Differences between two populations of bollworm, Helicoverpa zea (Lepidoptera: Noctuidae), with variable measurements of laboratory susceptibilities to Bt toxins exposed to non-Bt and Bt cottons in large field cages

Abstract

Interpreting variable laboratory measurements of Helicoverpa zea Boddie susceptibility to toxins from Bacillus thuringiensis Berliner (Bt) has been challenging due to a lack of clear evidence to document declining field control. Research that links laboratory measurements of susceptibility to survival on Bt crops is vital for accurate characterization and any subsequent response to the occurrence of an implied H. zea resistance event. In this study, H. zea survival and the resultant damage to plant fruiting structures of non-Bt, Bollgard II, and Bollgard III cottons from two insect colonies with differing levels of laboratory susceptibility to Bt toxins were evaluated in large field cages. Laboratory bioassays revealed resistance ratios of 2.04 and 622.14 between the two H. zea colonies for Dipel DF and Cry1Ac, respectively. Differences between the two H. zea colonies measured via bioassays with Bollgard II and Bollgard III cotton leaf tissue in the laboratory were not statistically discernable. However, there was 17.6% and 5.3% lower larval mortality in Bollgard II and Bollgard III for the feral relative to the laboratory colony of H. zea, respectively. Although H. zea larval numbers in cages infested with the laboratory susceptible colony did not differ between the two Bt cottons, there were fewer larvae per 25 plants in Bollgard III than in Bollgard II cotton in cages containing tolerant insects. Cages infested with tolerant H. zea moths had higher numbers of total larvae than those containing the laboratory susceptible colony in both Bollgard II and Bollgard III cottons. Bollgard II and Bollgard III cottons received 77.4% and 82.7% more total damage to total plant fruiting structures in cages infested with tolerant insects relative to those containing the laboratory susceptible colony. The damage inflicted to fruiting structures on Bollgard III cotton by a feral H. zea colony with decreased measurements of laboratory susceptibility to Dipel DF and Cry1Ac indicate that the addition of Vip3A to third generation Bt cottons may not provide sufficient control in situations where infestations levels are high.

Early Warning of Resistance to Bt Toxin Vip3Aa in Helicoverpa zea

“…The TM strain (called SS in [ 36 ]) was started from more than 150 larvae collected from non-Bt corn in May 2016 at the LSUAC near Winnsboro, LA. The SIMRU strain has been maintained in the lab since 1971 [ 18 ].…”

“…Some populations of at least nine major lepidopteran and coleopteran pests have evolved practical resistance to Bt crystalline (Cry) toxins produced by transgenic crops [ 6 , 9 , 10 ], including Helicoverpa zea (corn earworm and bollworm), one of the most damaging crop pests in the United States [ 3 , 11 ]. Throughout much of the United States where this polyphagous lepidopteran is an important pest, it has evolved practical resistance to the Cry toxins produced by Bt corn and cotton, including Cry1Ab, Cry1Ac, Cry1A.105, Cry1Fa, and Cry2Ab [ 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 ]. Driven in part by this resistance, farmers have planted Bt corn and cotton that produce the Bt vegetative insecticidal protein Vip3Aa in addition to various Cry proteins [ 15 , 23 ].…”

“…Despite no strong cross-resistance between Cry toxins and Vip3Aa and the initial high efficacy of Vip3Aa against H. zea [ 33 , 34 ], some previously reported evidence suggests this pest has begun to evolve resistance to Vip3Aa in the southern United States [ 18 , 19 , 20 , 22 , 35 ]. For example, in a 2018 field trial at the Texas A&M University (TAMU) field laboratory in Snook, Texas, H. zea larvae damaged 67.5% of corn producing Cry1Ab + Cry1Fa + Vip3Aa [ 19 ].…”

Genetic Knockouts Indicate That the ABCC2 Protein in the Bollworm Helicoverpa zea Is Not a Major Receptor for the Cry1Ac Insecticidal Protein

“…The Bacillus thuringiensis subsp. kurstaki HD-73 strain obtained from the Bacillus Genetic Stock Center (Columbus, OH, USA) was used to produce full length Cry1Ac protoxin as described in Little et al [26]. Bioassays were performed in 128-well plastic trays (B-D International, Franklin Lakes, NJ, USA) with Cry1Ac protein incorporated in meridic diet.…”

Genetic Knockouts Indicate That the ABCC2 Gene in the Bollworm Helicoverpa Zea Is Not a Major Receptor for the Cry1Ac Insecticidal Protein.

“…The Bacillus thuringiensis subsp. kurstaki HD-73 strain obtained from the Bacillus Genetic Stock Center (Columbus, OH, USA) was used to produce the Cry1Ac protoxin as described in Little et al (2019). Bioassays were performed in 128-well plastic trays (B-D International, Franklin Lakes, NJ, USA) with Cry1Ac protein incorporated in meridic diet.…”