Efficacy and nontarget effects of broadcast treatments to manage spotted lanternfly (Hemiptera: Fulgoridae) nymphs.

Management to control the spotted lanternfly, Lycorma delicatula (White), would ideally achieve managers' goals while limiting impacts on nontarget organisms. In a large-scale field study with 45 plots at least 711 m2, we tested foliar applications of dinotefuran and 2 formulations of Beauveria bassiana (Balsamo) Vuillemin, each applied from the ground and separately by helicopter. Applications targeted early instar nymphs. For both application methods, a single treatment with dinotefuran significantly reduced L. delicatula numbers, as measured by catch on sticky bands (91% reduction by air and 84% reduction by ground 19 days after application) and by timed counts (89% reduction by air and 72% reduction by ground 17 days after application). None of the B. bassiana treatments significantly reduced L. delicatula numbers, even after 3 applications. Beauveria bassiana infection in field-collected nymphs ranged from 0.4% to 39.7%, with higher mortality and infection among nymphs collected from ground application plots. Beauveria bassiana conidia did not persist for long on foliage which probably contributed to low population reduction. Nontarget effects were not observed among arthropods captured in blue vane flight intercept traps, San Jose Scale pheromone sticky traps or pitfall traps, but power analysis revealed that small reductions of less than 40% may not be detected despite extensive sampling of 48,804 specimens. These results demonstrate that dinotefuran can markedly reduce local abundance of L. delicatula with little apparent effect on nontarget insects when applied shortly after hatch, and that aerial applications can match or exceed the effectiveness of applications from the ground.

Applications of Beauveria bassiana (Hypocreales: Cordycipitaceae) to Control Populations of Spotted Lanternfly (Hemiptera: Fulgoridae), in Semi-Natural Landscapes and on Grapevines.

Spotted lanternfly, Lycorma delicatula (White), is an invasive Asian insect that was initially found in Berks County, Pennsylvania, in 2014. As of early 2020, this pest had been found in five more eastern states and it is expected to continue to expand its geographical range. Lycorma delicatula is highly polyphagous but seems to prefer tree-of-heaven, Ailanthus altissima. However, grape growers in Pennsylvania have reported significant damage and loss of vines caused by L. delicatula adults. In fall 2018, two fungal entomopathogens (Beauveria bassiana and Batkoa major) drove localized collapses in L. delicatula populations in Berks County, Pennsylvania. In 2019, we tested applications of a commercialized mycoinsecticide based on B. bassiana strain GHA on L. delicatula populations in a public park in southeastern Pennsylvania. A single application of B. bassiana reduced fourth instar nymphs by 48% after 14 d. Applications of B. bassiana to L. delicatula adults in the same park resulted in 43% mortality after 14 d. Beauveria bassiana spores remained viable on foliage for 5-7 d after spraying. We also conducted semi-field bioassays with B. bassiana GHA (formulated as BoteGHA and Aprehend) and another mycoinsecticide containing Isaria fumosorosea Apopka Strain 97 against L. delicatula adults feeding on potted grapes. All the mycoinsecticides killed ≥90% of adults after 9 d using direct applications. Aprehend killed 99% of adults after 9 d with exposure to residues on sprayed grapes. These data show that fungal entomopathogens can help to suppress populations of L. delicatula in agroecosystems and natural areas.

Insect resistance management in GM crops: past, present and future.

Transgenic plants expressing insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) were first commercialized in 1996 amid concern from some scientists, regulators and environmentalists that the widespread use of Bt crops would inevitably lead to resistance and the loss of a 'public good,' specifically, the susceptibility of insect pests to Bt proteins. Eight years later, Bt corn and cotton have been grown on a cumulative area >80 million ha worldwide. Despite dire predictions to the contrary, resistance to a Bt crop has yet to be documented, suggesting that resistance management strategies have been effective thus far. However, current strategies to delay resistance remain far from ideal. Eight years without resistance provides a timely opportunity for researchers, regulators and industry to reassess the risk of resistance and the most effective strategies to preserve Bt and other novel insect-resistant crops in development.

Transgenic plants expressing two Bacillus thuringiensis toxins delay insect resistance evolution.

Preventing insect pests from developing resistance to Bacillus thuringiensis (Bt) toxins produced by transgenic crops is a major challenge for agriculture. Theoretical models suggest that plants containing two dissimilar Bt toxin genes ('pyramided' plants) have the potential to delay resistance more effectively than single-toxin plants used sequentially or in mosaics. To test these predictions, we developed a unique model system consisting of Bt transgenic broccoli plants and the diamondback moth, Plutella xylostella. We conducted a greenhouse study using an artificial population of diamondback moths carrying genes for resistance to the Bt toxins Cry1Ac and Cry1C at frequencies of about 0.10 and 0.20, respectively. After 24 generations of selection, resistance to pyramided two-gene plants was significantly delayed as compared with resistance to single-gene plants deployed in mosaics, and to Cry1Ac toxin when it was the first used in a sequence. These results have important implications for the development and regulation of transgenic insecticidal plants.

Evaluation of a chemically inducible promoter for developing a within-plant refuge for resistance management.

Chemically inducible production of Bacillus thuringiensis (Bt) toxins in transgenic plants may provide considerable benefits in preventing or delaying the evolution of insect resistance to Bt crops by creating within-plant temporal refuges. We examined the effect of inducible cry1Ab expression on survival of different genotypes (RR, RS, and SS) of diamondback moth, Plutella xylostella (L.), in transgenic broccoli, Brassica oleracea L., plants transformed with a PR-1a/cry1Ab expression cassette. Spraying leaves of these plants with the inducer acibenzolar-s-methyl [= benzo (1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester] (ASM) resulted in high levels of Bt toxin, and detached leaves from fully induced plants caused 100% mortality to all instars of P. xylostella SS and RS genotypes. When plants infested with larvae were treated with ASM, only a few larvae that were nearing completion of their development were able to survive the induction process. Signal transduction from ASM-treated leaves to new plant tissue also was evaluated using a larval assay. New foliage that emerged after plants were induced remained toxic to > or = 80% of RS larvae up to the fourth new leaf. In whole plant tests, however, induced plants remained protected from larval damage for > or = 3 wk. Uninduced PR-1a/cry1Ab plants seemed to produce low levels of Bt that were undetected by an enzyme-linked immunosorbent assay but that resulted in significant fitness costs for susceptible insects. The suitability of PR-1a/cry1Ab broccoli plants for insect resistance management and the requirements of an appropriate inducible promoter are discussed.

Evidence for oil-induced oxidative stress to larvae of the lightbrown apple moth, Epiphyas postvittana Walker (Lepidoptera: Tortricidae).

For the purpose of understanding better the mode of action of alkanes on insects, the relationship between mortality, weight loss in oxygen enriched atmospheres and dietary antioxidants was examined using an alkane, C15 Ampol CPD and a spray oil, C23 DC-Tron NR, on lightbrown apple moth, Epiphyas postvittana Walker, (LBAM). The results showed that the surfactant blend used in CPD was an insignificant contributor to the overall toxicity of dilute oil/water emulsions. Higher weight loss occurred in CPD-dipped larvae than C23 DC-Tron NR-dipped larvae, which suggests that alkanes disrupt tracheal waxes and render insects more prone to desiccation. High levels of oxygen increased the toxicity of CPD to LBAM larvae. In addition, dietary supplements of anti-oxidant, alpha-tocopherol, fed to LBAM larvae were successful in reducing the toxicity of CPD. These results suggest that the alkane may contribute to oxidative injury. The potential role of oil-induced oxidative stress in acute and chronic toxicity in insects is discussed.

Insect resistance to transgenic Bt crops: lessons from the laboratory and field.

Transgenic crops that produce insecticidal toxins from the bacterium Bacillus thuringiensis (Bt) grew on >62 million ha worldwide from 1996 to 2002. Despite expectations that pests would rapidly evolve resistance to such Bt crops, increases in the frequency of resistance caused by exposure to Bt crops in the field have not yet been documented. In laboratory and greenhouse tests, however, at least seven resistant laboratory strains of three pests (Plutella xylostella [L.], Pectinophora gossypiella [Saunders], and Helicoverpa armigera [Hübner]) have completed development on Bt crops. In contrast, several other laboratory strains with 70- to 10,100-fold resistance to Bt toxins in diet did not survive on Bt crops. Monitoring of field populations in regions with high adoption of Bt crops has not yet detected increases in resistance frequency. Resistance monitoring examples include Ostrinia nubilalis (Hübner) in the United States (6 yr), P. gossypiella in Arizona (5 yr), H. armigera in northern China (3 yr), and Helicoverpa zea (Boddie) in North Carolina (2 yr). Key factors delaying resistance to Bt crops are probably refuges of non-Bt host plants that enable survival of susceptible pests, low initial resistance allele frequencies, recessive inheritance of resistance to Bt crops, costs associated with resistance that reduce fitness of resistant individuals relative to susceptible individuals on non-Bt hosts ("fitness costs"), and disadvantages suffered by resistant strains on Bt hosts relative to their performance on non-Bt hosts ("incomplete resistance"). The relative importance of these factors varies among pest-Bt crop systems, and violations of key assumptions of the refuge strategy (low resistance allele frequency and recessive inheritance) may occur in some cases. The success of Bt crops exceeds expectations of many, but does not preclude resistance problems in the future.

Natural enemies delay insect resistance to Bt crops.

We investigated whether development of resistance to a Bt crop in the presence of a natural enemy would be slower than without the natural enemy and whether biological control, in conjunction with a Bt crop, could effectively suppress the pest population. Additionally, we investigated whether insecticide-sprayed refuges of non-Bt crops would delay or accelerate resistance to the Bt crop. We used a system of Bt broccoli expressing Cry1Ac, a population of the pest Plutella xylostella with a low frequency of individuals resistant to Cry1Ac and the insecticide spinosad, and a natural enemy, Coleomegilla maculata, to conduct experiments over multiple generations. The results demonstrated that after 6 generations P. xylostella populations were very low in the treatment containing C. maculata and unsprayed non-Bt refuge plants. Furthermore, resistance to Bt plants evolved significantly slower in this treatment. In contrast, Bt plants with no refuge were completely defoliated in treatments without C. maculata after 4-5 generations. In the treatment containing sprayed non-Bt refuge plants and C. maculata, the P. xylostella population was low, although the speed of resistance selection to Cry1Ac was significantly increased. These data demonstrate that natural enemies can delay resistance to Bt plants and have significant implications for integrated pest management (IPM) with Bt crops.

Concurrent use of transgenic plants expressing a single and two Bacillus thuringiensis genes speeds insect adaptation to pyramided plants.

Transgenic plants expressing insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) were grown on over 13 million ha in the United States and 22.4 million ha worldwide in 2004. Preventing or slowing the evolution of resistance by insects ("resistance management") is critical for the sustainable use of Bt crops. Plants containing two dissimilar Bt toxin genes in the same plant ("pyramided") have the potential to delay insect resistance. However, the advantage of pyramided Bt plants for resistance management may be compromised if they share similar toxins with single-gene plants that are deployed simultaneously. We tested this hypothesis using a unique model system composed of broccoli plants transformed to express different Cry toxins (Cry1Ac, Cry1C, or both) and a synthetic population of the diamondback moth (Plutella xylostella) carrying genes for resistance to Cry1Ac and Cry1C at frequencies of approximately 0.10 and 0.34, respectively. After 24-26 generations of selection in the greenhouse, the concurrent use of one- and two-gene plants resulted in control failure of both types of Bt plants. When only two-gene plants were used in the selection, no or few insects survived on one- or two-gene Bt plants, indicating that concurrent use of transgenic plants expressing a single and two Bt genes will select for resistance to two-gene plants more rapidly than the use of two-gene plants alone. The results of this experiment agree with the predictions of a Mendelian deterministic simulation model and have important implications for the regulation and deployment of pyramided Bt plants.

Pollen-mediated movement of herbicide resistance between commercial canola fields.

There is considerable public and scientific debate for and against genetically modified (GM) crops. One of the first GM crops, Brassica napus (oilseed rape or canola) is now widely grown in North America, with proposed commercial release into Australia and Europe. Among concerns of opponents to these crops are claims that pollen movement will cause unacceptable levels of gene flow from GM to non-GM crops or to related weedy species, resulting in genetic pollution of the environment. Therefore, quantifying pollen-mediated gene flow is vital for assessing the environmental impact of GM crops. This study quantifies at a landscape level the gene flow that occurs from herbicide-resistant canola crops to nearby crops not containing herbicide resistance genes.

"Active" refuges can inhibit the evolution of resistance in insects towards transgenic insect-resistant plants.

Negative cross-resistance (NCR) toxins that hitherto have not been thought to have practical uses may indeed be useful in the management of resistance alleles. Practical applications of NCR for pest management have been limited (i) by the scarcity of high toxicity NCR toxins among pesticides, (ii) by the lack of systematic methodologies to discover and develop such toxins, as well as (iii) by the lack of deployment tactics that would make NCR attractive. Here we present the concept that NCR toxins can improve the effectiveness of refuges in delaying the evolution of resistance by herbivorous insect pests to transgenic host plants containing insecticidal toxins. In our concept, NCR toxins are deployed in the refuge, and thus are physically separated from the transgenic plants containing the primary plant-protectant gene (PPPG) encoding an insecticidal toxin. Our models show: (i) that use of NCR toxins in the refuge dramatically delays the increase in the frequency of resistance alleles in the insect population; and (ii) that NCR toxins that are only moderately effective in killing insects resistant to the PPPG can greatly improve the durability of transgenic insecticidal toxins. Moderately toxic NCR toxins are more effective in minimizing resistance development in the field when they are deployed in the refuge than when they are pyramided with the PPPG. We explore the potential strengths and weaknesses of deploying NCR toxins in refuges.