Molasses Grass Induces Direct and Indirect Defense Responses in Neighbouring Maize Plants.

Plants have evolved intricate defence strategies against herbivore attack which can include activation of defence in response to stress-related volatile organic compounds (VOCs) emitted by neighbouring plants. VOCs released by intact molasses grass (Melinis minutiflora), have been shown to repel stemborer, Chilo partellus (Swinhoe), from maize and enhance parasitism by Cotesia sesamiae (Cameron). In this study, we tested whether the molasses grass VOCs have a role in plant-plant communication by exposing different maize cultivars to molasses grass for a 3-week induction period and then observing insect responses to the exposed plants. In bioassays, C. partellus preferred non-exposed maize landrace plants for egg deposition to those exposed to molasses grass. Conversely, C. sesamiae parasitoid wasps preferred volatiles from molasses grass exposed maize landraces compared to volatiles from unexposed control plants. Interestingly, the molasses grass induced defence responses were not observed on hybrid maize varieties tested, suggesting that the effect was not simply due to absorption and re-emission of VOCs. Chemical and electrophysiological analyses revealed strong induction of bioactive compounds such as (R)-linalool, (E)-4,8-dimethyl-1,3,7-nonatriene and (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene from maize landraces exposed to molasses grass volatiles. Our results suggest that constitutively emitted molasses grass VOCs can induce direct and indirect defence responses in neighbouring maize landraces. Plants activating defences by VOC exposure alone could realize enhanced levels of resistance and fitness compared to those that launch defence responses upon herbivore attack. Opportunities for exploiting plant-plant signalling to develop ecologically sustainable crop protection strategies against devastating insect pests such as stemborer C. partellus are discussed.

Characterization of Male-Produced Aggregation Pheromone of the Bean Flower Thrips Megalurothrips sjostedti (Thysanoptera: Thripidae).

Aggregation of the bean flower thrips, Megalurothrips sjostedti (Trybom) (Thysanoptera: Thripidae), has been observed on cowpea, Vigna unguiculata (L.) Walp. To understand the mechanism underpinning this behavior, we studied the responses of M. sjostedti to headspace volatiles from conspecifics in a four-arm olfactometer. Both male and female M. sjostedti were attracted to male, but not to female odor. Gas chromatography/mass spectrometry (GC/MS) analyses revealed the presence of two distinct compounds in male M. sjostedti headspace, namely (R)-lavandulyl 3-methylbutanoate (major compound) and (R)-lavandulol (minor compound); by contrast, both compounds were only present in trace amounts in female headspace collections. A behavioral assay using synthetic compounds showed that male M. sjostedti was attracted to both (R)-lavandulyl 3-methylbutanoate and (R)-lavandulol, while females responded only to (R)-lavandulyl 3-methylbutanoate. This is the first report of a male-produced aggregation pheromone in the genus Megalurothrips. The bean flower thrips is the primary pest of cowpea, which is widely grown in sub-Saharan Africa. The attraction of male and female M. sjostedti to these compounds offers an opportunity to develop ecologically sustainable management methods for M. sjostedti in Africa.

Maize Chlorotic Mottle Virus Induces Changes in Host Plant Volatiles that Attract Vector Thrips Species.

Maize lethal necrosis is one of the most devastating diseases of maize causing yield losses reaching up to 90% in sub-Saharan Africa. The disease is caused by a combination of maize chlorotic mottle virus (MCMV) and any one of cereal viruses in the Potyviridae group such as sugarcane mosaic virus. MCMV has been reported to be transmitted mainly by maize thrips (Frankliniella williamsi) and onion thrips (Thrips tabaci). To better understand the role of thrips vectors in the epidemiology of the disease, we investigated behavioral responses of F. williamsi and T. tabaci, to volatiles collected from maize seedlings infected with MCMV in a four-arm olfactometer bioassay. Volatile profiles from MCMV-infected and healthy maize plants were compared by gas chromatography (GC) and GC coupled mass spectrometry analyses. In the bioassays, both sexes of F. williamsi and male T. tabaci were significantly attracted to volatiles from maize plants infected with MCMV compared to healthy plants and solvent controls. Moreover, volatile analysis revealed strong induction of (E)-4,8-dimethyl-1,3,7-nonatriene, methyl salicylate and (E,E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene in MCMV-infected maize seedlings. Our findings demonstrate MCMV induces changes in volatile profiles of host plants to elicit attraction of thrips vectors. The increased vector contact rates with MCMV-infected host plants could enhance virus transmission if thrips feed on the infected plants and acquire the pathogen prior to dispersal. Uncovering the mechanisms mediating interactions between vectors, host plants and pathogens provides useful insights for understanding the vector ecology and disease epidemiology, which in turn may contribute in designing integrated vector management strategies.

Pheromonal variation and mating between two mitotypes of fall armyworm (Spodoptera frugiperda) in Africa.

In the Americas, the fall armyworm (Spodoptera frugiperda) exists in two genetically distinct strains, the corn (C) and rice (R) strains. Despite their names, these strains are not associated with host plant preferences but have been shown to vary in pheromone composition and male responses. Recently, S. frugiperda was detected in Africa as an invasive species, but knowledge about variation in strain types, pheromone composition and inter-strain mating of populations of the pest in the continent has not been fully examined. Therefore, this study aimed to investigate variations, if any in the pheromone composition of female moths, male moth responses, and mating between C and R mitotypes of S. frugiperda populations in Kenya, as well as their geographic distribution. Strains (mitotypes) of S. frugiperda were identified using mitochondrial DNA (mtDNA) markers, and their pheromonal composition determined by coupled gas chromatography-mass spectrometric (GC-MS) analysis. Male moth responses to these compounds were evaluated using GC-electroantennographic detection (EAD), electroantennogram (EAG), and wind tunnel assays. Oviposition assays were used to determine whether R and C mitotype moths could mate and produce eggs. The results showed that both the R and C mitotypes were present, and there were no statistically significant differences in their distribution across all sampled locations. Five pheromone compounds including (Z)-7-dodecenyl acetate (Z7-12:OAc), (Z)-7-tetradecenyl acetate (Z7-14:OAc), (Z)-9-tetradecenyl acetate (Z9-14:OAc), (Z)-11-tetradecenyl acetate (Z11-14:OAc) and (Z)-11-hexadecenyl acetate (Z11-16:OAc), were detected in the pheromone glands of female moths of both mitotypes, with Z9-14:OAc being the most abundant. The relative percentage composition of Z9-14:OAc was similar in both mitotypes. However, the R mitotype had a 2.7 times higher relative percentage composition of Z7-12:OAc compared to the C mitotype moth, while the C mitotype moth had a 2.4 times higher relative percentage composition of Z11-16:OAc than the R mitotype moth. Male moths of both mitotypes exhibited similar responses to the pheromone compounds, showing the strongest responses to Z9-14:OAc and Z7-12:OAc in electrophysiological and behavioural assays. There was mating between R and C mitotypes with egg production comparable to mating within the same mitotype. Our results revealed that differences between the two S. frugiperda mitotypes are characterized by female moth pheromone composition rather than male moth responses to the pheromones, and that this does not prevent hybridisation between the mitotypes, which may have implications for their management.

Responses of the fall armyworm (Spodoptera frugiperda) to different host plants: Implications for its management strategy.

BACKGROUND: The selection of suitable host plants for oviposition is critical for herbivorous insects to maximise survival of their offspring. Olfaction plays an important role in this process. However, little is known about how olfaction shapes the interaction between the fall armyworm (Spodoptera frugiperda) and host plants. In this study, we tested the hypothesis that olfaction guides the host selection process in the fall armyworm using oviposition and wind tunnel bioassays. RESULTS: In no-choice and dual-choice assays, female moths oviposited on all seven host plants that were tested (maize, sorghum, wheat, bean, cowpea, tomato and cabbage). However, in multiple-choice assays, no eggs were deposited on cowpea and cabbage. We found that maize, sorghum and wheat were most preferred for oviposition, whereas cowpea was least preferred. Wind tunnel assays confirmed these divergent oviposition preferences, with maize, sorghum and wheat odours being the most attractive. Gas chromatography-mass spectrometry analysis followed by random forest classification identified terpenes as the potential host-plant attractants. CONCLUSION: Our results improve our understanding of the chemical ecology of the fall armyworm and suggest that some of these host plants could offer potential for use in an intercropping strategy to manage S. frugiperda. © 2022 Society of Chemical Industry.

Differences in the biochemical content and radical scavenging activity of propolis from different parts of a Meliponula ferruginea hive.

Research on stingless bee products has increased in recent years, and of particular interest is propolis because of its biological activities such as antioxidant and antimicrobial. However, there is paucity of information regarding intra-hive variations in the biochemical composition and biofunctional properties of this propolis. In this study, we investigated the phytochemicals and radical scavenging activity (RSA) of Meliponula ferruginea propolis from 10 wooden hives (n = 49). The samples were collected from five different locations comprising the entrance, involucrum, pillars, pots and sealant. Principal component analysis showed that there is an intra-hive variation in phytochemical content and RSA. Phenolic content constituted the highest phytochemical content in all the locations. The sealant and entrance had the highest amounts of phytochemicals compared to the involucrum, pillars and pots. Further analysis of propolis extracts by gas chromatography-mass spectrometry revealed occurrence of different compounds such as monoterpenoids, hydrocarbons, triterpenoids and alkaloids. Hydrocarbons were common in all parts while monoterpenes and triterpenes were present in the entrance. The findings of our study indicates that there is an intra-hive variation in propolis of M. ferruginea and hence this information will provide further insight into better understanding of stingless bee propolis.

Oviposition induced volatile emissions from African smallholder farmers' maize varieties.

Maize (corn), Zea mays, is a genetically diverse crop, and we have recently shown that certain open pollinated varieties (OPVs) of Latin American origin possess a trait not present in mainstream commercial varieties: they produce volatiles in response to stemborer oviposition that are attractive to stemborer parasitoids. Here, we tested whether a similar tritrophic effect occurs in the African OPVs 'Nyamula' and 'Jowi'. Herbivore induced plant volatiles (HIPVs) were collected from plants exposed to egg deposition by the stemborer Chilo partellus. In a four-arm olfactometer bioassay, the parasitic wasp Cotesia sesamiae preferred samples containing HIPVs from plants with eggs to samples collected from plants without eggs. EAG-active compounds, including (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT), were released in higher amounts from the egg induced headspace samples. Our results suggest that this oviposition trait is not limited to S. American Z. mays germplasm, and that it could be used to increase indirect defense against attack by stemborers.

Maize landraces recruit egg and larval parasitoids in response to egg deposition by a herbivore.

Natural enemies respond to herbivore-induced plant volatiles (HIPVs), but an often overlooked aspect is that there may be genotypic variation in these 'indirect' plant defence traits within plant species. We found that egg deposition by stemborer moths (Chilo partellus) on maize landrace varieties caused emission of HIPVs that attract parasitic wasps. Notably, however, the oviposition-induced release of parasitoid attractants was completely absent in commercial hybrid maize varieties. In the landraces, not only were egg parasitoids (Trichogramma bournieri) attracted but also larval parasitoids (Cotesia sesamiae). This implies a sophisticated defence strategy whereby parasitoids are recruited in anticipation of egg hatching. The effect was systemic and caused by an elicitor, which could be extracted from egg materials associated with attachment to leaves. Our findings suggest that indirect plant defence traits may have become lost during crop breeding and could be valuable in new resistance breeding for sustainable agriculture.

Mating behaviour, mate choice and female resistance in the bean flower thrips (Megalurothrips sjostedti).

Many species of thrips (Thysanoptera) in the family Thripidae form mating aggregations, but the adaptive significance of these aggregations and the extent of male and female mate choice is poorly understood. We studied the mating behaviour of the bean flower thrips Megalurothrips sjostedti (Trybom) (Thysanoptera: Thripidae), which forms male aggregations and occurs across sub-Saharan Africa. We tested whether males choose mates by female age or mating status. No-choice mating bioassays with one male and one female were used to simulate the way males usually encounter only one female at a time in aggregations in the field. Virgin females violently resisted mating attempts by males, but we found no compelling evidence to establish whether this was indiscriminate or was screening suitable males. Younger males (1-2 days old) did not discriminate females by age (1-2 or 7-10 days old), but older males (7-10 days old) avoided mating with older females. Any male choice by female mating status (virgin or mated) was weak or absent. The mating behaviour of M. sjostedti shows broad similarities with that of other thrips species that form aggregations, but also shows some distinct and novel differences, which can help our understanding of the adaptive significance of aggregations.

Evaluation of African Maize Cultivars for Resistance to Fall Armyworm Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) Larvae.

The fall armyworm (FAW) has recently invaded and become an important pest of maize in Africa causing yield losses reaching up to a third of maize annual production. The present study evaluated different aspects of resistance of six maize cultivars, cropped by farmers in Kenya, to FAW larvae feeding under laboratory and field conditions. We assessed the arrestment and feeding of FAW neonate larvae in no-choice and choice experiments, development of larvae-pupae, food assimilation under laboratory conditions and plant damage in a field experiment. We did not find complete resistance to FAW feeding in the evaluated maize cultivars, but we detected differences in acceptance and preference when FAW larvae were given a choice between certain cultivars. Moreover, the smallest pupal weight and the lowest growth index were found on 'SC Duma 43' leaves, which suggests an effect of antibiosis of this maize hybrid against FAW larvae. In contrast, the highest growth index was recorded on 'Rachar' and the greatest pupal weight was found on 'Nyamula' and 'Rachar'. The density of trichomes on the leaves of these maize cultivars seems not to be directly related to the preference of neonates for feeding. Plant damage scores were not statistically different between cultivars in the field neither under natural nor artificial infestation. However, plant damage scores in 'Nyamula' and 'Jowi' tended to be lower in the two last samplings of the season compared to the two initial samplings under artificial infestation. Our study provides insight into FAW larval preferences and performance on some African maize cultivars, showing that there are differences between cultivars in these variables; but high levels of resistance to larvae feeding were not found.

Genome wide association analysis of a stemborer egg induced "call-for-help" defence trait in maize.

Tritrophic interactions allow plants to recruit natural enemies for protection against herbivory. Here we investigated genetic variability in induced responses to stemborer egg-laying in maize Zea mays (L.) (Poaceae). We conducted a genome wide association study (GWAS) of 146 maize genotypes comprising of landraces, inbred lines and commercial hybrids. Plants were phenotyped in bioassays measuring parasitic wasp Cotesia sesamiae (Cameron) (Hymenoptera: Braconidae) attraction to volatiles collected from plants exposed to stemborer Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) eggs. Genotyping-by-sequencing was used to generate maize germplasm SNP data for GWAS. The egg-induced parasitoid attraction trait was more common in landraces than in improved inbred lines and hybrids. GWAS identified 101 marker-trait associations (MTAs), some of which were adjacent to genes involved in the JA-defence pathway (opr7, aos1, 2, 3), terpene biosynthesis (fps3, tps2, 3, 4, 5, 7, 9, 10), benzoxazinone synthesis (bx7, 9) and known resistance genes (e.g. maize insect resistance 1, mir1). Intriguingly, there was also association with a transmembrane protein kinase that may function as a receptor for the egg elicitor and other genes implicated in early plant defence signalling. We report maize genomic regions associated with indirect defence and provide a valuable resource for future studies of tritrophic interactions in maize. The markers identified may facilitate selection of indirect defence by maize breeders.

A maize landrace that emits defense volatiles in response to herbivore eggs possesses a strongly inducible terpene synthase gene.

Maize (Zea mays) emits volatile terpenes in response to insect feeding and egg deposition to defend itself against harmful pests. However, maize cultivars differ strongly in their ability to produce the defense signal. To further understand the agroecological role and underlying genetic mechanisms for variation in terpene emission among maize cultivars, we studied the production of an important signaling component (E)-caryophyllene in a South American maize landrace Braz1006 possessing stemborer Chilo partellus egg inducible defense trait, in comparison with the European maize line Delprim and North American inbred line B73. The (E)-caryophyllene production level and transcript abundance of TPS23, terpene synthase responsible for (E)-caryophyllene formation, were compared between Braz1006, Delprim, and B73 after mimicked herbivory. Braz1006-TPS23 was heterologously expressed in E. coli, and amino acid sequences were determined. Furthermore, electrophysiological and behavioral responses of a key parasitic wasp Cotesia sesamiae to C. partellus egg-induced Braz1006 volatiles were determined using coupled gas chromatography electroantennography and olfactometer bioassay studies. After elicitor treatment, Braz1006 released eightfold higher (E)-caryophyllene than Delprim, whereas no (E)-caryophyllene was detected in B73. The superior (E)-caryophyllene production by Braz1006 was positively correlated with high transcript levels of TPS23 in the landrace compared to Delprim. TPS23 alleles from Braz1006 showed dissimilarities at different sequence positions with Delprim and B73 and encodes an active enzyme. Cotesia sesamiae was attracted to egg-induced volatiles from Braz1006 and synthetic (E)-caryophyllene. The variation in (E)-caryophyllene emission between Braz1006 and Delprim is positively correlated with induced levels of TPS23 transcripts. The enhanced TPS23 activity and corresponding (E)-caryophyllene production by the maize landrace could be attributed to the differences in amino acid sequence with the other maize lines. This study suggested that the same analogous genes could have contrasting expression patterns in different maize genetic backgrounds. The current findings provide valuable insight not only into genetic mechanisms underlying variation in defense signal production but also the prospect of introgressing the novel defense traits into elite maize varieties for effective and ecologically sound protection of crops against damaging insect pests.

New directions for improving crop resistance to insects by breeding for egg induced defence.

Plant defence responses to insect oviposition, including tritrophic interactions with natural enemies of herbivores, have rarely been targeted in crop breeding programmes. Emission of herbivore induced plant volatiles (HIPVs) that attract natural enemies early on at the egg-laying stage of herbivore attack could provide timely biological control of pests and deter subsequent oviposition. This is needed in an agroecological context where the third trophic level often does not keep pace with the growth rate of pests. Our very recent data, using maize as an example, show that herbivore egg induced volatile emission is very rare in commercial hybrids but common in farmer selected landraces. Strategies for crop genetic improvement to enhance such responses to insect attack are considered.