Oviposition patterns of primary lepidopteran defoliators in soybean and the impact on structured refuge recommendations.

BACKGROUND: Transgenic Bt technology in soybean, with plants expressing Cry1Ac, has been adopted as an insect pest management tool. It was first adopted in large areas of South America and Asia in 2013. The risk of resistance in target pests to this technology demands insect resistance management (IRM) programs. In Brazil, a structured refuge (area of non-Bt soybean) planted adjacent to the Bt soybean crop has been an important IRM recommendation, particularly for the primary lepidopteran defoliators Anticarsia gemmatalis (Lepidoptera: Erebidae) and Chrysodeixis includens (Lepidoptera: Noctuidae). The overall goal of this study was to validate IRM recommendations to Bt soybean. The objectives were to document the impact of soybean phenology, cultivar choice and non-Bt soybean defoliation on moth oviposition. In addition, a mark-release-recapture study estimated the dispersal capacity of these species. Five field experiments per species were performed for 3 years. RESULTS: Our results revealed an increase in A. gemmatalis and C. includens oviposition, respectively, on Bt plants as a consequence of the difference in plant growth stage at the time of oviposition. Defoliation of non-Bt plants significantly increased the oviposition preference of both moth species for Bt plants. The mark-release-recapture experiment indicated an average dispersal distance of ~300 m from the release point for A. gemmatalis, with maximum recapture at 1000 m. CONCLUSION: Overall, our findings emphasize the importance of planting synchronization of Bt soybean and the structured refuge. In addition, when operational aspects in large soybean areas challenge this recommendation, the priority should be for planting the refuge area first. This approach will minimize the impact of selective oviposition of A. gemmatalis and C. includens. © 2024 Society of Chemical Industry.

Emergence Response of Telenomus podisi (Hymenoptera: Scelionidae) to Field Temperature Originated from Colonies Reared at Different Temperature Regimes.

We studied the impact of field temperature on the emergence of Telenomus podisi Ashmead (Hymenoptera: Scelionidae) after being released as either protected (encapsulated) or unprotected pupae close to adult emergence. Two independent bioassays (protected pupae vs. unprotected pupae) were carried out in a randomized block experimental design in a 2 (parasitoid rearing temperature regimes) ×4 (release positions within plant canopy) factorial scheme. One of the T. podisi colonies was reared at a constant temperature (25 °C) and 12:12 h L:D (Light: Dark) photoperiod. The second T. podisi colony was reared at fluctuating temperatures (20 °C during 12 h D and 30 °C during 12 h of L). Each bioassay had five replicates. Each replicate contained 40 pupae from each rearing temperature and release position within plant canopy. Parasitoid pupae were released in the bottom third, middle third, or upper third of the plant's canopy, as well as between planting lines. Each bioassay was repeated six times (in different plant developmental stages: R1, R2, R3, R4, R5 and R6). There was no difference in emergence of parasitoids between colonies reared at different temperatures after exposure to field conditions. Emergence of T. podisi was reduced after pupae exposure to temperatures above 35 °C in the field, regardless of the rearing temperature. Telenomus podisi tolerates temperature fluctuations for emergence as long as they fit the optimal range (20 to 30 °C).

Release strategies of Telenomus podisi for control of Euschistus heros: a computational modeling approach.

BACKGROUND: The egg-parasitoid wasp Telenomus podisi has received attention as a biological-control agent for one of the most important soybean pests in Brazil, the stink bug Euschistus heros. As yet, no studies have conclusively established strategies for the release of T. podisi. We developed a computational model using cellular automata in the C programming language to investigate release strategies for T. podisi in soybean crops, in order to optimize the use of these wasps in managing E. heros, assuming a two-dimensional grid of cells corresponding to a soybean field. RESULTS: The release strategies capable of maintaining an E. heros population below the Economic Threshold level involved releasing a total of at least 15 000 female parasitoids per hectare, in three or four releases of 5000 or more (equivalent to approximately 7142 or more male and female parasitoids per hectare, assuming a sex ratio of 0.70). A 25-m spacing between release points or strips was indicated. The model is very sensitive to the variation in the number of parasitoids per release and in the number of releases, but little sensitive to the release mode and spacing values. CONCLUSION: The theoretical results produced by the computational model are expected to guide future field studies to improve T. podisi release plans for managing E. heros in soybeans. Therefore, we recommend the release strategy of three to four releases of 5000 or more female parasitoids per hectare, at points or strips spaced 25 m apart, to be tested in field experiments for proper implementation by producers. © 2022 Society of Chemical Industry.

Impact of Environmental Variables on Parasitism and Emergence of Trichogramma pretiosum, Telenomus remus and Telenomus podisi.

Environmental variables may markedly influence egg parasitoid performance and must be considered when choosing the best release strategy. Thus, this study aimed to investigate the effects of light, temperature, soil moisture, and precipitation on the emergence and parasitism of parasitoid releases of unprotected and encapsulated pupae. The presence of light favored the parasitism of Trichogramma pretiosum Riley, 1879, and Telenomus remus (Nixon, 1937) but did not impact the parasitism of Telenomus podisi (Ashmead, 1893). The release strategy adopted (release of encapsulated or unprotected pupae) also impacted the results. On one hand, card capsules gave protection against rainfall, limited to 10 mm regarding Tr. pretiosum, while on the other hand, card capsules led to a reduction in the number of captured adults for Te. podisi at 25°C and 30°C. Therefore, Tr. pretiosum, Te. remus, and Te. podisi can be released using both encapsulated and unprotected pupae, with advantages and disadvantages for each strategy, depending on each studied environmental variable. In addition, parasitoid pupae should be released so that the majority emerge during daylight, especially for Te. remus and Tr. pretiosum, since parasitism was greatly reduced in dark environments. Telenomus podisi is not affected by this variable as it demonstrated similar parasitism in light and dark environments.

Are economic thresholds for IPM decisions the same for low LAI soybean cultivars in Brazil?

BACKGROUND: Economic thresholds (ETs) are well-established for defoliation of soybean, Glycine max, and have been updated for many of the newer cultivars; however, there is increasing grower adoption of cultivars with a reduced leaf area index (LAI). It is of theoretical and practical interest to determine low LAI cultivar tolerance to defoliation. We conducted experiments during two consecutive crop seasons (2017/2018 and 2018/2019) using three soybean cultivars (NS 5959 IPRO, NS 5445 IPRO, and DON MARIO 5.8i) and three defoliation levels (0%, 16.7%, and 33.3%) to evaluate the tolerance of reduced LAI soybean cultivars under different defoliation levels. RESULTS: We observed differences among cultivar's LAI during plant development during both years. Soybean LAI was reduced with increasing defoliation intensity. Tested continuous defoliation levels from plant development stages of V2 to R6 reduced the weight of 1000 seeds and yield but did not impact oil or protein content. CONCLUSIONS: Despite our findings that current ET for defoliators in soybean (30% defoliation during vegetative stage and 15% defoliation during reproductive stage) are valid, it is important to consider that continuous defoliation injury impacts the capacity of the plant to respond to injury and must be further evaluated for ET refinement in future research. © 2020 Society of Chemical Industry.

Yield Response of Determinate Chipping Potato to Artificial Defoliation.

The relationship between canopy defoliation by insects and yield loss in potato (Solanum tuberosum L.) has been extensively researched. However, detailed analysis of varieties with different properties and examination of methods of defoliation are still lacking. Absence of research results has led many producers to use a very conservative economic threshold (ET) of 10% defoliation in the management of insect defoliators. A series of leaf removal experiments was conducted on a determinate chipping variety of potato at different plant stages. In 2002, the highest levels of defoliation induced upon the plant canopy were 50, 30, and 75% for tuber initiation, full bloom, and plant maturity stages, respectively. In 2003, defoliation levels were increased to 70, 80, and 90% for tuber initiation, full bloom, and plant maturity, respectively. Total yield was unaffected, and no significant changes in market sized yield, or number of marketable tubers were observed at any growth stage in either year. Because the determinate variety showed remarkable ability to recover from substantial defoliation, two different methods of defoliation: hand cut (cutting the edge of leaves with scissors while avoiding the midrib) and hole punching (hole-punching leaf tissue [15.08-mm diameter] while avoiding midrib) were tested. Although total yield was unaffected, marketable yield and small potato yield were reduced, Thus, although defoliation methods must be considered, a 10% defoliation ET is exceedingly conservative and an ET of 60% for all stages of determinate chipping potato could be adopted by growers.

Euschistus heros (Hemiptera: Pentatomidae) Fitness on Artificial Diets: An Approach to Optimize Mass Rearing of Telenomus podisi (Hymenoptera: Platygastridae) for Augmentative Biological Control.

In order to optimize the mass rearing of Telenomus podisi Ashmead (Hymenoptera: Platygastridae) for augmentative biological control we tested the suitability of different artificial diets with respect to egg production of Euschistus heros (F.) (Hemiptera: Pentatomidae). For this purpose, biological parameters of E. heros and T. podisi were evaluated in experimental trial with these diets. All tested artificial diets supported the rearing of E. heros. However, female and adult life history traits including longevity, total oviposition, nymph developmental time, and fertility were altered, with poorer results than under the natural diet. However, because of the higher economic cost of the natural diet, its replacement with artificial diets could be beneficial. Since stink bug reared on artificial diet has shorter lifespan, an alternative to overcome this issue is to replace earlier the insects in the insect facility what could still be cheaper than insects reared on natural diet. It was also noted the possibility of using artificial diets without lyophilized components what can further reduce the cost of rearing.

Cost of Production of Telenomus remus (Hymenoptera: Platygastridae) Grown in Natural and Alternative Hosts.

The study of the technical and economic aspects of rearing natural enemies is essential for its effective use as a biological control agent in the field. The aim of this study was to determine the cost of production of the parasitoid, Telenomus remus (Nixon; Hymenoptera: Platygastridae), reared in eggs of its natural host, Spodoptera frugiperda (J. E. Smith; Lepidoptera: Noctuidae), and the alternative host, Corcyra cephalonica (Stainton; Lepidoptera: Pyralidae). The insects were reared in accordance with their respective methodologies of laboratory rearing. The cost of production of this biocontrol agent is US$ 0.0004 when reared with S. frugiperda eggs and US$ 0.0002 with C. cephalonica eggs.

Biological characteristics of black armyworm Spodoptera cosmioides on genetically modified soybean and corn crops that express insecticide Cry proteins

ABSTRACT This study aimed to evaluate the development and reproduction of the black armyworm, Spodoptera cosmioides when larvae fed on leaves of Bt-corn hybrids, expressing a single Cry1F and also Cry1F, Cry1A.105 and Cry2Ab2 in pyramided corn and their non-Bt-isoline (hybrid 2B688), as well as on leaves of two soybean isolines expressing the Cry1Ac protein and its non-Bt isoline (A5547-227). We also assessed the effect of these Bt and non-Bt plants on the leaf consumption rate of S. cosmioides larvae. This pest was unable to develop when fed on any of the corn isolines (Bt and non-Bt). When both 1st and 3rd instar larvae were fed on corn leaf, mortality was 100% in both Bt and non-Bt corn. In contrast, when corn leaves were offered to 5th instar larvae, there were survivors. Defoliation and leaf consumption was higher with non-Bt corn than with both of the Bt corn isolines. There was no negative effect of Bt soybean leaves on the development and reproduction of S. cosmioides with respect to all evaluated parameters. Our study indicates that both Bt and non-Bt corn adversely affect the development of S. cosmioides while Bt soybean did not affect its biology, suggesting that this lepidopteran has major potential to become an important pest in Bt soybean crops.