Exploring levels of egg parasitism and variation in egg cuticular chemistry in different Clavigralla spp.

Clavigralla spp. (Hemiptera: Coreidae) are major pests of cowpea (Vigna unguiculata (L.) Walp, Fabacae), common bean (Phaseolus vulgaris L., Fabacae) and pigeon pea (Cajanus cajan L., Fabacae) in Africa. Clavigralla spp. egg parasitoids, Gryon spp. (Hymenoptera: Scelionidae), have previously been reported as potential biological control candidates. Little is known about the parasitism levels and their potential relationship with cuticular chemistry of Clavigralla spp. The aims of this study were to determine parasitism levels of Clavigralla tomentosicollis Stål (Hemiptera: Coreidae) and C. elongata Signoret (Hemiptera: Coreidae) eggs, and to explore the relationship between egg parasitism and egg cuticular chemistry. High parasitism levels were determined for C. tomentosicollis by collecting eggs from plants in mono‐cropping and multi‐cropping systems in farmers’ fields in Bénin and Kenya between April and June 2016. Three species of Clavigralla were recorded: C. tomentosicollis, C. shadabi and C. elongata. Clavigralla tomentosicollis was the most common in both countries, while C. shadabi and C. elongata were only collected in Bénin and Kenya, respectively. An egg parasitoid (Gryon sp.) was recovered from egg batches collected from both countries. In parasitism assays using Gryon sp., the incidence of parasitism was higher in C. tomentosicollis eggs than that of C. elongata. Chemical analysis by coupled gas chromatography/mass spectrometry (GC/MS) of cuticular extracts obtained from C. tomentosicollis and C. elongata eggs identified fifteen compounds including ten alkanes of which the amounts varied between the two species. We speculate that Clavigralla spp. cuticular chemistry may serve as potential host location cues for Gryon sp.     

Insect vectors of cowpea mosaic virus in Nigeria

A yellow strain of cowpea mosaic virus (CPMV) was transmitted in cowpea by two thrips, Sericothrips occipitalis and Taeniothrips sjostedti; two chrysomelid beetles, Ootheca mutabilis and Paraluperodes quaternus; a curculionid beetle, Nematocerus acerbus; and two acridid grasshoppers, Catantops spissus spissus and Zonocerus variegatus. Summarizing trials with single insects, the efficiency of transmission of CPMV averaged 18—21% for N. acerbus and the two grasshoppers, 55% for P. quaternus, and 71% for O. mutabilis. Twenty-two and 40% of the plants exposed to large populations of S. occipitalis and T. sjostedti, respectively, were infected. In three trials with an aphid, Aphis craccivora, 4 of 49 plants were infected with CPMV, but these infections were considered spurious because no infections occurred in any of 63 plants exposed to this insect in four other trials. A coreid bug, Riptortus dentipes, did not transmit CPMV. Mosaic symptoms in infected plants appeared 5—39 days after they were exposed to vectors. Infective virus was recovered from fresh faecal pellets of each grasshopper vector.     

Temperature influence on cohort parameters and demographic characteristics of the two cowpea coreids Clavigralla tomentosicollis and C. shadabi

Age-specific life tables of two important pests of cowpea, Vigna unguiculata (L.) Walp., the pod sucking bugs Clavigralla tomentosicollis Stål and C. shadabi Dolling (Heteroptera: Coreidae), were obtained from observations carried out at different temperatures. A biophysical model was found satisfactory to describe the temperature-response of developmental and mortality rates of egg and nymphal stages, with a peak developmental rate around 34°C in both species. The variability in development times was small and the experimental data did not permit any conclusion with regard to the Erlang probability density function. Survival of eggs and nymphs remained high between 20° and 30°C for both species. At temperatures above 34°C, C. tomentosicollis survivorship and fecundity was higher than that of C. shadabi, which in turn laid more eggs at temperatures between 20° and 30°C. Maximum fecundity is estimated to be at 29°C for C. tomentosicollis (99 eggs/female) and 26°C for C. shadabi (261 eggs/female). At 30°C, the intrinsic rate of increase reached a maximum in both species, 0.152 per day for C. tomentosicollis and 0.145 per day for C. shadabi, and remained high for C. tomentosicollis until 36°C. C. tomentosicollis performed significantly better on pigeonpea, Cajanus cajan Millsp., than on cowpea at higher temperatures.     

Repellent activity of Cymbopogon citratus and Tagetes minuta and their specific volatiles against Megalurothrips sjostedti

Cowpea is an important source of protein for people in Africa. However, the crop suffers major damage and yield losses due to bean flower thrips, Megalurothrips sjostedti Trybom (Thysanoptera: Thripidae). Although companion plants are known to reduce the damage caused by insect pests, the role of their volatiles in repelling pests from target plants has been the subject of few investigations. Here, we used the Y‐tube olfactometer experiments and chemical analyses to investigate the effect of volatiles from cowpea flowers and two companion plants; lemongrass, Cymbopogon citratus and Mexican marigold, Tagetes minuta on the olfactory responses of M. sjostedti. The results revealed that M. sjostedti males and females were repelled by the volatiles from freshly cut leaves of C. citratus. The combination of freshly cut leaves of C. citratus and cowpea flower was repellent to females but not to males. The female thrips, but not males, were repelled by the volatiles from the vegetative stage of T. minuta. Fifty‐four compounds were identified in the volatiles from two herbal plants. Among the major compounds, citral and a 4‐component blend comprised of dihydrotagetone, (Z)‐3‐hexenyl acetate, limonene and (Z)‐β‐ocimene repelled females but dihydrotagetone alone attracted females. While myrcene combined with cowpea flower volatiles enhanced the attraction of females M. sjostedti, when tested alone was not attractive. These results highlight the potential of volatiles from C. citratus and T. minuta to repel M. sjostedti females. The use of these plants as companion plants in a cowpea cropping system could reduce M. sjostedti infestation.     

Behavioural response of alate Aphis craccivora Koch (Homoptera: Aphididae) to volatiles from different cowpea cultivars

The cowpea aphid, Aphis craccivora, is a major insect pest of cowpea in Africa. Volatile organic compounds (VOCs) mediate plant–arthropod interactions that could be used in the management of insect pests. In this study, we established the VOC profile involved in the interaction between A. craccivora and four cowpea cultivars, namely Ex‐Luanda, Katumani 80, Machakos 66 and Ken Kunde 1. Behavioural assays were conducted to study host preference and gas chromatography‐mass spectrometry (GC/MS) for chemical analysis of volatiles. In preference assays, alate A. craccivora had no significant preference for any of the four cowpea cultivars tested. However, in the olfactometer assays, the aphids showed a significant preference for odours from cultivar Ex‐Luanda compared to Katumani 80. Machakos 66 and Ken Kunde 1 elicited neutral responses. In pairwise comparisons, alate A. craccivora did not distinguish between odours of respective cowpea cultivars. GC/MS analysis identified 23 compounds in the volatiles of the four cowpea cultivars. Not all compounds were detected in all cowpea cultivars, and the detected compounds amounts varied in each cultivar. Of these, only four compounds (hexanal, (E)‐2‐hexenal, 1‐octen‐3‐ol and p‐xylene) were emitted in significantly different quantities in the four cultivars. A blend of hexanal and (E)‐2‐hexenal added to cowpea cultivar Ex‐Luanda decreased its attractiveness to A. craccivora compared to the control. Our findings showed differential attractiveness of VOCs of cowpea cultivars to A. craccivora, suggesting that VOCs could be used in the management of A. craccivora.     

Pod hairs as a factor in Vigna vexillata resistance to the pod-sucking bug, Clavigralla tomentosicollis

Two varieties of Vigna vexillata Benth proved more highly resistant, both under field (free-choice) and screenhouse (no-choice) conditions, to infestation and damage by the pod-sucking bug Clavigralla tomentosicollis (Stål.) (Hemiptera: Coreidae) than cowpea (Vigna unguiculata Walp.) varieties. Trichomes on the pod of V. vexillata partly accounted for the resistance. However, other factors in the pod or seeds are involved in the resistance. Because the pod wall of V. vexillata is not tougher than that of cowpeas, biochemical characteristics of pods and seeds of V. vexillata will be further investigated.

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Résumé Deux variétés de Vigna vexillata se sont avérées plus résistantes aux infestations et dégâts causés par la punaise des cosses Clavigralla tomentosicollis (Stål.) (Hemiptera: Coreidae) que les variétés de Vigna unguiculata, tant en champ (condition de libre choix) qu'en serre (sans choix possible). Les trichomes des cosses de V. vexillata sont en partie responsables de cette résistance. Cependant d'autres facteurs de la cosses ou des fèves sont impliqués dan la résistance. Comme la paroi de la cosse de V. vexillata n'offre pas plus de résistance mécanique que celle de V. unguiculata, les études futures se porteront sur les caractéristiques biochimiques des cosses et des fèves de V. vexillata.

     

Influence of thrips prey species on the life-history and behaviour of Orius albidipennis

The effects of two diets, i.e., cowpea leaves plus Megalurothrips sjostedti larvae versus cowpea leaves alone, on nymphal development and mortality of Orius albidipennis were studied under controlled laboratory conditions in Benin, West Africa. Nymphal development was longer when O. albidipennis was fed with cowpea leaves only, compared to the diet complemented with M. sjostedti larvae (17.6 days versus 14.8, respectively). Nymphal mortality was very high (78.6%) if fed with cowpea leaves only, and still relatively high if the diet was complemented with thrips larvae (44.4%).In a separate study on the longevity and fecundity of O. albidipennis adult females feeding on M. sjostedtilarvae, an average of 6.8 eggs per day (maximum 16 eggs) and a mean total fecundity of 61.1 eggs were observed. The females lived on average 13.5 days.Finally, observations on the predation by O. albidipennis adults on three different thrips species revealed that M. sjostedti larvae were killed at a lower rate than larvae of Ceratothripoides cameroni and Frankliniella schultzei. Higher predation rates were measured using unmated females and males than in mated females. However, the rate of first attack, measured as first larva attacked in dual-choice assays, was higher for M. sjostedti when a F. schultzei larva was offered simultaneously, and not different when a C. cameroni or Sericothrips adolfifridericilarva was offered.The results of this study are discussed with regard to the lack of efficacy of O. albidipennis as biological control agent for M. sjostedti.     

Effect of application of different concentrations and appropriate schedules of aqueous garlic (Allium sativum L.) bulb extracts against Maruca vitrata and Clavigralla tomentosicollis on cowpea,Vigna unguiculata (L.) Walp

Abstract

Three concentrations (5, 10 and 20% w/v) and spraying schedules (2×, 4× and 6× weekly applications) of garlic, Allium sativum L. bulb extracts were assayed for efficacy against Maruca vitrata Fab. and Clavigralla tomentosicollis Stal. on cowpea under rain fed conditions in two seasons. The results showed that application of aqueous garlic bulb extracts at all the treatment levels significantly (p < 0.05) reduced the populations of the target pests compared with the untreated check. Application of 10 and 20% garlic bulb extracts with 4 or 6 weekly applications gave better control of these pests, protected the pods from damage and increased grain yields on treated cowpea compared with 5% extract at all three application schedules and were comparable to the synthetic insecticide treatment. This material could play a role in the integrated management of M. vitrata and C. tomentosicollis on cowpea fields managed by limited resource farmers in developing countries. The plant material is readily available, cheap, safe, non phytotoxic and technologically simple to use.     

Response of old and new cowpea varieties to insecticide spray regimes in the Sudan savanna of Nigeria

Insect pests are major constraint to cowpea production in northern Nigeria causing yield losses up to 70%. Several cowpea varieties have been developed and delivered to farmers by IITA over the past four decades. These varieties have varying degrees of resistance to insect pest attacks. A field study was established in northern Nigeria to determine the response to insecticide spraying of old cowpea varieties developed in the late 1970s and new varieties developed in the 2000s in order to determine whether new varieties have lesser requirement for spraying with insecticides than the old ones. The result revealed that the new cowpea varieties developed in the 2000s require more spraying than the old varieties developed in the 1970s. Infestations by the insect pests, maruca (Maruca virata Fabricius) and flower thrips (Megalurothrips sjostedti Trybom) were significantly less at zero and 1 spray for old varieties than for the new varieties. Old varieties also produced significant higher grain yield at zero and 1 spray compared with new varieties. Newer varieties require more spray to maximise yield gain.     

Worldwide Populations of the Aphid Aphis craccivora Are Infected with Diverse Facultative Bacterial Symbionts

Facultative bacterial endosymbionts can play an important role in the evolutionary trajectory of their hosts. Aphids (Hemiptera: Aphididae) are infected with a wide variety of facultative endosymbionts that can confer ecologically relevant traits, which in turn may drive microevolutionary processes in a dynamic selective environment. However, relatively little is known about how symbiont diversity is structured in most aphid species. Here, we investigate facultative symbiont species richness and prevalence among worldwide populations of the cowpea aphid, Aphis craccivora Koch. We surveyed 44 populations of A. craccivora, and detected 11 strains of facultative symbiotic bacteria, representing six genera. There were two significant associations between facultative symbiont and aphid food plant: the symbiont Arsenophonus was found at high prevalence in A. craccivora populations collected from Robinia sp. (locust), whereas the symbiont Hamiltonella was almost exclusively found in A. craccivora populations from Medicago sativa (alfalfa). Aphids collected from these two food plants also had divergent mitochondrial haplotypes, potentially indicating the formation of specialized aphid lineages associated with food plant (host-associated differentiation). The role of facultative symbionts in this process remains to be determined. Overall, observed facultative symbiont prevalence in A. craccivora was lower than that of some other well-studied aphids (e.g., Aphis fabae and Acyrthosiphon pisum), possibly as a consequence of A. craccivora's almost purely parthenogenetic life history. Finally, most (70 %) of the surveyed populations were polymorphic for facultative symbiont infection, indicating that even when symbiont prevalence is relatively low, symbiont-associated phenotypic variation may allow population-level evolutionary responses to local selection.     

Bionomics of the parasitoid Apanteles taragamae as influenced by different diets fed to its host,Maruca vitrata

The parasitoid Apanteles taragamae Viereck (Hymenoptera: Braconidae) has been introduced into Benin by the International Institute of Tropical Agriculture (IITA-Benin station) from Taiwan in an attempt to control the cowpea pod borer, Maruca vitrata Fabricius (Lepidoptera: Crambidae), a serious pest of flowers and pods of cowpea Vigna unguiculata Walp. (Leguminosae: Fabaceae). Both the pest and its parasitoid are currently mass-reared using a cowpea and maize flour-based meridic diet. In this study, the suitability of germinating seeds from three cowpea varieties, Kpodji-guèguè, Tawa and TVX3236, as substrates, was tested for rearing A. taragamae under laboratory conditions. The seeds that were soaked for 48 h were the most suitable for M. vitrata larval rearing with up to 87 % survival. The highest parasitism rate of M. vitrata larvae by A. taragamae was observed on the local variety Tawa. Compared to meridic diet, the development cycle of the parasitoid was shorter on Tawa sprouts which also lead to the highest lifetime fecundity. The parasitoid sex ratio varied significantly between feeding substrates with a higher proportion of female wasps on the improved varieties TVX2336 and local variety Kpodji-guèguè. Rearing M. vitrata larvae on germinating cowpea grains is 4.38 times cheaper than on meridic diet. The data are discussed with emphasis on the effects of feeding substrates quality on life history parameters of insect pests and their parasitoids.     

Importance of timing of application of the entomopathogenic fungus,metarhizium anisopliae for the control of legume flower thrips,megalurothrips sjostedti and its persistence on cowpea

Field experiments were conducted for two seasons to evaluate the timing of application of the entomopathogenic fungus, Metarhizium anisopliae for the control of legume flower thrips, Megalurothrips sjostedti on cowpea. One application of M. unisopliae timed at flower bud stage and another at flowering stage did not protect cowpea yield against M. sjostedti as does chemical insecticide, Karate (Lambda‐cyhalothrin). Instead, one application of the fungus given at flower bud stage and two applications given at flowering were required to keep M. sjostedti population in check through these stages, which are very sensitive to thrips damage with a concomitant increase in cowpea yield which was significantly higher than the Karate treatment. Studies of persistence showed that M. anisopliae remained active in the field for 3–4 days.     

Annual cycle of the legume pod borer Maruca vitrata Fabricius (Lepidoptera: Crambidae) in southwestern Burkina Faso

Maruca vitrata is an economically significant insect pest of cowpea in sub-Saharan Africa. Understanding the seasonal population patterns of M. vitrata is essential for the establishment of effective pest management strategies. M. vitrata larval populations on cultivated cowpea and adult flying activities were monitored in addition to scouting for host plants and parasitoids during 2 consecutive years in 2010 and 2011 in southwestern Burkina Faso. Our data suggest that M. vitrata populations overlapped on cultivated cowpea and alternate host plants during the rainy season. During the cowpea off-season, M. vitrata maintained a permanent population on the wild host plants Mucuna poggei and Daniella oliveri. The parasitoid fauna include three species, Phanerotoma leucobasis Kri., Braunsia kriegeri End. and Bracon sp. Implications of these finding for pest management strategies are discussed.     

Field evaluation of some selected plant materials for the control of major insect pests of cowpea (Vigna unguiculata(L.) Walp.) in the Northern Guinea Savannah of Nigeria

Abstract

Field experiments were carried out at the Teaching and Research Farm of the Abubakar Tafawa Balewa University, Bauchi during the rainy season of 2005. The study was carried out with sole objective of evaluating the efficacy of six selected plant materials (sweetsop, red pepper, garlic, neem, mahogany and gmalina) against the major insect pests of cowpea variety, Dan Sokoto. The experiment was laid down in randomised complete block design with seven treatments. Each treatment was replicated three times. The results of the study showed that all the plant materials used were significantly (p < 0.05) better than control where no plant material was used in controlling the population of B. tabaci, E. dolichi, M. sjostedti and C. tomentosicollis at 1, 2 and 3 days after application of the treatments. Similarly the effects of these plant materials on the number of seeds/pod showed a significant (p < 0.05) difference between plots treated with plant materials and control. On the grain yield of the crop, all the plant materials showed significant effect except mahogany and the control which were statistically similar. Furthermore, the order of level of control indicated that sweet sop has (70.7%), garlic (69.3%), neem (61.0%), red pepper (54.0%), ash (30.9%), and mahogany (3.5%). The result of the present finding therefore recommends the use of sweet sop, garlic and neem as they were found to be the most promising in the control of major cowpea insect pests.     

Effect of maize plants on colonisation of cowpea plants by bean flower thrips,Megalurothrips sjostedti

The effect of non-host maize plants on colonisation of cowpea byMegalurothrips sjostedti (Trybom) (Thysanoptera; Thripidae) was investigated. There were no differences in population density and activity ofM. sjostedti on sole cowpea crop and mixed cowpea/maize crop during the colonisation phase (i.e. 10–30 days after emergence of the plants. However, subsequentlyM. sjostedti numbers were lower in the mixed than in the sole crop, suggesting that maize did not interfere with colonisation of cowpea crop by thrips. In a choice situation, higher numbers ofM. sjostedti oriented towards, and settled on, sole cowpea plants than on those mixed with maize. Olfactory tests indicated that fewer thrips oriented towards a cowpea/maize mixed odour source. When equal numbers of thrips were introduced into the centre of sole- and mixed-cropped cowpea plots, the thrips became randomly distributed in each plot. Fewer thrips were recovered from the mixture than from the sole crop. It is concluded that, although non-host plant odours do not reduce thrips colonisation they interfere with host plant utilisation.     

Factors Limiting the Spread of the Protective Symbiont Hamiltonella defensa in Aphis craccivora Aphids

Many insects are associated with heritable symbionts that mediate ecological interactions, including host protection against natural enemies. The cowpea aphid, Aphis craccivora, is a polyphagous pest that harbors Hamiltonella defensa, which defends against parasitic wasps. Despite this protective benefit, this symbiont occurs only at intermediate frequencies in field populations. To identify factors constraining H. defensa invasion in Ap. craccivora, we estimated symbiont transmission rates, performed fitness assays, and measured infection dynamics in population cages to evaluate effects of infection. Similar to results with the pea aphid, Acyrthosiphon pisum, we found no consistent costs to infection using component fitness assays, but we did identify clear costs to infection in population cages when no enemies were present. Maternal transmission rates of H. defensa in Ap. craccivora were high (ca. 99%) but not perfect. Transmission failures and infection costs likely limit the spread of protective H. defensa in Ap. craccivora. We also characterized several parameters of H. defensa infection potentially relevant to the protective phenotype. We confirmed the presence of H. defensa in aphid hemolymph, where it potentially interacts with endoparasites, and performed real-time quantitative PCR (qPCR) to estimate symbiont and phage abundance during aphid development. We also examined strain variation of H. defensa and its bacteriophage at multiple loci, and despite our lines being collected in different regions of North America, they were infected with a nearly identical strains of H. defensa and APSE4 phage. The limited strain diversity observed for these defensive elements may result in relatively static protection profile for this defensive symbiosis.     

Soil pH alters the biological parameters of cowpea aphid Aphis craccivora Koch (Hemiptera: Aphididae) on its host plant Vicia faba

The biotic and abiotic factors including the agricultural implementation can modify soil acidification. We hypothesized that soil pH should as repercussion, alter the plant physiological and physical properties and eventually affect insect herbivores including agricultural pests. This study aimed to evaluate the impact of seven levels of soil pH on the performance of cowpea aphid Aphis craccivora on Vicia faba. Significant relationships between soil pH and growth of host bean seedlings or development and reproduction of the aphid were detected. Data demonstrated significant differences in the total longevity, the pre-reproductive, reproductive, post-reproductive and pre-viviparity periods. Within a suitable range of pH for bean growth between pH 5.3 and pH 7.2, the aphid performance was worse on seedlings growing better, however, under unfavorable extreme pH conditions, plant quality measured as height did not affect the aphids anymore and their performance was uniformly low except the case in pH 8.1 condition in which the best aphid reproduction was observed. The results confirm that soil pH affect the performance of cowpea aphid A. craccivora and also exhibited strong influence on the growth of broad bean plants.