Lethal and sublethal effects of synthetic and bio-insecticides on Trichogramma brassicae parasitizing Tuta absoluta

The tomato leaf miner (TLM), Tuta absoluta (Meyrick), is an invasive tomato pest found worldwide. Sustainable control strategies aimed at increasing biological control approaches and decreasing chemical inputs are required, due to the tendency to develop insecticide resistance. In this study, the lethal and sublethal effects of four chemical insecticides (abamectin, indoxacarb, chlorantraniliprole, and spinosad) and the sublethal effects of the entomopathogenic fungus Metarhizium anisopliae (Metschnikoff) on a widespread TLM egg parasitoid, Trichogramma brassicae Bezdenko, were estimated. Concentration mortality response bioassays enabled the estimation of lethal concentrations of the tested insecticides for the parasitoids, with chlorantraniliprole having the lowest LC50 and indoxacarb the highest. The LC25 and LC50 of the tested insecticides on the TLM were sprayed on eggs and then offered at three time intervals to the parasitoids. The fertility and other life table parameters of the individuals emerging from the treated eggs were estimated. All of the chemical insecticides, but not the fungus, had harmful effects on T. brassicae. The insecticide applications caused a 3.84–5.17 times reduction in the net reproductive rate (R0) compared with the control. No parameters were affected by spraying the fungus in the 0h treatment, but effects were recorded at 24 and/or 48h, except for the gross reproduction rate (GRR). The value of the intrinsic rate of increase (rm) also decreased to 0.528–0.617 after the insecticide treatments. The doubling time (DT) increased in all treatments compared to the control. Nevertheless, the generation time (T) was only very slightly affected. In addition, in the combination experiments, M. anisopliae showed a remarkable synergism with T. brassicae in controlling TLM eggs. These results indicate that low levels of lethal effects on key biological control agents should be considered in the choice of insecticides to be included in sustainable TLM control packages.

Rapid molecular identification of Trichogramma (Hymenoptera: Trichogrammatidae) parasitizing the eggs of the European corn borer, Ostrinia nubilalis (Lepidoptera: Crambidae) in Serbia

Background The European corn borer (ECB), Ostrinia nubilalis (Hübner, 1796) (Lepidoptera: Crambidae), is the major pest of maize (Zea mays Linnaeus, 1753) in Serbia. One potential method for managing this pest is the augmentative release of naturally occuring egg parasitoids of the genus Trichogramma. The first step in this process is accurately identifying the naturally occuring species and estimating their natural distribution and abundance. Molecular identification, based on differences in DNA sequences, has commonly been employed for the identification of Trichogramma species. A simple, quick, and accurate molecular assay is urgently required for the identification of two common Trichogramma species, associated with ECB in Serbia: T. brassicae Bezdenko, 1968 and Trichogramma evanescens Westwood, 1833. Such an assay will facilitate an expansive survey of resident populations of Trichogramma associated with ECB across agricultural growing regions of Vojvodina province. Results A species-specific multiplex PCR assay for the 2 species was developed and validated that assay using a sample of 79 parasitoid wasps reared from ECB egg masses collected from sample sites across Vojvodina province. Trichogramma brassicae was confirmed as the dominant egg parasitoid of ECB in this region, accounting for 77 of the 79 wasps (97.47%). The remaining 2 were confirmed as T. evanescens. Trichogramma brassicae was detected at all 12 sample sites, while T. evanescens was detected at only 2 plots, Mokrin and Nakovo. Conclusions The species-specific multiplex PCR assay presented herein can provide the basis of a quick, cheap, and reliable means for identifying the species of Trichogramma that parasitize ECB egg masses in Serbia. Two currently documented species, T. brassicae and T. evanescens, are readily diagnosed by the size of the PCR product they produce in the assay. Any additional species are expected to not produce a band of a diagnostic size. Such species would subsequently be identified by sequencing, which may also allow them to be promptly incorporated into a revised assay.

Wolbachia manipulate fitness benefits of olfactory associative learning in a parasitoid wasp

Upon encountering a host, a female parasitoid wasp has to decide whether to learn positive or negative cues related to a host. The optimal female decision will depend on the fitness costs and benefits of learned stimuli. Reward quality is positively related to the rate of behavioral acquisition in processes such as associative learning. Wolbachia, an endosymbiotic bacterium, often plays an impressive role in the manipulation of its arthropod host's biology. Here we studied the responses of two natural Wolbachia infected/uninfected Trichogramma brassicae populations to theoretically high- and low- reward values during a conditioning process and the consequences of their responses in terms of memory duration. According to our results, uninfected wasps showed an attraction response to high value rewards, but showed aversive learning in response to low value rewards. Memory span of uninfected wasps after conditioning by low-value rewards was significantly shorter compared to high-value rewards. As our results revealed, responses to high quality hosts will bring more benefits (bigger size, increased fecundity and enhanced survival) compared to low-quality hosts for uninfected wasps. Infected wasps were attracted to conditioned stimuli with the same memory duration after conditioning by both types of hosts. This was linked to the fact that parasitoids emerging from both types of hosts present the same life-history traits. Therefore, these hosts represent the same quality reward for infected wasps. According to obtained results it can be concluded that Wolbachia manipulates the learning ability of its host resulting in the wasp responding to all reward values similarly.

Landscape connectivity alters the evolution of density-dependent dispersal during pushed range expansions

As human influence reshapes communities worldwide, many species expand or shift their ranges as a result, with extensive consequences across levels of biological organization. Range expansions can be ranked on a continuum going from pulled dynamics, in which low-density edge populations provide the "fuel" for the advance, to pushed dynamics in which high-density rear populations "push" the expansion forward. While theory suggests that evolution by spatial sorting, a common feature of range expansions, could lead pushed expansions to become pulled with time, empirical comparisons of phenotypic divergence in pushed vs. pulled contexts are lacking. In a previous experiment using Trichogramma brassicae wasps as a model, we showed that expansions were more pushed when connectivity was lower. Here we used descendants from these experimental landscapes to look at how the range expansion process and connectivity interact to shape phenotypic evolution. Interestingly, we found no clear and consistent phenotypic shifts, whether along expansion gradients or between treatments, when we focused on low-density trait expression. However, we found evidence of changes in density-dependence, in particular regarding dispersal: populations went from positive to negative density-dependent dispersal at the expansion edge, but only when connectivity was high. As positive density-dependent dispersal leads to pushed expansions, our results confirm predictions that evolution during range expansions may lead pushed expansions to become pulled, but add nuance by showing environmental context may slow down or cancel this process. This shows we need to jointly consider evolution and ecological context to accurately predict range expansion dynamics and their consequences.

Integration of resistant variety and biological agent to control tomato leaf miner, Tuta absoluta (Meyrick), under greenhouse conditions

Tuta absoluta (Meyrick) is considered as a major pest of tomato worldwide that causes significant losses in the crop production. This study aimed to evaluate integration of two effective and environmentally safe methods (host plant resistant and biological control) for sustainable management of the pest under greenhouse conditions. The experiment was conducted based on the factorial design with ten replicates under greenhouse conditions (22 ± 3°C, 50 ± 10 RH and 14 L:10 D photoperiod). Infestation to T. absoluta was conducted at the first-flowering stage of the plants by introducing a pair of newly emerged adults (one female and one male) per plant. Ten days later, the biological agent, Trichogramma brassicae, was released on the treatments by hanging a card contained 50 parasitized eggs in each replicate. Observation was performed weekly on ratio of infested leaves per plant (%), number of larvae/plant, number of mines/leaf and ratio of infested fruits/plant (%). Results indicated that the susceptible variety alone (Izmir) supported the highest ratio of infested leaves (42.92 ± 1.95%), number of larvae/plant (12.86 ± 0.71), number of mines/leaf (1.29 ± 0.07) and infested fruits/plant (18.8 ± 1.10%), whereas the lowest (6.12 ± 0.42%, 1.85 ± 0.13, 0.18 ± 0.02 and 0.12 ± 0.06%, respectively) were observed in combined resistant variety (Cherry) and parasitoid released treatment. Integration of these methods not only decreases damage caused on tomato leaflets and fruits, but also reduces insecticide applications which are adversely impact human health and environment.

Lethal and sublethal effects of synthetic and bio-insecticides on Trichogramma brassicae parasitizing Tuta absoluta

The invasive tomato leaf miner (TLM), Tuta absoluta (Meyrick) is an invasive pest on tomatoes worldwide. The main control measure against the pest has been chemical insecticides, but the pest developed resistance to many chemical classes. So alternative methods, such as biological control agents, alone or combined to chemical compounds must be evaluated to validate their synergistic actions. In this study, both lethal (concentration-mortality response) and sublethal effects of three synthetic insecticides, the bioinsecticide spinosad, as well as the entomopathogenic fungus Metarhizium anisopliae (Metschnikoff) Sorokin were studied on Trichogramma brassicae Bezdenko within T. absoluta eggs. To assess the sublethal effects, the lethal concentration 25% (LC25) of chlorantraniliprole, spinosad, abamectin and indoxacarb and LC50 value of M. anisopliae was sprayed on eggs and then offered at three time intervals to the parasitoids. Fertility and other life table parameters of the individuals emerged from treated eggs were estimated. The results showed that indoxacarb showed the highest deleterious sublethal effects on T. brassicae. On the other hand, M. anisopliae was the safest treatment to combine to Trichogramma with no significant effect on some parameters. The lowest LC50 value for T. brassicae was obtained for chlorantraniliprole followed by spinosad. Synergistic effect was observed when M. anisopliae and T. brassicae used together. Hence, this will be a promising integration against T. absoluta.

Hybrid Genome Assembly and Evidence-Based Annotation of the Egg Parasitoid and Biological Control Agent Trichogramma brassicae

Trichogramma brassicae (Bezdenko) are egg parasitoids that are used throughout the world as biological control agents and in laboratories as model species. Despite this ubiquity, few genetic resources exist beyond COI, ITS2, and RAPD markers. Aided by a Wolbachia infection, a wild-caught strain from Germany was reared for low heterozygosity and sequenced in a hybrid de novo strategy, after which several assembling strategies were evaluated. The best assembly, derived from a DBG2OLC-based pipeline, yielded a genome of 235 Mbp made up of 1,572 contigs with an N50 of 556,663 bp. Following a rigorous ab initio-, homology-, and evidence-based annotation, 16,905 genes were annotated and functionally described. As an example of the utility of the genome, a simple ortholog cluster analysis was performed with sister species T. pretiosum, revealing over 6000 shared clusters and under 400 clusters unique to each species. The genome and transcriptome presented here provides an essential resource for comparative genomics of the commercially relevant genus Trichogramma, but also for research into molecular evolution, ecology, and breeding of T. brassicae.