Woolly apple aphid Eriosoma lanigerum Hausmann ecology and its relationship with climatic variables and natural enemies in Mediterranean areas

Implications of cell wall immunocytochemical profiles on the structural and functional traits of root and stem galls induced by Eriosoma lanigerum on Malus domestica

Alterations in cell wall composition imply in new structural and functional traits in gall developmental sites, even when the inducer is a sucking exophytophagous insect with strict feeding sites as the aphid associated to Malus domestica Borkh. This host plant is an economically important, fruit-bearing species, susceptible to gall induction by the sucking aphid Eriosoma lanigerum Hausmann, 1802. Herein, the immunocytochemical detection of arabinogalactan-proteins (AGPs), pectins, and hemicelluloses using monoclonal antibodies was performed in samples of non-galled roots and stems, and of root and stem galls on M. domestica. The dynamics of these cell wall components was discussed under the structural and functional traits of the galls proximal, median, and distal regions, according to the proximity of E. lanigerum colony feeding site. In the proximal region, the epitopes of AGPs and homogalacturonans (HGs) are related to cell growth and divisions, which result in the overproduction of parenchyma cells both in root and stem galls. In the proximal and median regions, the co-occurrence of HGs and arabinans in the cell walls of parenchyma and secondary tissues favors the nutrient flow and water-holding capacity, while the xylogalacturonans and hemicelluloses may function as additional carbohydrate resources to E. lanigerum. The immunocytochemical profile of the cell walls support the feeding activity of E. lanigerum mainly in the gall proximal region. The similarity of the cell wall components of the gall distal region and the non-galled portions, both in roots and stems, relates to the decrease of the cecidogenetic field the more distant the E. lanigerum colony is.

Lethal and behavioural toxicity of differently aged insecticide residues on European earwigs (Forficula auricularia) in the laboratory and in the field

Pesticide residues are an important topic in many environmental studies, but little is known about the effects of pesticide residues of different ages on beneficial arthropods. Therefore, in this study the activity of residues of three different ages of several commonly used insecticides on the behaviour and mortality of European earwigs was evaluated in the laboratory and the effect of the insecticides was verified in the field. All residues of the biorational SpinTor® (spinosad), Radiant SC (spinetoram), and the conventional Steward® (indoxacarb) showed significantly faster mortality progression compared to the control in the laboratory. All the Steward® residues caused a significant wave of erratic behaviour as the earwigs went through a deep and relatively long moribund stage and resurrected. We verified the effects of SpinTor® and Steward® on changes in earwig abundance and their behaviour in the orchard. Earwigs abundance was significantly lower 16 days after application of biorational SpinTor® and conventional Steward® which had a significantly more pronounced and longer lasting effect. The earwig population stabilised by day 53 after the insecticide applications. Field applications of insecticides had no effect on earwig behaviour and sex ratio. Our results show that older residues have a negative effect on European earwigs equal to that of fresh residues, although their degradation should occur. It is interesting to note that biorational insecticides may not be safer than chemical insecticides.

Earwig Releases Provide Accumulative Biological Control of the Woolly Apple Aphid over the Years

Nature-based solutions, such as biological control, can strongly contribute to reducing the use of plant protection products. In our study, we assessed the effect of augmentative releases of the European earwig (Forficula auricularia) to control the woolly apple aphid (Eriosoma lanigerum), a worldwide pest that causes serious damage to apple trees. The trials were carried out in two organic apple orchards located in Catalonia (NE Spain) from 2017 to 2020. Two treatments were compared: with vs. without earwig release. For the treatment, 30 earwigs per tree were released by means of a corrugated cardboard shelter. These releases were performed once per season and were repeated every year. We periodically assessed the length of the woolly apple aphid colonies, the number of colonies per tree, the percentage of aphids parasitized by Aphelinus mali, and the number of earwigs per shelter. Our results showed that earwig releases reduced the length of the colonies, but this effect was noticeable only for the second year onwards. Moreover, we found that those releases were compatible with A. mali. Overall, we demonstrated the positive impact of earwig releases on the woolly apple aphid control and the importance of considering time on augmentative biological control strategies.

Differences in morphology, mitochondrial genomes, and reproductive compatibility between two clades of parasitic wasps Aphelinus mali (Hymenoptera: Aphelindae) in China

Aphelinus mali (Haldeman) (Hymenoptera: Aphelinidae) in China is comprised of two clades (termed, the Shandong and Liaoning clades). In order to clarify the genetic relationship between these two clades, we compared and analyzed the morphological characteristics and the mitochondrial genome of each, and performed a hybridization experiment. Morphological results showed that both males and females of the Liaoning clade were larger than Shandong clade, in terms of whole body, abdominal, wing and antennal lengths, however, there were no significant differences between clades for total length of the middle or hind leg of females. The length of the mitochondrial genome of the Shandong clade was 14415 bp and, for the Liaoning clade, it was 14804 bp. Each contained 31 genes, including 13 protein-encoded genes, 16 tRNA genes, and 2 rRNA genes. The highest AT level among the 13 protein-coding genes for the two clades were the same gene (ATP8) (Shandong clade, 91.52%; Liaoning clade, 90.91%). By hybridization and backcrossing, we found that there was no cross incompatibility between these two clades of A. mali. Our results indicate that the historic geographical isolation between these clades has not yet caused reproductive isolation of these populations, and they belong to the same species.

Efficacy of RNA interference using nanocarrier-based transdermal dsRNA delivery system in the woolly apple aphid, Eriosoma lanigerum

RNA interference (RNAi) is an essential approach for studying gene function and has been considered as a promising strategy for pest control. However, RNAi method has not been conducted in Woolly apple aphid (Eriosoma lanigerum Hausmann), one of the most damaging apple pests in the world. In the study, we investigated the efficacy of RNAi of V-ATPase subunit D (ATPD), an efficacious target for RNAi in other insects, in E. lanigerum by a transdermal double-stranded RNA (dsRNA) delivery system with nanocarriers. Our results showed although topical application of dsATPD in E. lanigerum for 24 h produced 40.5% gene silencing, the additional help of nanocarriers extremely improved the interference efficiency with 98.5% gene silencing. Moreover, a 55.75% mortality was observed 5 days after topical application of nanocarriers and dsATPD, relative to the control (topical application of nanocarriers and double-stranded green fluorescent protein [dsGFP]). The nanocarrier-based transdermal dsRNA delivery system will promote the development of functional analysis of vital genes and also provide a potential target for RNAi-based management of E. lanigerum.

Selection of insectary plants for the conservation of biological control agents of aphids and thrips in fruit orchards

This study evaluated the potential of flowering plant species naturally occurring to promote the conservation and early establishment of key natural enemies of aphids and thrips in apple and peach orchards. Flowering plants present in the North East of Spain, a main fruit production area in Europe, were sampled to determine their flowering period and to identify potential natural enemies present on each plant species. Thirty-six plant species were found blooming from early March to late May and provided an array of flowers that might ensure food resources for natural enemies. Among them, six species - Eruca vesicaria (L.) Cav., Cardaria draba (L.) Desv., Euphorbia serrata (L.) S.G. Gmel., Malva sylvestris L., Anacyclus clavatus (Desf.) Pers. and Diplotaxis erucoides (L.) DC. - hosted a high diversity of potential natural enemies of aphids and thrips. Their blooming started early in the season and lasted for several sampling weeks and they were widely distributed. Moreover, they had available nectar even in those species with protected nectaries. Therefore, these plant species can be considered as promising candidates for inclusion in the ecological infrastructure designed for fruit orchards in the study area to promote the conservation of the biological control agents of aphids and thrips.

Towards a Knowledge-Based Decision Support System for Integrated Control of Woolly Apple Aphid, Eriosoma lanigerum, with Maximal Biological Suppression by the Parasitoid Aphelinus mali

Simple Summary

The woolly apple aphid Eriosoma lanigerum is an important pest in apple orchards worldwide. At present, effective limitation of woolly aphid populations relies on a good synergy between chemical control treatments and biological suppression by beneficial insects, especially by its main specific natural enemy, the parasitic wasp (parasitoid) Aphelinus mali. In order to reach maximum control levels on woolly apple aphids and avoid negative side effects on A. mali, decision support for the optimal timing and positioning of control treatments is needed. In this study, we developed prediction models that based on the weather conditions (temperature data) can reasonably accurately predict crucial development/activity phases of both insects in the orchard. These prediction models can be utilized to target insecticide sprayings at the most sensitive stage of the pest (woolly apple aphids) and/or to avoid insecticide sprayings with detrimental side effects at the vulnerable stage of the beneficial insect (parasitoid A. mali), as was demonstrated by the outcomes of a field trial in this study.

Abstract

The woolly apple aphid Eriosoma lanigerum (Homoptera: Aphidiae) is an important pest in apple orchards worldwide. Since the withdrawal or restricted use of certain broad-spectrum insecticides, E. lanigerum has become one of the most severe pests in apple growing areas across Western Europe. At present, effective limitation of woolly aphid populations relies on a good synergy between chemical control treatments and biological suppression by beneficial arthropods, especially by its main specific natural enemy, the parasitoid Aphelinus mali (Hymenoptera: Aphelinidae). To develop a knowledge-based decision support system, detailed monitoring data of both species were collected in the field (region of Sint-Truiden, Belgium) for a period of ten years (2010–2020). Aphelinus mali flights were monitored in the field, starting before flowering until the end of the second-generation flight at minimum. The seasonal occurrence of the most important management stages of E. lanigerum, e.g., start of wool production or activity on aerial parts in spring and migration of crawlers from colonies towards flower clusters or shoots, were thoroughly monitored. All obtained data were compared with historical and literature data and analysed in a population dynamics phenological model. Our outcomes showed that the emergence of first-generation A. mali adults (critical for the first parasitation activity and the basis for following A. mali generations in the continuation of the season) can be accurately predicted by the developed model. Hence, this information can be utilized to avoid insecticide sprayings with detrimental side effects at this particular moment as demonstrated by the outcomes of a field trial. In addition, the start of migration of E. lanigerum crawlers towards flower clusters or shoots is accurately predicted by the model. In conclusion, our results demonstrate that the model can be used as decision support system for the optimal timing of control treatments in order to achieve effective control of E. lanigerum with maximal biological suppression by its main natural enemy.

The Contribution of Surrounding Margins in the Promotion of Natural Enemies in Mediterranean Apple Orchards

(1) Habitat management can enhance beneficial arthropod populations and provide ecosystem services such as biological control. However, the implementation of ecological infrastructures inside orchards has a number of practical limitations. Therefore, planting/growing insectary plants in the margins of orchards should be considered as an alternative approach. (2) Here, we assessed the efficacy of a flower margin composed by four insectary plant species (Achillea millefolium, Lobularia maritima, Moricandia arvensis and Sinapis alba), which was placed on an edge of four Mediterranean apple orchards to attract natural enemies of two apple tree aphids (Dysaphis plantaginea and Eriosoma lanigerum). We also characterized the natural enemies present in the aphid colonies. (3) Our results show that the implementation of a flower margin at the edge of apple orchards attracts predators (Syrphidae, Thysanoptera, Araneae, Heteroptera, Coleoptera) and parasitoids. Parasitoids are the main natural enemies present in aphid colonies in our area. (4) The implementation of the flower margins successfully recruited natural enemy populations, and the presence of parasitoids in the surroundings of the orchards increased the parasitism of D. plantaginea colonies.

Effects of restricting movement between root and canopy populations of woolly apple aphid

Movement of insect pests between spatially subdivided populations can allow them to recolonize areas where local extinction has occurred, increasing pest persistence. Populations of woolly apple aphid (Eriosoma lanigerum [Hausmann]; Hemiptera: Aphididae), a worldwide pest of apple (Malus domestica [Borkhausen]), occur both below- and aboveground. These spatially subdivided subpopulations encounter different abiotic conditions, natural enemies, and control tactics. Restricting movement between them might be an effective management tactic to decrease woolly apple aphid persistence and abundance. We examined this possibility in the field, using sticky barriers to restrict upward woolly apple aphid movement to tree canopies, and in the greenhouse, using mulches and sand amendments to restrict downward movement to roots. In the field, blocking aphid movement up tree trunks did not decrease the number of colonies in tree canopies. Instead, sticky-banded apple trees had higher aphid colony counts late in the study. Earwigs, which are woolly apple aphid predators, were excluded from tree canopies by sticky bands. In the greenhouse, fewer root galls (indicative of aphid feeding) occurred on trees in sandy potting media and on those with mulch (wood chips or paper slurry). Our results suggest that upward movement is less important than other factors that affect aboveground aerial woolly apple aphid population dynamics. In addition, apple orchards planted in sandier soils or with mulches may be partially protected from woolly apple aphid root feeding.

Laboratory comparison of two Aphelinus mali clades for control of woolly apple aphid from Hebei Province, China

Aphelinus mali (Haldeman) is an effective natural enemy of woolly apple aphid (WAA), Eriosoma lanigerum (Hausmann). Previous studies have found that, with WAA from Shandong Province (Qingdao) as the host, there are significant differences in various biological characteristics between a Shandong clade and Liaoning clade of A. mali. The ability of the Shandong clade to control this aphid was significantly higher than that of the Liaoning clade in Shandong Province. In order to determine whether differences were caused by better adaptation of the Shandong parasitoid clade to the population of the host in that province or if it represents a more general fitness of this clade to control the host regardless of location, we compared the same parasitoid clades with hosts from Hebei Province. We found no significant differences in the developmental threshold temperature, effective accumulated temperature, fecundity, longevity, and oviposition period of the two clades, but the duration of host searching of the Shandong clade was significantly longer than that of the Liaoning clade. The instantaneous attack rate, the control ability (a/Th), the search parameter (Q) of the Shandong clade (0.0946, 0.543, 0.0725) of A. mali were higher than that of the Liaoning clade (0.0713, 0.382, 0.0381), and therefore, with WAA from Hebei Province as the host, the host adaptability of the Shandong clade of A. mali was not worse than that of the Liaoning clade, while the pest control ability of the Shandong clade was still greater than that of the Liaoning clade.

Relative Efficacy of Two Clades of Aphelinus mali (Hymenoptera: Aphelinidae) for Control of Woolly Apple Aphid (Hemiptera: Aphididae) in China

Aphelinus mali (Haldeman) is an effective natural enemy used in China to control woolly apple aphid. Previous studies have found that populations of A. mali in China fall into two distinct genetic clades. We found that one of these, the Shandong clade, is more effective as a biological control agent than the Liaoning clade. The control ability of the Shandong clade of A. mali (a/Th = 2.447) was found to be stronger than that of the Liaoning clade (a/Th = 1.278); the search parameter (Q) and the mutual interference parameter (m) of the Shandong clade (0.1166, 0.2567) were both significantly higher than that of the Liaoning clade (0.076, 0.185). The egg production and life (64.4 ± 10.44 eggs and 16.26 ± 1.04 (female), 13.54 ± 0.75 (male) days, respectively) of the Shandong clade were both significantly higher than they were for the Liaoning clade (30.67 ± 4.02 eggs and 9.34 ± 0.67 (female), 7.64 ± 0.48 (male) days, respectively), and the Shandong clade had a longer oviposition period for one oviposition (Shandong 86.6 s, Liaoning 61.4 s). We expect, in Shandong Province, the pest control ability of the Shandong clade of A. mali to be better than the Liaoning clade.