Dively GP, Crossley MS, Schoville SD, Steinhauer N, Hawthorne DJ. 2020. Pest Management Science. 2020 Jul. DOI: 10.1002/ps.5992.

BACKGROUND: Agricultural insect pests frequently exhibit geographic variation in levels of insecticide resistance, which are often presumed to be due to the intensity of insecticide use for pest management. However, regional differences in the evolution of resistance to novel insecticides suggests that other factors are influencing rates of adaptation. We examined median lethal concentration (LC50) bioassay data spanning 15 years and six insecticides (abamectin, imidacloprid, spinosad, cyantraniliprole, chlorantraniliprole, and metaflumizone) for evidence of regional differences in Leptinotarsa decemlineata baseline sensitivity to insecticides as they became commercially available.

RESULTS: We consistently found that larvae from Colorado potato beetle populations from the northwestern USA had the highest baseline sensitivity to novel insecticides, while populations from the eastern USA had the lowest. Comparisons of gene expression between populations from these regions revealed constitutively elevated expression of an array of detoxification genes in the East, but no evidence of additional induction when exposed to imidacloprid.

CONCLUSIONS: Our results suggest a mechanism for geographic variation in rates of adaptation to insecticides, whereby baseline levels of gene expression determine a population's response to novel insecticides. These findings have implications for the regional development of insecticide resistance management strategies and for the fundamental question of what determines the rate of adaptation to insecticides.

Yoon JS, Kim K, Palli SR. 2020. Journal of Asia-Pacific Entomology. 23(4):1160–1164.

Despite extensive research during the past decade elucidating the mechanism of RNA interference (RNAi) in insects, it is not clear how ingested or injected double-stranded RNA (dsRNA) triggers RNAi response in the whole body or even its progeny, which is referred to as systemic RNAi. In the present study, we aim to understand how the dsRNA delivered into cells causes systemic RNAi using Colorado potato beetle cells (Lepd-SL1). We first tested if dsRNA treatment induces systemic RNAi in Lepd-SL1 cells. Exposure of a new batch of Lepd-SL1 cells to the conditioned medium where Lepd-SL1 cells treated with dsRNA targeting inhibitor of apoptosis were grown for 6 h induced apoptosis in these new batch of cells. We hypothesized the exosomes in the conditioned medium are responsible for RNAi-inducing effect. To test this hypothesis, we isolated exosomes from the conditioned medium from Lepd-SL1 cells that had been treated with dsGFP (dsRNA targeting gene coding for green fluorescent protein) or dsLuc (dsRNA targeting gene coding for the luciferase) were grown. RNA present in the purified exosomes was analyzed to check if long dsRNA or siRNA is accumulated in them. The results from the electrophoretic mobility shift assay clearly showed that the long dsRNAs are present in the exosomes. By knockdown of candidate genes involved in endosome recycling and generation pathways, we found that Rab4 and Rab35 are involved in exosome production and transport.

Wu JJ, Mu LL, Chen ZC, Fu KY, Guo WC, Li C, Li GQ. 2019. Journal of Asia-Pacific Entomology. 22(2):443–452.

Chitin deacetylases (CDAs) catalyze N-deacetylation of chitin, a crucial process for chitin modification. In the present paper, LdCDA1 was identified in Leptinotarsa decemlineata. It was copiously expressed in larval foregut, hindgut and epidermis. Just before the molt in the first, second and third larval instars, the mRNA levels of LdCDA1 were high. In the fourth (final)-instar larvae, a peak occurred 4 days after ecdysis. In vivo results revealed that LdCDA1 transcriptionally responded, positively and negatively respectively, to 20-hydroxyecdysone and juvenile hormone titers. Moreover, knockdown of LdCDA1 significantly reduced foliage consumption, lengthened developing period and prevented growth in the final instar larvae. Three distinct lethal phenotypes were noted in the LdCDA1 RNAi larvae. About 30% of the RNAi larvae became moribund and finally died; approximately 50% of deformed pupae died as pharate adults; and around 20% of LdCDA1 depleted pupae finally emerged as abnormal adults and eventually died within 1 week after emergence. Furthermore, chitin content was low and the mRNA levels of five chitin biosynthesis transcripts (LdUAP1, LdUAP2, LdChSAa, LdChSAb and LdChSB) were significantly declined in the LdCDA1 RNAi larvae. In addition, glucose, trehalose and glycogen contents were increased in the LdCDA1 depleted hypomorphs, along with highly expressed genes coding for trehalose and glycogen synthesis enzymes. The findings provide a compelling piece of evidence that CDA1 is critical for chitin deposition in L. decemlineata. Moreover, LdCDA1 may be a potential target for control of the larvae.

Rainio MJ, Margus A, Virtanen V, Lindström L, Salminen JP, Saikkonen K, Helander M. 2020. Chemosphere. 258:127254.

Glyphosate is the most used herbicide worldwide, targeting physiological pathways in plants. Recent studies have shown that glyphosate can also cause toxic effects in animals. We investigated the glyphosate-based herbicide (GBH)-induced changes in potato (Solanum tuberosum) plant chemistry and the effects of a GBH on the survival rate and oxidative status of the Colorado potato beetle (Leptinotarsa decemlineata). The beetles were reared on potato plants grown in pots containing soil treated with a GBH (Roundup Gold, 450 g/l) or untreated soil (water control). The 2nd instar larvae were introduced to the potato plants and then collected in 2 phases: as 4th instar larvae and as adults. The main glycoalkaloids of the potato plants, α-solanine and α-chaconine, were measured twice during the experiment. The α-solanine was reduced in potato plants grown in GBH-treated soil, which can be detrimental to plant defenses against herbivores. GBH treatment had no effect on the survival rate or body mass of the larvae or the adult beetles. In the larvae, total glutathione (tGSH) concentration and the enzyme activity of catalase (CAT), superoxide dismutase, and glutathione-S-transferase were increased in the GBH treatment group. In the adult beetles, CAT activity and tGSH levels were affected by the interactive effect of GBH treatment and the body mass. To conclude, environmentally relevant concentrations of a GBH can affect the potato plant's glycoalkaloid concentrations, but are not likely to directly affect the survival rate of the Colorado potato beetle, but instead, modify the antioxidant defense of the beetles via diet.

Miller T, Crossley MS, Fu Z, Meier AR, Crowder DW, Snyder WE. 2020. Biological Control. 151:104403.

Prey commonly must compete with conspecifics for resources while also defending themselves against predators. Both competition and defense can reduce feeding opportunities, or otherwise strain prey energy reserves, even when the prey is not killed. This suggests that stress from competition and anti-predator defense might yield non-lethal harm that differs quantitatively rather than qualitatively. We examined this possibility for an herbivorous prey species, the Colorado potato beetle (Leptinotarsa decemlineata), feeding among differing numbers of intraspecific competitors while also avoiding predation by Nabis alternatus and Hippodamia convergens. In a field cage experiment, predators and intraspecific competitors similarly reduced the herbivores’ larval survival. Yet, only exposure to predators heightened the risk of beetles later being killed by entomopathogenic nematodes and fungi, as the beetles pupated in the soil. This suggests that the threat of predation was exerting physiological harm to the prey, although it was also possible that a tradeoff existed where those beetles most resistant to predator attack were most susceptible to pathogen infection. In a second experiment, we found that regular touching of larvae by predatory Nabis or Hippodamia, in the absence of actual predation, consistently induced predator-avoidance behaviors by the larvae (e.g., rearing, vomiting, wiggling, etc.) that might be energetically costly. Indeed, only being touched by predators, but not by a wooden dowel as a disturbance control, led to greater susceptibility to entomopathogens as pupae. Altogether, our findings suggest that non-lethal interactions with predators might carry a particular cost to beetles in heightened pathogen susceptibility, not seen when facing competitors or simple disturbance. Additional work is needed to determine the specific physiological mechanism underlying this apparent synergism in the effects of predators and entomopathogens, which spans prey life stages.

Máximo WPF, Howell JL, Mogilicherla K, Basij M, Chereddy SCRR, Palli SR. 2020. Archives of Insect Biochemistry and Physiology. 104:e21685.

The Colorado potato beetle (CPB; Leptinotarsa decemlineata) is one of the most notorious and difficult to control pests of potato and other solanaceous crops in North America. This insect has evolved a remarkable ability to detoxify both plant and synthetic toxins, allowing it to feed on solanaceous plants containing toxic alkaloids and to develop resistance to synthetic chemicals used for its control. RNA interference (RNAi) is a natural mechanism that evolved as an immune response to double-stranded RNA (dsRNA) viruses where dsRNA triggers silencing of target gene expression. RNAi is being developed as a method to control CPB. Here, we evaluated four CPB-specific genes to identify targets for RNAi-mediated control of this insect. Out of the four dsRNAs evaluated in CPB larvae and adults, dsIAP (dsRNA targeting inhibitor of apoptosis, iap gene) performed better than dsActin, dsHSP70, and dsDynamin in inducing larval mortality. However, in adults, the mortality induced by dsActin is significantly higher than the mortality induced by dsIAP, dsHSP70, and dsDynamin. Interestingly, a combination of dsIAP and dsActin performed better than either dsIAP or dsActin alone by inducing feeding inhibition in 24 hr and mortality in 48 hr in larvae. When the dsIAP and dsActin were expressed in the Escherichia coli HT115 strain and applied as a heat-killed bacterial spray on potato plants, it protected the plants from CPB damage. These studies show that the combination of dsIAP and dsActin shows promise as an insecticide to control CPB.

Lazarević J, Kostić I, Milanović S, Šešlija Jovanović D, Krnjajić S, Ćalić D, Stanković S, Kostić M. 2020.  Bulletin of Entomological Research. First View.

The Colorado potato beetle, Leptinotarsa decemlineata (Say), is one of the most destructive pest species to have developed resistance to most chemical insecticides. We determined the composition and evaluated the potential of Tanacetum parthenium L. and Tanacetum vulgare L. (Asteraceae family) essential oil (EO) application as an alternative eco-friendly control strategy against L. decemlineata. We assessed the antifeedant activity for L. decemlineata larvae and adults by estimating the damage to potato leaves treated with three concentrations of EOs dissolved in ethanol (0.125, 0.25 and 0.5%). Results showed that T. parthenium EO was more effective against larvae, and T. vulgare was more effective against adults. In an olfactometer assay, the time required to choose an untreated leaf disc did not depend on the Tanacetum species, or life stage examined. However, the concentration of EO exhibited a significant effect on the behaviour of both developmental stages. At higher EO concentrations, both third instar larvae and adults require less time to choose an untreated leaf disc. Additionally, T. parthenium EO provoked more rapid movement away from the treated leaf disc than T. vulgare, especially at the highest concentration. Successful modification of L. decemlineata behaviour by the two Tanacetum oils suggests that they possess the potential for use in potato protection.

Kang W-N, Fu K-Y, Guo W-C, Li G-Q. 2020.  Microbiology Resource Announcements. 9(25).

Bacteria of the genus Stenotrophomonas are opportunistic and have been documented in the guts of several insect species. Here, we present the complete genome sequence of S. maltophilia strain CPBW01, isolated from the wings of the Colorado potato beetle, Leptinotarsa decemlineata, collected from potato fields in Urumqi (43.71N, 87.39E), Xinjiang, China.

Kaiser N, Manrique-Carpintero NC, DiFonzo C, Coombs J, Douches D. 2020. Theoretical and Applied Genetics. 133(9):2583–2603.

Key message: A major QTL on chromosome 2 associated with leptine biosynthesis and Colorado potato beetle resistance was identified in a diploid S. chacoense F2 population using linkage mapping and bulk-segregant analysis. Abstract: We examined the genetic features underlying leptine glycoalkaloid mediated Colorado potato beetle (Leptinotarsa decemlineata) host plant resistance in a diploid F2 mapping population of 233 individuals derived from Solanum chacoense lines USDA8380-1 and M6. The presence of foliar leptine glycoalkaloids in this population segregated as a single dominant gene and displayed continuous distribution of accumulated quantity in those individuals producing the compound. Using biparental linkage mapping, a major overlapping QTL region with partial dominance effects was identified on chromosome 2 explaining 49.3% and 34.1% of the variance in Colorado potato beetle field resistance and leptine accumulation, respectively. Association of this putative resistance region on chromosome 2 was further studied in an expanded F2 population in a subsequent field season. Loci significantly associated with leptine synthesis colocalized to chromosome 2. Significant correlation between increased leptine content and decreased Colorado potato beetle defoliation suggests a single QTL on chromosome 2. Additionally, a minor QTL with overdominance effects explaining 6.2% associated with Colorado potato beetle resistance donated by susceptible parent M6 was identified on chromosome 7. Bulk segregant whole genome sequencing of the same F2 population detected QTL associated with Colorado potato beetle resistance on chromosomes 2, 4, 6, 7, and 12. Weighted gene co-expression network analysis of parental lines and resistant and susceptible F2 individuals identified a tetratricopeptide repeat containing protein with a putative regulatory function and a previously uncharacterized acetyltransferase within the QTL region on chromosome 2, possibly under the control of a regulatory Tap46 subunit within the minor QTL on chromosome 12.

Kadoić Balaško M, Mikac KM, Bažok R, Lemic D. 2020. Insects. 11(9):581

Colorado potato beetle, CPB (Leptinotarsa decemlineata Say), is one of the most important pests of the potato globally. Larvae and adults can cause complete defoliation of potato plant leaves and can lead to a large yield loss. The insect has been successfully suppressed by insecticides; however, over time, has developed resistance to insecticides from various chemical groups, and its once successful control has diminished. The number of available active chemical control substances is decreasing with the process of testing, and registering new products on the market are time-consuming and expensive, with the possibility of resistance ever present. All of these concerns have led to the search for new methods to control CPB and efficient tools to assist with the detection of resistant variants and monitoring of resistant populations. Current strategies that may aid in slowing resistance include gene silencing by RNA interference (RNAi). RNAi, besides providing an efficient tool for gene functional studies, represents a safe, efficient, and eco-friendly strategy for CPB control. Genetically modified (GM) crops that produce the toxins of Bacillus thuringiensis (Bt) have many advantages over agro-technical, mechanical, biological, and chemical measures. However, pest resistance that may occur and public acceptance of GM modified food crops are the main problems associated with Bt crops. Recent developments in the speed, cost, and accuracy of next generation sequencing are revolutionizing the discovery of single nucleotide polymorphisms (SNPs) and field of population genomics. There is a need for effective resistance monitoring programs that are capable of the early detection of resistance and successful implementation of integrated resistance management (IRM). The main focus of this review is on new technologies for CPB control (RNAi) and tools (SNPs) for detection of resistant CPB populations.