A matrix model describing host-parasitoid population dynamics: The case of Aphelinus certus and soybean aphid

Low concentrations of selenium nanoparticles enhance the performance of a generalist parasitoid and its host, with no net effect on host suppression

BACKGROUND

The environmental and economic costs of conventional insecticides have stirred an interest in alternative management tactics, including the use of nanotechnologies. Selenium nanoparticles (SeNPs) have many applications in agriculture but may not be compatible with biological control; however, low concentrations of SeNPs may benefit natural enemies via hormesis. This study investigates the concentration-dependent effects of SeNPs (0-1000 mg L-1 ) on Anisopteromalus calandrae (Howard) (Hymenoptera: Pteromalidae), a generalist parasitoid of stored product pests.

RESULTS

The LC50 of SeNPs was 1540 mg L-1 for female parasitoids and 1164 mg L-1 for males. SeNPs had a significant hormetic effect; average lifespan increased by 10% at a concentration of 4.03 mg L-1 for females and by 35% at 13.83 mg L-1 for males. In a bioassay including hosts [the azuki bean beetle, Callosobruchus chinensis (L.) (Coleoptera: Chrysomelidae: Bruchinae)], a low concentration of SeNPs (25 mg L-1 ) enhanced the performance of female parasitoids; lifespan increased by 23% and the number of offspring increased by 88%. However, the number of emerging hosts did not significantly decrease; in the absence of parasitism, SeNPs actually improved host emergence by 17%.

CONCLUSION

Because higher concentrations of SeNPs reduced parasitoid lifespan, whereas low concentrations enhanced not only parasitoid performance but also host emergence, practitioners should exercise caution when considering SeNPs for use in integrated pest management. © 2023 Society of Chemical Industry.

Cold tolerance and overwintering survival of Aphelinus certus (Hymenoptera: Aphelinidae), a parasitoid of the soybean aphid (Hemiptera: Aphididae) in North America

Broad-spectrum insecticides are the main control measure of the invasive and economically damaging soybean aphid (Aphis glycines) in North America, although biological control by resident natural enemies can also greatly diminish population levels. One such natural enemy is the accidentally introduced Eurasian parasitoid Aphelinus certus (Hymenoptera: Aphelinidae), though its impact appears to be limited by low rates of parasitism early in the growing season. We tested the hypothesis that A. certus might experience high overwintering mortality. In the laboratory, we used thermocouple thermometry to measure the supercooling points of diapausing parasitoids and assessed parasitoid survival after exposure to ecologically relevant durations of low temperature. We found A. certus to be freeze-intolerant with a median supercooling point of -28°C. When exposed to temperatures of 0°C for up to 7 months, adults emerged only after exposures of at least 60 days and survival decreased with durations beyond 150 days. We also conducted in-field studies at sites from northern Minnesota to southern Iowa to determine if diapausing A. certus could overwinter above and below the snowpack. Survival was negatively correlated with increasing latitude and was greater for parasitoids placed on the ground than 1 meter off the ground, likely due to the warmer and stabler temperatures of the subnivean microclimate. Our results suggest that A. certus is capable of overwintering in the region inhabited by soybean aphid but may experience substantial mortality even under ideal conditions. Climate change is predicted to bring warmer, drier winters to the North American Midwest, with decreased depth and duration of snow cover, which may further reduce overwintering survival.

The Peculiarities of Metopolophium dirhodum (Walk.) Population Formation Depending on Its Clonal and Morphotypic Organization during the Summer Period

The ecological plasticity of aphid populations is determined by their clonal and morphotypic diversity. Clones will be successful when the development of their component morphotypes is optimized. The purpose of this work was to reveal the peculiarities of clonal composition and the developmental characteristics of different summer morphotypes for the rose-grass aphid, Metopolophium dirhodum (Walk.), which is an important host-alternating cereal pest and a useful model species. During the experiments, aphids were kept under ambient conditions on wheat seedlings at natural temperatures and humidity levels. An analysis of the reproduction of summer morphotypes and the resulting composition of offspring found that variation among the clones and morphotypes, as well as generational effects and an influence of sexual reproduction (and interactions between all factors) influenced the population structure of M. dirhodum. The reproduction of emigrants was less among the clones than that of the apterous or alate exules. The number of offspring produced by apterous exules differed throughout the growing season (generational effects) and between years, with different clones exhibiting different responses. There were dispersing aphids only among the offspring of apterous exules. These results can contribute to future advances in the forecasting and monitoring of aphid populations.

A model for colour preference behaviour of spring migrant aphids

Aphids are economically and ecologically important herbivorous insects. A critical step in their life cycle is the visually guided host finding behaviour. To elucidate the role of colour in host finding of aphid spring migrants we conducted large colour trap experiments in the field and analysed aphid catch data, using trap spectral reflectance data as input. Based on known and putative photoreceptor sensitivities we developed and optimized a simple empirical colour choice model for spring migrants of different aphid species which confirmed and explained the yellow preference of these insects. In a further step, we applied multivariate statistical methods to behavioural and reflectance data, but without data on photoreceptor sensitivities, to find the wavelengths of greatest importance for the aphids' behavioural responses. This analysis confirmed the position of the green photoreceptor peak previously obtained independently with electrophysiological methods. In a final step, we applied the colour preference model to a dataset of leaf spectra. This showed that aphid visual preference would be dependent on the plants' nutritional status, with lower nitrogen input being associated with stronger preference, despite known benefits of high nitrogen levels for aphid reproduction and fitness. Ecological and evolutionary implications of these results are discussed. This article is part of the theme issue 'Understanding colour vision: molecular, physiological, neuronal and behavioural studies in arthropods'.

Towards Predictions of Interaction Dynamics between Cereal Aphids and Their Natural Enemies: A Review

Simple Summary

Understanding how pests and their natural enemies interact dynamically during the growing season and what drivers act on those interactions will help to develop efficient pest control strategies. We reviewed empirical and modeling publications on the drivers influencing the aphids–natural enemy dynamics. We found disparities between what is known empirically and what is used as main drivers in the models. Predation and parasitism are rarely measured empirically but are often represented in models, while plant phenology is supposed to be a strong driver of aphids’ dynamics while it is rarely used in models. Since modelers and empirical scientists do not share a lot of publications, we incite more crossover works between both communities to elaborate (i) new empirical settings based on simulation results and (ii) build more accurate and robust models integrating more key drivers of the aphid dynamics. These models could be integrated into decision support systems to help advisors and farmers to design more effective integrated pest management systems.

Abstract

(1) Although most past studies are based on static analyses of the pest regulation drivers, evidence shows that a greater focus on the temporal dynamics of these interactions is urgently required to develop more efficient strategies. (2) Focusing on aphids, we systematically reviewed (i) empirical knowledge on the drivers influencing the dynamics of aphid–natural enemy interactions and (ii) models developed to simulate temporal or spatio-temporal aphid dynamics. (3) Reviewed studies mainly focus on the abundance dynamics of aphids and their natural enemies, and on aphid population growth rates. The dynamics of parasitism and predation are rarely measured empirically, although it is often represented in models. Temperature is mostly positively correlated with aphid population growth rates. Plant phenology and landscape effects are poorly represented in models. (4) We propose a research agenda to progress towards models and empirical knowledge usable to design effective CBC strategies. We claim that crossover works between empirical and modeling community will help design new empirical settings based on simulation results and build more accurate and robust models integrating more key drivers of aphid dynamics. Such models, turned into decision support systems, are urgently needed by farmers and advisors in order to design effective integrated pest management.

Neonicotinoids from coated seeds toxic for honeydew-feeding biological control agents

Seed coating ('seed treatment') is the leading delivery method of neonicotinoid insecticides in major crops such as soybean, wheat, cotton and maize. However, this prophylactic use of neonicotinoids is widely discussed from the standpoint of environmental costs. Growing soybean plants from neonicotinoid-coated seeds in field, we demonstrate that soybean aphids (Aphis glycines) survived the treatment, and excreted honeydew containing neonicotinoids. Biochemical analyses demonstrated that honeydew excreted by the soybean aphid contained substantial concentrations of neonicotinoids even one month after sowing of the crop. Consuming this honeydew reduced the longevity of two biological control agents of the soybean aphid, the predatory midge Aphidoletes aphidimyza and the parasitic wasp Aphelinus certus. These results have important environmental and economic implications because honeydew is the main carbohydrate source for many beneficial insects in agricultural landscapes.

Parasitoid-mediated indirect interactions between unsuitable and suitable hosts generate apparent predation in microcosm and modeling studies

Parasitoids used as biological control agents often parasitize more than a single host species and these hosts tend to vary in suitability for offspring development. The population dynamics of parasitoids and hosts may be altered by these interactions, with outcomes dependent on the levels of suitability and acceptance of both host species. Parasitism of individuals of an unsuitable host species may indirectly increase populations of a suitable host species if eggs laid into unsuitable hosts do not develop into adult parasitoids. In this case, the unsuitable host is acting as an egg sink for parasitoids and this can reduce parasitism of suitable hosts under conditions of egg limitation. We studied parasitoid-mediated indirect interactions between two aphid hosts, Aphis glycines (the soybean aphid) and A. nerii (the milkweed, or oleander aphid), sharing the parasitoid Aphelinus certus. While both of these aphid species are accepted by A. certus, soybean aphid is a much more suitable host than milkweed aphid is. We observed a drastic reduction of parasitoid offspring production (45%) on the suitable host in the presence of the unsuitable host in microcosm assays. Aphelinus certus females laid eggs into the unsuitable hosts (Aphis nerii) in the presence of the suitable host leading to egg and/or time limitation and reduced fitness. The impact of these interactions on the equilibrium population sizes of the three interacting species was analyzed using a consumer-resource modeling approach. Both the results from the laboratory experiment and the modeling approaches identified apparent predation between soybean aphid and milkweed aphid, in which milkweed aphid acts as a sink for parasitoid eggs leading to an increase in the soybean aphid population. The presence of soybean aphids had the opposite effect on milkweed aphid populations as it supported increases in parasitoid abundance and thus reduced the fitness and abundance of this aphid species.

Modeling the combined impacts of host plant resistance and biological control on the population dynamics of a major pest of wheat

BACKGROUND

Single-tool approaches often fail to provide effective long-term suppression of pest populations, such that combining several tools into an integrated management strategy is critical. Yet studies that harness the power of population models to explore the relative efficacy of various management tools and their combinations remain rare. We constructed a Leslie matrix population model to evaluate the potential of crop resistance, acting alone or in combination with biological control, to reduce populations of the wheat stem sawfly, Cephus cinctus Norton, a major pest of wheat in North America.

RESULTS

Our model projections indicated that crop resistance reduced, but did not stop, C. cinctus population growth, suggesting that implementing multiple management tools will be necessary for longer term control of this pest. The levels of parasitism needed to curtail population growth were much lower in model projections for resistant solid-stemmed compared with susceptible hollow-stemmed cultivars (22% versus 86%). Furthermore, even when accounting for the reduced levels of parasitism observed in resistant cultivars, projected population growth rates for C. cinctus were always lower in resistant compared with susceptible wheat cultivars.

CONCLUSION

Despite some empirical evidence for antagonistic interactions between resistance and biological control, our models suggest that combining these two approaches will always reduce population growth rates to lower levels than implementing either strategy alone. More work focused on integrating biological control into crop resistance breeding programs, and determining how these approaches affect performance of limiting life stages, will be important to optimize sustainable approaches to integrated pest management in this system and more broadly. Published 2020. This article is a U.S. Government work and is in the public domain in the USA.

Density-dependent lifespan and estimation of life expectancy for a parasitoid with implications for population dynamics

Parasitoid lifespan is influenced by nutrient availability, thus the lifespan of parasitoids that rely on their hosts for nutritional resources (either via host feeding or by consuming honeydew) should vary with host density. We assessed the survival and reproduction of one such species, Aphelinus certus-a parasitoid of the soybean aphid, Aphis glycines-over a range of host densities using a laboratory assay. We found a positive, asymptotic relationship between host density and the lifespan and fecundity of A. certus that was supported by a traditional survivorship analysis as well as a logistic model. Parasitoids from this assay were also used to develop a wing wear index relating setae damage to parasitoid age. This index was used to estimate the life expectancy of field-collected parasitoids, which was shorter than the life expectancy of laboratory-reared female parasitoids. Finally, host-density-dependent parasitoid lifespan was incorporated into a coupled-equations matrix population model that revealed that decreasing the degree of host density dependence leads to higher equilibrium host densities and changes in the quality of equilibrium (e.g. stable limit cycles). These results detail the relatively unstudied phenomenon of host-density-dependent parasitoid lifespan and suggest that differences between laboratory- and field-determined parasitoid life expectancy have important implications for population dynamics and the biological control of insects.