Rearing history affects behaviour and performance of two virulent Nasonovia ribisnigri populations on two lettuce cultivars

Many aphid species have become virulent to host‐plant resistance, which limits the sustainability of insect resistance breeding. However, when this adaptation to resistant plants is associated with fitness costs for the aphids, virulence can be lost in the absence of resistant plants. For two populations of the lettuce aphid, Nasonovia ribisnigri (Mosely) (Hemiptera: Aphididae), we evaluated whether virulence to Nr‐gene‐based resistance was lost on a susceptible lettuce, Lactuca sativa L. (Asteraceae), and assessed possible costs of virulence. The feeding behaviour and performance of these aphids, reared and tested on susceptible and resistant lettuce, were investigated. The rearing plant affected feeding behaviour and performance of the aphids. Temporary reduction and long‐term loss of virulence were found. The total duration of phloem intake was shorter after being reared on susceptible lettuce and tested on resistant lettuce. In addition, one population had a lower survival on resistant lettuce after being reared on susceptible lettuce. There were also indications of fitness costs of the virulence in both populations.     

Feeding behavior and performance of Nasonovia ribisnigri on grafts,detached leaves,and leaf disks of resistant and susceptible lettuce

Aphids are dependent on the phloem sap of plants as their only source of nutrients. Host‐plant resistance in lettuce, Lactuca sativa L. (Asteraceae), mediated by the Nr gene is used to control the lettuce aphid Nasonovia ribisnigri (Mosely) (Hemiptera: Aphididae). The resistance is located in the phloem; however, the exact mechanism of resistance is unknown. In this study, we investigated whether the resistance factor (or factors) is synthesized in the root or in the shoot. The feeding behavior and performance of avirulent N. ribisnigri were studied on grafts of resistant and susceptible lettuce. In addition, the persistence of resistance in excised lettuce tissue was measured, by studying the feeding behavior and performance of N. ribisnigri on detached leaves and leaf disks of resistant lettuce. It appears that the resistance factor encoded by the Nr gene is produced in the shoots: aphid feeding was reduced on resistant shoots grafted on susceptible roots, whereas aphids were able to feed on grafts of susceptible shoots on resistant roots. Partial loss of resistance was observed after detachment of leaves and excision of leaf disks from resistant plants. Aphids fed longer on excised resistant plant tissue compared with intact resistant plants; however, compared with excised plant tissue of the susceptible cultivar, the time spent on feeding was shorter, indicating resistance was not completely lost. Our findings caution against the use of excised leaf material for aphid resistance bioassays.     

New sources of resistance to lettuce aphids in Lactuca spp.

The use of resistant cultivars is one of the best ways to protect lettuce from aphid pests. At present, there are cultivars available with nearly complete resistance to Nasonovia ribisnigri biotype Nr:0 (based on the Nr gene) and partial resistance to Macrosiphum euphorbiae. Nevertheless, a new biotype of N. ribisnigri (Nr:1) able to overcome the resistance based on the Nr gene is expanding around Europe and has become a major threat of lettuce. In the present work, we report the presence of this new biotype in southeastern Spain, a major lettuce-producing region. Furthermore, a pool of 264 germplasm accessions belonging to Lactuca genus was tested in a greenhouse assay to search for new resistance sources to N. ribisnigri. The most promising accessions were retested in the laboratory to characterize the resistance by means of free-choice and antibiosis assays against biotypes Nr:0 and Nr:1 of N. ribisnigri and against a clone of M. euphorbiae. Three accessions of L. virosa showed resistance against the target aphid species and could be of interest for ongoing breeding programs. The accessions CGN16272 and CGN13361 were both partially resistant to the Nr:1 biotype of N. ribisnigri and to M. euphorbiae, and CGN13355, in spite of not being resistant to N. ribisnigri, showed a near complete resistance to M. euphorbiae. The study of the feeding behavior of N. ribisnigri biotypes showed that the Nr:1 biotype is able to maintain a similar phloem feeding ingestion pattern on genotypes bearing the Nr gene and on N. ribisnigri-susceptible lettuce genotypes. Moreover, as aphids rejected L. virosa as a feeding source due to superficial factors (high level of antixenosis), no differences in the level of antibiosis between such genotypes were detected. A second set of screening assays were conducted on 40 accessions of L. virosa in order to select for resistance against the Nr:1 biotype. The results showed three accessions with high levels of resistance (CGN05148, CGN21399 and CGN16274) against Nr:1 that could be of interest in lettuce breeding programs.     

The effect of co-infestation by conspecific and heterospecific aphids on the feeding behaviour of <Emphasis Type="Italic">Nasonovia ribisnigri</Emphasis> on resistant and susceptible lettuce cultivars

Aphid saliva can suppress the blocking of sieve elements, a reaction that plants employ to inhibit aphid feeding, but aphid saliva can also elicit plant defence responses. Such plant responses might affect interactions between different aphid species and intraspecifically, e.g. among different biotypes. The objectives of our study were to investigate if feeding behaviour and performance of two biotypes of the lettuce aphid Nasonovia ribisnigri are affected by (1) feeding by the other biotype and (2) feeding by the green peach aphid Myzus persicae or the potato aphid Macrosiphum euphorbiae. Additionally the effect of feeding in a group was studied. All experiments were performed on both a resistant and an isogenic susceptible lettuce cultivar. Feeding or probing by conspecific or heterospecific aphids had different effects on Nasonovia ribisnigri biotypes. Aphids were only slightly affected by feeding or probing of the same biotype on both susceptible and resistant lettuce. N. ribisnigri virulent biotype Nr:1 suppressed the resistance against Nr:0 in the resistant cultivar. In contrast, defence was induced by Nr:1 against Nr:0 in susceptible lettuce. Co-infestation by M. euphorbiae and M. persicae had minor effects on Nr:0. Defence against Nr:1 was induced on both susceptible lettuce and resistant lettuce by Nr:0 and M. euphorbiae. Additionally, M. persicae induced defence in resistant lettuce against Nr:1. Effectors in the saliva of Nr:1 aphids are likely responsible for the defence suppression in lettuce. Identification of these effectors could lead to a better understanding of the mechanism of virulence in N. ribisnigri.     

The resistance of lettuce (Lactuca sativa L.) toNasonovia ribisnigri: Bionomics ofN. ribisnigri on near isogenic lettuce lines

The intrinsic rate of increase (r_m-values), mean relative growth rates and mortality ofNasonovia ribisnigri (Mosley) (Homoptera, Aphididae) on different lines of lettuce (Lactuca sativa L.) were determined. Near isogenic resistant and susceptible lines, plus their ancestors, were used. Bionomics ofN. ribisnigri on the resistant lines (NrNr) with the dominantNasonovia resistance gene (Nr-gene) differed clearly from the susceptible lines (nrnr). Mortality was high, no larvae reached adulthood, and no reproduction nor honeydew production was seen on the resistant lines. Transfer of aphids to susceptible plants after a period of 2 days on the resistant lines showed no signs of intoxication of aphids. Apparently there is no feeding on the resistant lines. It is not clear whether the aphids can not reach the phloem or do not accept it on the resistant line.     

The impact of insecticide resistance in the currant-lettuce aphid, Nasonovia ribisnigri, on pest management in lettuce

Abstract 1 This paper reports on experiments to determine how two different insecticide resistance phenotypes in the aphid Nasonovia ribisnigri (Mosley), which is a major pest of lettuce, change its susceptibility to pyrethroid insecticides and the carbamate pirimicarb. 2 A novel statistical approach determined how the effectiveness of different insecticides was changed by the two resistance phenotypes. This compared the between‐plant distribution of aphid numbers, as opposed to the mean number of aphids per plant. 3 Results from field cage experiments showed that the effect of the resistances differed. Pyrethroid resistance resulted in lower mortality immediately after application of pyrethroids, whereas resistance to pirimicarb shortened the time over which the chemical was effective. 4 The results of laboratory bioassays suggested that these two resistances were not found together in N. ribisnigri. However, the results reported here contradict this assertion. 5 Experiments with insecticide residues showed that reproduction of resistant N. ribisnigri was greater than that of susceptible N. ribisnigri on plants with ageing insecticide residues, even in circumstances where mortality of resistant and susceptible clones of N. ribisnigri were similar. 6 If more than a few aphids are found on a plant then a whole consignment can be rejected for processing. The results reported here suggest that the effect of both insecticide resistances in N. ribsinigri will be to increase the proportion of lettuce heads with an unacceptable number of aphids on them, leading to increased rejection of plants for processing.     

Trophic impacts of a rolled-rye cover crop on the lettuce aphid (Nasonovia ribisnigri) and associated insect fauna in cultivated Histosols

Cultivated peatland (Histosol) in Southern Québec (Canada) is a rapidly declining non-renewable resource used to grow most Canadian lettuce (Lactuca sativa L., Asteraceae). Rolled-rye (Secale cereale L., Poaceae) cover crop is one of the conservation practices proposed to reach a more sustainable lettuce production, but the overall impact on the agroecosystem remains poorly studied in Histosols. We assessed multiple effects of rolled-rye cover crop on the trophic chain associated with the lettuce aphid, Nasonovia ribisnigri (Mosley) (Hemiptera: Aphididae), a major pest of lettuce. During one growing season and through two consecutive lettuce crops, we monitored in situ the impacts of rolled-rye cover crop on insect fauna and lettuce quality. We used visual scouting and yellow pan traps to assess plant colonization by N. ribisnigri, its natural enemies, and alternative prey. Exclusion cage experiments were also conducted to measure aphid fitness and population growth. Under greenhouse conditions, following cover crop removal, we examined potential lingering effects of rye within the soil on lettuce plants and N. ribisnigri. In situ experiments showed that rolled-rye cover crop has the potential to inhibit N. ribisnigri field colonization and recruit natural enemies and alternative prey in the first lettuce crop. Rye also reduced aphid fitness as well as lettuce foliar amino acid concentration and weight. For the second lettuce crop, an almost 50% reduction in N. ribisnigri abundance was observed with the use of rolled-rye cover crop. In greenhouse experiments, no persistent effect of rye was observed on the quality of lettuce grown with soil collected under a cover crop, nor on the fitness of aphids inoculated on these lettuces. This study highlights the diversity of trophic effects rolled-rye cover crop may have on lettuce production in Histosols and the potential of rolled-rye cover crop as a cultural practice to reduce aphid populations. Mechanisms at play while underlining agronomic challenges regarding proper rye termination must be further explored to maintain high-performing lettuce yields.     

Growth, development and consumption by four syrphid species associated with the lettuce aphid, Nasonovia ribisnigri, in California

The lettuce aphid, Nasonovia ribisnigri Mosley, was accidentally introduced into California from Europe during the late 1990s and soon became an economic pest of Romaine lettuce along California’s central coast region. Indigenous syrphid larvae attack the lettuce aphid and are believed to be effective in the management of this invasive pest, although there have been no studies on the capacity of the syrphid larvae to kill and consume lettuce aphids. We focused on four syrphid species commonly found in central coast lettuce fields: Allograpta obliqua (Say), Eupeodes fumipennis (Thomson), Sphaerophoria sulphuripes (Thomson), and Toxomerus marginatus (Say). Laboratory feeding experiments were conducted to estimate the development times of all juvenile stages, the daily growth rate of larvae, the number of third instar aphids killed, the aphid biomass killed, and the aphid biomass consumed as measures of predator performance. Results show that during larval development E. fumipennis killed the most third-instar aphids (507 aphids, 88 mg biomass killed) and reached the largest size, followed by A. obliqua (228 aphids, 39 mg killed), S. sulphuripes (194 aphids, 31 mg killed) and T. marginatus (132 aphids, 20 mg killed). Body size alone did not account for species differences in per-capita larval consumption rates. This information is discussed in relation to the predation potential of syrphids through the short cropping cycle of lettuce, and the choice of plant species to use for floral resource provisioning to enhance the activity of syrphids needed for effective management of lettuce aphids in California’s central coast fields.     

Tissue localisation of lettuce resistance to the aphidNasonovia ribisnigri using electrical penetration graphs

Electrical penetration graphs (EPG's) ofNasonovia ribisnigri (Mosley) (Homoptera, Aphididae) on resistant and susceptible lettuce (Lactuca sativa, Compositae) showed a large reduction in the duration of the food uptake pattern (E2) on the resistant line. No differences in EPG's were observed before the phloem was reached. Therefore, resistance is believed to be located in the phloem vessel. Both mechanical blocking of the sieve element after puncturing and a difference in composition of the phloem sap are possible resistance factors. However, a chemical factor seems more likely because of the specificity of the resistance againstN. ribisnigri.     

Performance and prospects of Rag genes for management of soybean aphid

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is an invasive insect pest of soybean [Glycine max (L.) Merr. (Fabaceae)] in North America, and it has led to extensive insecticide use in northern soybean‐growing regions there. Host plant resistance is one potential alternative strategy for managing soybean aphid. Several Rag genes that show antibiosis and antixenosis to soybean aphid have been recently identified in soybean, and field‐testing and commercial release of resistant soybean lines have followed. In this article, we review results of field tests with soybean lines containing Rag genes in North America, then present results from a coordinated regional test across several field sites in the north‐central USA, and finally discuss prospects for use of Rag genes to manage soybean aphids. Field tests conducted independently at multiple sites showed that soybean aphid populations peaked in late summer on lines with Rag1 or Rag2 and reached economically injurious levels on susceptible lines, whereas lines with a pyramid of Rag1 + Rag2 held soybean aphid populations below economic levels. In the regional test, aphid populations were generally suppressed by lines containing one of the Rag genes. Aphids reached putative economic levels on Rag1 lines for some site years, but yield loss was moderated, indicating that Rag1 may confer tolerance to soybean aphid in addition to antibiosis and antixenosis. Moreover, no yield penalty has been found for lines with Rag1, Rag2, or pyramids. Results suggest that use of aphid‐resistant soybean lines with Rag genes may be viable for managing soybean aphids. However, virulent biotypes of soybean aphid were identified before release of aphid‐resistant soybean, and thus a strategy for optimal deployment of aphid‐resistant soybean is needed to ensure sustainability of this technology.     

Study on inheritance of resistance to root aphid, Pemphigus bursarius, in lettuce

The inheritance of resistance to lettuce root aphid, Pemphigus bursarius, was studied in lettuce using the Wellesbourne cultivars Avondefiance and Avoncrisp as resistant parents and Borough Wonder and Webb's Wonderful as aphid-susceptible parents. All four cultivars were crossed in all possible combinations including reciprocals and the response to root aphid of plants in the P₁F₁F₂ and BC generations was assessed using apterae of P. bursarius from the lettuce cv. Iceberg. Resistance to attack was clearly inherited and the parents appeared to be homozygous for their resistance or susceptibility. In the F₁ generation, however, in all crosses between resistant and susceptible parents, segregation into susceptible, resistant and some slightly less resistant plants occurred. This and the highly significant differences in segregation between pairs of reciprocal crosses in the F₁ and other generations indicate that the inheritance of resistance to root aphid is controlled by extra-nuclear factors. Modifying genes might also be involved but there appears to be no linkage of root aphid resistance with resistance to downy mildew, for which the Wellesbourne lettuces were bred.     

Characterisation of bird cherry-oat aphid (Rhopalosiphum padi L.) behaviour and aphid host preference in relation to partially resistant and susceptible wheat landraces

The bird cherry-oat aphid (Rhopalosiphum padi L.) is a major pest of wheat (Triticum aestivum L.) and can cause up to 30% yield losses. Heritable plant resistance to aphids is both an economically and ecologically sound method for managing aphids. Here we report how the behaviour and performance of R. padi differs on two resistant, one susceptible wheat landrace and a susceptible elite wheat variety. Feeding behaviour differed among the genotypes, with aphids on resistant lines spending longer in the pathway phase and less time phloem feeding. These behaviours suggest that both inter- and intracellular factors encountered during pathway and phloem feeding phases could be linked to the observed aphid resistance. Locomotion and antennal positioning choice tests also revealed a clear preference for susceptible lines. Although feeding studies revealed differences in the first probe indicating that the resistance factors might also be located in the peripheral layers of the plant tissue, scanning electron microscopy revealed no difference in trichrome length and density on the surface of leaves. Aphids are phloem feeders and limiting the nutrient uptake by the aphids may negatively affect their growth and development as shown here in lower weight and survival of nymphs on resistant genotypes and decreased reproductive potential, with lowest mean numbers of nymphs produced by aphids on W064 (54.8) compared to Solstice (71.9). The results indicate that resistant lines markedly alter the behaviour, reproduction and development potential of R. padi and possess both antixenosis and antibiosis type of resistance.     

Mi‐mediated aphid resistance in tomato: tissue localization and impact on the feeding behavior of two potato aphid clones with differing levels of virulence

The Mi‐1.2 gene in tomato, Solanum lycopersicum L. (Solanaceae), confers resistance against several herbivores, including the potato aphid, Macrosiphum euphorbiae (Thomas) (Hemiptera: Sternorrhyncha: Aphididae) and the sweetpotato whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Sternorrhyncha: Aleyrodidae). Previous studies on the tissue localization of resistance have given varying results; whitefly resistance was attributed to factors localized in the mesophyll or epidermis, whereas aphid resistance was attributed to factors localized in the phloem. Our study utilizes the direct current electrical penetration graph (DC‐EPG) technique to compare aphid feeding behavior on resistant (Mi‐1.2+) and susceptible (Mi‐1.2?) tomato plants. This study also compares the impact of resistance on the feeding behavior of two aphid clones that vary in their virulence, or their ability to survive and reproduce on resistant plants. Previous work had shown that the avirulent WU11 clone is almost completely inhibited by resistance, whereas the semi‐virulent WU12 clone can colonize resistant hosts. Here, DC‐EPG analysis shows that both aphid clones take longer to initiate cell sampling and to establish a confirmed sieve element phase on resistant plants than on susceptible hosts, and have shorter ingestion periods on resistant plants. However, the magnitude of these deterrent effects is far less for the semi‐virulent clone than for the avirulent aphids. In particular, the WU12 clone is less sensitive to factors that limit sieve element ingestion, showing shorter non‐probe duration and rapidly establishing sustained phloem ingestion on resistant plants when compared to the WU11 clone. We conclude that, in addition to previously described factors in the phloem that inhibit ingestion, Mi‐mediated aphid resistance also involves factors (possibly in the mesophyll and/or epidermis) that delay initiation of phloem salivation, and that act in the intercellular spaces to deter the first cell sampling. Furthermore, the relative effectiveness of these components of resistance differs among insect populations.     

Stylet penetration by two aphid species on susceptible and resistant lettuce

The probing behavior of two aphid species, Myzus persicae (Sulz.) and Nasonovia ribisnigri (Mosley), was electronically monitored on susceptible and resistant lettuce lines using a DC amplifier. A waveform pattern associated with extracellular stylet pathway activities, pattern C, occurred for longer periods when either aphid species probed resistant plants. This pattern is usually regularly interrupted by drops in electrical potential lasting a few seconds, reflecting cell membrane punctures followed by rapid withdrawal of the stylet tips. For M. persicae on resistant lettuce a large increase in pattern C without these potential drops accounted for the increased duration of this pattern. For N. ribisnigri the increase in pathway activity on resistant plants was due to an increase in the more typical pattern C with potential drops, as well as to an increased duration of pattern F, associated with a curious type of stylet penetration within cell walls. Both aphids made more but shorter probes on resistant than on susceptible plants, and these probes led less frequently to periods of sieve element contact and ingestion. The effects of resistance appear to involve both mesophyll and phloem factors. The underlying mechanisms, however, remain unclear. The results indicate which stylet penetration activities or waveform patterns are of interest for further investigation of resistance mechanisms.

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Résumé Le comportement de sondage de variétés sensibles et résistantes de laitues par deux espèces de pucerons, Myzus persicae (Sulz.) et Nasonovia ribisnigri (Mosley), a été enregistré électroniquement en utilisant un amplificateur DC. Les deux espèces de pucerons ont présenté une amplification de l'onde de type C associée au cheminement extracellulaire des stylets lors du sondage des variétés résistantes. Cette onde est normalement interrompue par des chutes de quelques secondes du potentiel électrique, traduisant des ponctions de la membrane cellulaire suivies par un rapide retrait de l'extrémité des stylets. La forte augmentation de l'onde C mais sans chutes de potentiel rend compte de la plus longue durée de cette onde chez M. persicae sur laitue résistante. L'accroissement du cheminement des stylets chez N. ribisnigri sur plantes résistantes est dû à une augmentation de l'onde C typique avec chutes de potentiel, ainsi qu'à une prolongation de l'onde F liée à la pénétration des stylets dans les parois cellulaires. Les deux espèces font des sondages plus brefs et plus nombreux sur variétés résistantes, et ces sondages entraînent des contacts moins fréquents avec les éléments criblés et débouchent moins souvent sur de l'ingestion. Les effects de la résistance semblent impliquer des facteurs liés à la fois au mésophylle et au phloème. Les mécanismes sous-jacents, cependant, ne sont pas encore clairs. Ces résultats Montrent que l'examen des ondes liées à la pénétration des stylets est important pour des études ultérieures sur les mécanismes de résistance.

     

Interactive effects between diet and genotypes of host and pathogen define the severity of infection

Host resistance and parasite virulence are influenced by multiple interacting factors in complex natural communities. Yet, these interactive effects are seldom studied concurrently, resulting in poor understanding of host‐pathogen‐environment dynamics. Here, we investigated how the level of opportunist pathogen virulence, strength of host immunity and the host condition manipulated via diet affect the survival of wood tiger moth Parasemia plantaginis (Arctidae). Larvae from “low cuticular melanin” and “high cuticular melanin” (considered as low and high pathogen resistance, respectively) selection lines were infected with moderately and highly virulent bacteria strains of Serratia marcescens, while simultaneously manipulating host diet (with or without antibacterial compounds). We measured host survival and food preference before and after infection to test whether the larvae “self‐medicate” by choosing an anti‐infection diet (Plantago major, i.e., plantain leaf) over lettuce (Lactuca sativa). “High melanin” larvae were more resistant than “low melanin” larvae to the less virulent strain that had slower growth and colonization rate compared with the more virulent strain. Cuticular melanin did not enhance survival when the larvae were infected with the highly virulent strain. Anti‐infection diet enhanced survival of the “high melanin” but not the “low melanin” hosts. Survival was dependent on family origin even within the melanin selection lines. Despite the intrinsic preference for lettuce, no evidence of self‐medication was found. These results demonstrate that the relative benefit of host cuticular melanin depends on both diet and pathogen virulence: plantain diet only boosted the immunity of already resistant “high melanin” hosts, and cuticular melanin increased host survival only when infected with moderately virulent pathogen. Moreover, there was considerable variation in host survival between families within both melanin lines suggesting genetic basis for resistance. These results indicate that although melanin is an important predictor of insect immunity, its effect on disease outcomes greatly depends on other interacting factors.     

Effect of UV‐C Radiation on Resistance of Romaine Lettuce (Lactuca sativa L.) Against Botrytis cinerea and Sclerotinia minor

The aim of this research was to examine the effect of UV‐C on resistance of lettuce to Botrytis cinerea and Sclerotinia minor. Analysis of the lesion surfaces showed that plants exposed to UV‐C were less susceptible to the two pathogens, especially on the fourth day after inoculation. Chlorophyll, carotenoid contents and malondialdehyde and hydrogen peroxide were assayed after 1 day and 4 days. Lettuces treated with UV‐C and inoculated showed an increase in chlorophyll and carotenoid content, especially 24 h after inoculation, and low values of the two indicators of oxidative stress as compared with lettuces which were inoculated but did not receive UV‐C treatment.     

Responses and adaptation by Nephotettix virescens to monogenic and pyramided rice lines with Grh‐resistance genes

The green leafhopper, Nephotettix virescens (Distant) (Hemiptera: Cicadellidae), occasionally damages rice in Asia either directly, by feeding on the host phloem, or indirectly by transmitting tungro virus. We assessed the nature of resistance against the leafhopper in monogenic and pyramided near‐isogenic rice lines containing the resistance genes Grh2 and Grh4. Only the pyramided line was resistant to leafhopper damage. Leafhopper nymphs and adults had high mortality and low weight gain when feeding on the pyramided line and adults laid few eggs. In contrast, although there was some minor resistance in 45‐day‐old plants that possessed either Grh2 or Grh4 genes, the monogenic lines were generally as susceptible to the leafhopper as the recurrent parent line Taichung65 (T65). Resistance in the pyramided line was stable as the plant aged and under high nitrogen, and affected each of five Philippine leafhopper populations equally. Furthermore, in a selection study, leafhoppers failed to adapt fully to the pyramided resistant line: nymph and adult survival did improve during the first five generations of selection and attained similar levels as on T65, but egg‐laying failed to improve over 10 generations. Our preliminary results suggested that resistance was associated with physiological costs to the plants in some experiments. The results of this study demonstrate the success of pyramiding resistance genes through marker‐assisted breeding, to achieve a strong and potentially durable resistance. We discuss the utility of gene pyramiding and the development of near‐isogenic lines for leafhopper management.     

Preference of Bt‐resistant and susceptible Busseola fusca moths and larvae for Bt and non‐Bt maize

The sustainability of genetically engineered insecticidal Bacillus thuringiensis Berliner (Bt) maize, Zea mays L. (Poaceae), is threatened by the evolution of resistance by target pest species. Several Lepidoptera species have evolved resistance to Cry proteins expressed by Bt maize over the last decade, including the African maize stem borer, Busseola fusca (Fuller) (Lepidoptera: Noctuidae). The insect resistance management (IRM) strategy (i.e., the high‐dose/refuge strategy) deployed to delay resistance evolution is grounded on certain assumptions about the biology and ecology of a pest species, for example, the interactions between the insect pest and crop plants. Should these assumptions be violated, the evolution of resistance within pest populations will be rapid. This study evaluated the assumption that B. fusca adults and larvae select and colonize maize plants at random, and do not show any preference for either Bt or non‐Bt maize. Gravid female B. fusca moths of a resistant and susceptible population were subjected to two‐choice oviposition preference tests using stems of Bt and non‐Bt maize plants. Both the number of egg batches as well as the total number of eggs laid on each stem were recorded. The feeding preference of Bt‐resistant and susceptible neonate B. fusca larvae were evaluated in choice test bioassays with whorl leaf samples of specific maize cultivars. Although no differential oviposition preference was observed for either resistant or susceptible female moths, leaf damage ratings indicated that neonate larvae were able to detect Bt toxins and that they displayed feeding avoidance behaviour on Bt maize leaf samples.     

A gene for resistance to the Varroa mite (Acari) in honey bee (Apis mellifera) pupae

Social insect colonies possess a range of defences which protect them against highly virulent parasites and colony collapse. The host–parasite interaction between honey bees (Apis mellifera) and the mite Varroa destructor is unusual, as honey bee colonies are relatively poorly defended against this parasite. The interaction has existed since the mid‐20th Century, when Varroa switched host to parasitize A. mellifera. The combination of a virulent parasite and relatively naïve host means that, without acaricides, honey bee colonies typically die within 3 years of Varroa infestation. A consequence of acaricide use has been a reduced selective pressure for the evolution of Varroa resistance in honey bee colonies. However, in the past 20 years, several natural‐selection‐based breeding programmes have resulted in the evolution of Varroa‐resistant populations. In these populations, the inhibition of Varroa's reproduction is a common trait. Using a high‐density genome‐wide association analysis in a Varroa‐resistant honey bee population, we identify an ecdysone‐induced gene significantly linked to resistance. Ecdysone both initiates metamorphosis in insects and reproduction in Varroa. Previously, using a less dense genetic map and a quantitative trait loci analysis, we have identified Ecdysone‐related genes at resistance loci in an independently evolved resistant population. Varroa cannot biosynthesize ecdysone but can acquire it from its diet. Using qPCR, we are able to link the expression of ecdysone‐linked resistance genes to Varroa's meals and reproduction. If Varroa co‐opts pupal compounds to initiate and time its own reproduction, mutations in the host's ecdysone pathway may represent a key selection tool for honey bee resistance and breeding.     

Plant‐mediated interactions between two herbivores differentially affect a subsequently arriving third herbivore in populations of wild cabbage

• Plants are part of biodiverse communities and frequently suffer from attack by multiple herbivorous insects. Plant responses to these herbivores are specific for insect feeding guilds: aphids and caterpillars induce different plant phenotypes. Moreover, plants respond differentially to single or dual herbivory, which may cascade into a chain of interactions in terms of resistance to other community members. Whether differential responses to single or dual herbivory have consequences for plant resistance to yet a third herbivore is unknown.
• We assessed the effects of single or dual herbivory by Brevicoryne brassicae aphids and/or Plutella xylostella caterpillars on resistance of plants from three natural populations of wild cabbage to feeding by caterpillars of Mamestra brassicae. We measured plant gene expression and phytohormone concentrations to illustrate mechanisms involved in induced responses.
• Performance of both B. brassicae and P. xylostella was reduced when feeding simultaneously with the other herbivore, compared to feeding alone. Gene expression and phytohormone concentrations in plants exposed to dual herbivory were different from those found in plants exposed to herbivory by either insect alone. Plants previously induced by both P. xylostella and B. brassicae negatively affected growth of the subsequently arriving M. brassicae. Furthermore, induced responses varied between wild cabbage populations.
• Feeding by multiple herbivores differentially activates plant defences, which has plant‐mediated negative consequences for a subsequently arriving herbivore. Plant population‐specific responses suggest that plant populations adapt to the specific communities of insect herbivores. Our study contributes to the understanding of plant defence plasticity in response to multiple insect attacks.