model or Kruskal-Wallis test).</p

Growth rate of spider mite populations on cotton, corn, and tomato plants.

<p>Growth rate of spider mite populations was measured on cotton (<i>N = </i>8), corn (<i>N = </i>10), and tomato plants (<i>N = </i>5) treated with the neonicotinoid insecticides in a greenhouse. Population growth rate was calculated by estimating the weekly change in density of spider mites per cm² of leaf area. Neonicotinoid applications resulted in significantly greater population growth rate of spider mites. Values are means±one standard error. Different letters indicate significant differences (<i>P</i><0.05; ANOVA, simple effects in mixed model).</p

Effect of the neonicotinoid insecticides on abundance of spider mites on cotton, corn, and tomato.

<p>Spider mites increased in abundance on all three plants exposed to the insecticides. Abundance of the herbivores on cotton (<i>N = </i>8) and corn (<i>N = </i>10) plants increased by nearly 30% (A) and 60% (B) following applications of the neonicotinoid insecticides. Tomato plants (<i>N = </i>5) treated with imidacloprid had over twice as many spider mites as untreated tomatoes (C). Values are means±one standard error. Asterisks mark means that are significantly different (<i>P</i><0.05; ANOVA, mixed model).</p