Behavioral Responses of Thrips (Thysanoptera: Thripidae) and Tarnished Plant Bug (Hemiptera: Miridae) to a New Bt Toxin, Cry51Aa2.834_16 in Cotton

Abstract

Thrips (Thysanoptera: Thripidae) and tarnished plant bug, Lygus lineolaris (Hemiptera: Miridae), are among the most important insect pests of cotton, Gosssypium hirsutum, in the mid-southern United States. These pests are currently managed primarily by insecticides; however, a new Bt toxin, Cry51Aa2.834_16 is under evaluation for control of thrips and tarnished plant bug. Experiments were conducted to evaluate the behavioral response of thrips and tarnished plant bug to Bt Cry51Aa2.834_16. Adult thrips avoided Bt Cry51Aa2.834_16 cotton in field choice tests and in separate field tests of Bt and non-Bt cotton not treated with insecticides. In a greenhouse choice test, approximately twice as many adult thrips and eggs were found on non-Bt compared with Bt Cry51Aa2.834_16 cotton. Similarly, in a field test of nontreated Bt Cry51Aa2.834_16 and non-Bt cotton, 68% of adult thrips collected were found on non-Bt cotton. In cotton that was not sprayed with insecticides, Bt Cry51Aa2.834_16 did not affect the distribution of tarnished plant bug within the canopy, although more square and flower injury was caused by tarnished plant bug in non-Bt cotton. Adult tarnished plant bug exhibited a nonpreference for diet containing lyophilized Bt Cry51Aa2.834_16 leaves and for excised Bt Cry51Aa2.834_16 squares in choice tests with non-Bt squares. The behavioral responses of these pests when exposed to this new Bt toxin will play a key role in the efficacy and potential resistance management strategies if this new technology is incorporated in an overall cotton insect pest management system.

Understanding the potential impact of continued seed treatment use for resistance management in Cry51Aa2.834_16 Bt cotton against Frankliniella fusca

“…However, the mechanism of MON 88702 activity creates added challenges to resistance management in the field. Specifically, recent studies have shown that MON 88702 acts by a non-lethal antixenosis leading to suppressed oviposition that results in fewer F. fusca larvae developing on MON 88702 cotton [13][14][15][16]. Similar non-lethal suppression of oviposition by adult F. fusca in conjunction with acute larval toxicity have been documented for neonicotinoid seed treated (NST) cotton [17][18][19].…”

“…Both MON 88702 cotton and neonicotinoid seed treatments have strong antixenotic effects on adult F. fusca resulting in large reductions in oviposition and larval establishment on cotton, even in the presence of alternative host plants. While MON 88702 alters feeding behavior and reduces oviposition of adults, it has minimal impacts on adult and larval survival [13][14][15][16]. Both imidacloprid and thiamethoxam applied as seed treatments also have minimal impact on survival of adult F. fusca and dramatically reduce oviposition on treated cotton seedlings; however, they also cause moderate to high levels of larval mortality [17][18][19].…”

“…Specifically, recent studies have shown that MON 88702 acts by a non-lethal antixenosis leading to suppressed oviposition that results in fewer F . fusca larvae developing on MON 88702 cotton [ 13 – 16 ]. Similar non-lethal suppression of oviposition by adult F .…”

Novel mechanism of thrips suppression by Cry51Aa2.834_16 Bt toxin expressed in cotton

“…Field trials have demonstrated that cotton seedlings expressing MON 88702 support lower F. fusca adult and larval infestations and sustain less feeding damage through the period of susceptibility from seedling emergence through the 4–5 true‐leaf stage . Further study of population development in a choice experiment revealed that the presence of MON 88702 was associated with reduced oviposition when compared to non‐ Bt cotton . Despite reduced oviposition and damage, the detection of larvae on cotton seedlings expressing the protein suggests that the toxin expression level may not meet the criteria for a sufficiently high dose to mitigate rapid selection for resistance.…”

“…11,12,14 Further study of population development in a choice experiment revealed that the presence of MON 88702 was associated with reduced oviposition when compared to non-Bt cotton. 15 Despite reduced oviposition and damage, the detection of larvae on cotton seedlings expressing the protein suggests that the toxin expression level may not meet the criteria for a sufficiently high dose to mitigate rapid selection for resistance. To our knowledge, detailed reports of impacts of MON 88702 on F. occidentalis populations have not been published, although it is an occasional pest of cotton in the U.S. [16][17][18][19] This raises the question: if MON 88702 does not kill targeted thrips at a rate consistent with lepidopteran-and coleopteran-active Bt toxins (i.e., high dose toxin), what are the consequences for long-term resistance management of this novel toxin?…”