Differential response to the South American rice water weevil (Coleoptera: Curculionidae) damage of rice cultivars with contrasting levels of resistance: implications for field management

BACKGROUND

Oryzophagus oryzae (Costa Lima) (Coleoptera: Curculionidae) is a major pest of flooded rice paddies throughout South America, and species with very similar life histories are present in many rice-producing regions of the world (collectively rice water weevils, RWWs). The damage caused by RWW larvae on rice cultivars with contrasting levels of resistance ('BRS Pampa CL'= 'Pamp': susceptible, 'BRS Atalanta' = 'Atal': resistant-antibiosis) was evaluated in two consecutive years in the field under seven infestation levels: 0, 2, 4, 6, 8, 10 and 20 adult couples per cage with plants.

RESULTS

Higher densities of adults increased the number of leaf-feeding scars and larvae on roots, respectively, at the rate ≤2.14 and 2.75 in Pamp and 2.23 and 2.48 in Atal, with the total number of larvae generated being lower in Atal. Grain yield was negatively impacted with increasing larval density on the roots, mainly by decreasing root volume and the number of grains per panicle. Root injury intensity and yield loss showed a nonlinear relationship. It was found that significant yield losses occurred with 8.37 larvae per plant in Pamp and 9.80 larvae per plant in Atal CONCLUSION: The results provided the first evidence of a tolerance limit for RWW larval damage that, even though similar between cultivars of contrasting resistance, corresponds to twice the current conventional level for larval control in Brazil. We also discuss here the potential of applying tolerance limits to manage the RWW adult population in rice fields. © 2022 Society of Chemical Industry.

Insecticide-mediated effects on mating success and reproductive output of Drosophila suzukii

Exposure to sublethal concentrations can have adverse effects on certain individuals, but, can also favor survival and reproduction of others. This study aimed to evaluate the effects of exposing Drosophila suzukii (Matsumura, 1931) (Diptera: Drosophilidae) adults to LC₅₀ of spinetoram and lambda-cyhalothrin and field rate of thiamethoxam on their mating, reproductive output and longevity. The LC₅₀ for lambda-cyhalothrin and for spinetoram were 7.83 and 32.91 ng a.i./cm², respectively. Thiamethoxam, in the concentrations tested, was not toxic to D. suzukii, therefore, we use the recommended concentration for strawberry, 231.25 ng a.i./cm², in the sublethal tests. Insects exposed to LC₅₀ of lambda-cyhalothrin were more likely to mate, but presented shorter copula. Sublethal exposure to thiamethoxam lead to an increase in total fecundity. Among the insecticides tested, spinetoram presented the least expressive effects. Longevity was not affected by any of the tested insecticides. Results presented in this study are relevant to D. suzukii management since the stimulatory effects shown on this pest species can collaborate to outbreaks and insecticide resistance.

Susceptibility of Brazilian Populations of Anastrepha fraterculus, Ceratitis capitata (Diptera: Tephritidae), and Drosophila suzukii (Diptera: Drosophilidae) to Selected Insecticides

Anastrepha fraterculus (Wiedemann), Ceratitis capitata (Wiedemann), and Drosophila suzukii (Matsumura) are the main fruit pests in Brazil. Here, we evaluated the susceptibility of Brazilian populations of A. fraterculus, C. capitata, and D. suzukii to selected insecticides. In ingestion bioassays, adults from a laboratory susceptible population of each species were exposed to five different modes of action of insecticide. Then, field populations of each species were exposed to the diagnostic concentrations to evaluate possible changes in susceptibility. Our findings indicate that lambda-cyhalothrin, malathion, and spinosad had similar levels of toxicity against a susceptible laboratory population of A. fraterculus, with LC50 values of 6.34, 6.54, and 8.76 µg a.i./ml, respectively. Ceratitis capitata had similar susceptibilities to spinosad (1.30 µg a.i./ml), spinetoram (2.76 µg a.i./ml), and malathion (7.10 µg a.i./ml), but a lower susceptibility to lambda-cyhalothrin (76.55 µg a.i./ml). For D. suzukii, the LC50 values of deltamethrin (0.67 µg a.i./ml), malathion (3.30 µg a.i./mL), spinosad (4.16 µg a.i./ml), and spinetoram (4.75 µg a.i./ml) were lower than for abamectin (15.02 µg a.i./ml), acetamiprid (39.38 a.i./ml), and thiamethoxam (70.15 µg a.i./ml). The diagnostic concentrations, based on LC99 values of the insecticides, caused more than 99% mortality for most field populations of each species. For lambda-cyhalothrin the populations RS-1 (A. fraterculus) and SC (C. capitata) showed approximately 10% of live insects. These differences represent the natural variation in population susceptibility and not due to insecticide selection pressure. The diagnostic concentrations defined here should be used in future resistance monitoring programs in Brazil.

Toxicity of insecticides on Neotropical stingless bees Plebeia emerina (Friese) and Tetragonisca fiebrigi (Schwarz) (Hymenoptera: Apidae: Meliponini)

Use of pesticides in agroecosystems is considered a major cause of bees diversity losses in the Neotropics, where Plebeia emerina (Friese) and Tetragonisca fiebrigi (Schwarz) (Hymenoptera: Apidae: Meliponini) are wild pollinators of native and crop plants. The aim of this study was to know the acute lethal toxicity of acetamiprid, malathion, phosmet and spinosad insecticides on P. emerina and T. fiebrigi. We obtained the mean concentration and mean lethal dose (LC₅₀ and LD₅₀) and the mean survival of workers after oral and topical exposure to insecticides, respectively. The LC₅₀ values (ng a.i./μl of diet) and the decreasing order of toxicity for P. emerina was spinosad (4.96) > malathion (18.75) > phosmet (97.33) > acetamiprid (4204.06), and for T. fiebrigi also was spinosad (5.65) > malathion (8.39) > phosmet (53.91) > acetamiprid (9841.32), when orally exposed. The LD₅₀ values (ng a.i./bee) and the decreasing order of toxicity for P. emerina was spinosad (1.90) > malathion (10.90) > phosmet (19.54) > acetamiprid (6216.55) and for T. fiebrigi was malathion (29.29) ≥ spinosad (29.79) > phosmet (41.95) > acetamiprid (1421.23), when topically exposed. The mean survival (hours) of contaminated bees by malathion, phosmet, and spinosad, was 11.81, 7.20, and 12.32 for P. emerina and 8.55, 7.20, and 13.34 for T. fiebrigi when orally exposed; and was 4.87, 9.87 and 11.17 for P. emerina, and 4.87, 4.76, and 19.05 for T. fiebrigi when topically exposed. Malathion, phosmet, and spinosad were highly toxic, while acetamiprid was moderately toxic. Our results indicated that the insecticides tested, mainly malathion, phosmet, and spinosad may be harmful to P. emerina and T. fiebrigi, making it essential to propose measures to minimize their impact on wild pollinators.

Traps and Baits for Luring Grapholita molesta (Busck) Adults in Mating Disruption-Treated Apple Orchards

Grapholita molesta (Busck) is one of the main pests in apple crops in Brazil, where it is controlled by mating disruption (MD) with the use of the synthetic sex pheromone. However, sex-pheromone-based monitoring is not effective in MD-treated areas and may result in losses in production. This work has defined a trap model and a bait for luring G. molesta adults in MD apple orchards. The experiments were conducted in commercial apple orchards located in São Joaquim, SC, Brazil. Three trap models-McPhail, Pot, and Ajar-and three baits-grape juice (25%) (GJ), sugarcane molasses (25%) (SM), and a solution containing brown sugar (8.69%) and terpinyl acetate (0.05%) (TAS)-were assessed for luring G. molesta adults in areas subjected to the mating disruption. The assessments were performed weekly by collecting the insects caught in the traps. In addition, time needed to replace traps was also assessed, as well as the selectivity of the trap/bait set. In the laboratory, G. molesta adults were sexed, and the females were dissected to confirm reproductive status. We discuss our results and sugarcane molasses (25%) captured the least number of G. molesta adults regardless of the tested traps. The Ajar/TAS, Pot/TAS, and McPhail/GJ captured the largest number of G. molesta adults. The Ajar/TAS was the most selective and easier to handle. TAS was efficient in catching G. molesta until 14 days after preparation of the solution. Ajar/TAS has potential to be used in the monitoring of G. molesta in apple orchards.