Sex pheromone biology and behavior of the cowpea weevilCallosobruchus maculatus (Coleoptera: Bruchidae)

Female cowpea weevils,Callosobruchus maculatus (F.), emitted a pheromone which excited males. Pheromone release began soon after emergence and continued for one week. Synchronization of pheromone release with calling behavior was demonstrated. Mating reduced pheromone release but not male response. Pheromone obtained by aeration collection was utilized for determining a quantitative dose-response relationship.Mention of a commercial product does not constitute an endorsement by the USDA.Visiting scholar on leave from Shanghai Institute of Entomology, PRC.     

Biological control of bruchids infesting cowpea by the introduction of Dinarmus basalis (Rondani) (Hymenoptera: Pteromalidae) adults into farmers’ stores in West Africa

Biological control of bruchids infesting cowpea stores was achieved by introducing adults of an endemic parasitoid Dinarmus basalis (Rondani) (Hymenoptera: Pteromalidae) into farmers’ storage systems in the guinean zone in Togo and the sudano-sahelian zone in Burkina Faso. In the guinean zone, introduction of 20 or 40 pairs of D. basalis adults at the beginning of storage and again 15 days later reduced the development of the bruchid populations after 5 months of storage. Consequently, weight losses from 25 kg batches of treated cowpea seeds were lower than 10% as compared with around 30% in controls. In the sudano-sahelian zone, an introduction of 40 pairs of D. basalis adults at the beginning of storage and again 15 days later was effective, also maintaining seed weight losses from 25 kg batches at less than 10%. These results illustrate that biological control of bruchids by the introduction of D. basalis adults into farmers’ cowpea storage systems is possible in the two climatic zones of Western Africa, and produces better cowpea seed quality. However, the introduction must take place right at the beginning of storage, and the number to be introduced must take into account the initial rate of seed infestation by the bruchids.     

The inhibitory effect of the natural seed storage protein arcelin on the development of Acanthoscelides obtectus

To protect stored beans from attack by Acanthoscelides obtectus and other bruchids by non-toxic means of control, it is desirable to combine mutually compatible methods in an integrated approach. A combination of an optimal level of host–plant resistance based on the natural storage protein arcelin and a larval parasitoid has been suggested as a promising two-component strategy. While a dose–response to arcelin is evident in the bruchid Zabrotes subfasciatus, the effects of different arcelin concentrations on A. obtectus are still virtually unknown, even though they might be important for the success of the proposed integrated control system. The current investigation quantified the effect of different arcelin-based resistance levels of beans on the development of A. obtectus in a series of bioassays. Our findings show that all seven tested bean lines with markedly different levels of arcelin-based resistance against Z. subfasciatus consistently prolonged development of A. obtectus. Results provide clear evidence that arcelin acts as a growth inhibitor on first instar larvae. Furthermore, arcelin affected bruchid female fitness through its effect on body mass, which was significantly reduced on all arcelin-containing bean lines. This study demonstrates that, irrespective of its relative concentration, arcelin holds the potential to affect development of A. obtectus sufficiently to support the proposed two-component integrated pest management system.     

Combining parasitoids and plant resistance for the control of the bruchid Acanthoscelides obtectus in stored beans

Acanthoscelides obtectus (Say) and Zabrotes subfasciatus (Boheman) are the main bruchid pests of stored beans in widespread regions of Latin America and Africa. Host-plant resistance based on the protein arcelin is effective in reducing damage caused by Z. subfasciatus, but beans containing arcelin remain susceptible to A. obtectus. The compatibility of combining arcelin resistance with biological control by Dinarmus basalis (Rondani) was investigated in climatic chambers. Three arcelin containing bean varieties with high and intermediate resistance towards Z. subfasciatus (RAZ 36, RAZ 94 and RAZ 104) and an arcelin-free standard (Calima, susceptible to both bruchids) were investigated. Immature development of A. obtectus in arcelin-containing beans was prolonged by 15% as compared to the standard, allowing D. basalis to have access to suitable host stages for a longer period of time. Over a 20-week storage period, the combined use of resistant host plants and biological control agents yielded best results with the host-plant varieties RAZ 94 and RAZ 104. In both varieties, parasitoids managed to keep bruchid damage below 1% as compared to 4.7% in the arcelin-free standard, and bruchids were eradicated in 80% of the replicates. Control levels in RAZ 36, the bean variety with the highest resistance to Z. subfasciatus, were not greater than the standard. Our results show that the combination of certain arcelin-enriched bean varieties with the parasitoid D. basalis is favourable for suppressing damage by A. obtectus.     

Bioactivity of endosperm extracts from susceptible and resistant bean seeds against Callosobruchus maculatus

Due to their high nutritional value, seeds are often the target of herbivory. In response, plants have evolved a host of adaptations that physically and/or chemically reduce the likelihood of granivory. In grain legumes, the main chemical defences within the endosperm derive from anti-nutritional proteins such as lectins and protease inhibitors. However, less is known about the bioactivity of secondary metabolites found within seeds. Here, solvent extraction was used to identify the major classes of phytochemicals found within the endosperm of both resistant beans (Phaseolus vulgaris) and susceptible beans (Vigna unguiculata). Phenols and terpenoids were recovered from the resistant P. vulgaris seeds only, whilst glycosides and flavonoids were recovered in greater amounts from P. vulgaris in comparison to V. unguiculata. To assess the bioactivity of the extracts against the seed beetle, Callosobruchus maculatus, the extracts were incorporated into artificial seeds and key life-history traits of the beetle determined. The results show the endosperm of both resistant and susceptible beans to contain an ensemble of secondary metabolites that deter oviposition (antixenosis) by adult beetles and disrupt the survival and development of the larvae (antibiosis). However, no extract resulted in complete resistance, indicating that the phytochemicals function alongside other anti-herbivory mechanisms such as α-amylase inhibitors and protease inhibitors. Breeding programmes could potentially select for key phytochemical groups such as terpenoids or glycosides in cultivars in order to optimise the trade-off between resistance and agronomic quality by harnessing the anti-herbivory properties of secondary metabolites in conjunction with anti-nutritional proteins.     

Dried leaves from rocky mountain plants decrease infestation by stored-product beetles

Leaves of two highly aromatic plants,Artemisia tridentata (Nutt.) andMonarda fistulosa L., prepared according to a patented process, inhibited oviposition by the Mexican bean weevil,Zabrotes subfasciatus (Boheman), in beans at concentrations less than 1% w/w. Both plant species were less effective against the rice weevil,Sitophilus oryzae L., in wheat, with onlyM. fistulosa exhibiting any concentration-dependent activity. The maximal control achieved against this species was less than 50% at 3% w/w. Two less aromatic plant species,Balsamorhiza sagittata (Pursh.) Nutt. andGeranium viscosissimum Fisch. and Mey., caused only low levels of inhibition against both insect species. Volatiles probably caused the response toA. tridentata andM. fistulosa, while the asymptotic concentration dependence for the less volatile plant material was likely caused by behavioral factors related to the physical presence of foreign particulate matter in the foodstuff. Chemical analysis indicated that most of the volatile components from the dried leaf material from all species were terpenoids, with camphor (9.7 mg/g) and 1,8-cineole (4.0 mg/g) being most abundant inA. tridentata and carvacrol (26.3 mg/g) being most abundant inM. fistulosa.Portions of this research were conducted by the U.S. Department of Agriculture. All programs and services of the U.S. Department of Agriculture are offered on a nondiscriminatory basis without regard to race, color, national origin, religion, sex, age, marital status, or handicap. This article reports the results of research only. Mention of a proprietary product does not constitute an endorsement or a recommendation for its use by USDA.     

Seed coat phytochemistry of both resistant and susceptible seeds affords some protection against the granivorous beetle Callosobruchus maculatus

The seed coat lies at the interface between the internal structures of the seed and the external environment and thus represents a key arena in the study of seed-herbivore interactions. Callosobruchus maculatus is a cosmopolitan pest of legume seeds, and under post-harvest conditions, females interact directly with the seed testa prior to laying their eggs. Here we investigate the effect of chemical extracts of the seed coat of the resistant Phaseolus vulgaris and the susceptible Vigna unguiculata beans on egg laying preferences and larval development of C. maculatus. Seed coat extracts contained phenolic, glycoside and alkaloid compounds. Upon re-incorporation of extracts into artificial host beans it was found that that several seed coat extracts from both the resistant and susceptible varieties reduced female oviposition and disrupted larval growth and development. However, none of the extracts assayed resulted in complete ovipositional or developmental failure suggesting that complete resistance in P. vulgaris is derived from other physical or chemical properties of the seed and/or seed coat that function either alone or synergistically. Further work is required to elucidate the importance of synergistic interactions between different physiological defence mechanisms on overall plant (seed) resistance.     

Response ofNp mutant of pea (Pisum sativum L.) to pea weevil (Bruchus pisorum L.) oviposition and extracts

TheNp mutant of pea (Pisum sativum L.) is characterized by two physiological responses: growth of callus under pea weevil (Bruchus pisorum L., Coleoptera: Bruchidae) oviposition on pods, and formation of neoplastic callus on pods of indoor-grown plants. Although these two responses are conditioned byNp, they are anatomically and physiologically distinguishable, based on sites of origin, distribution pattern, and sensitivity to plant hormones. Further characterization of the response to extracts of pea weevil showed that response of excised pods, measured by callus formation, was log-linear, and treatment with as little as 10^–4 weevil equivalents produced a detectable response. Mated and unmated females contained similar amounts of callus-inducing compound(s), and immature females contained significantly less of the compound(s). Female vetch bruchids (Bruchus brachialis F., Coleoptera: Bruchidae), a related species, contained callus-inducing compound(s), but usually less than pea weevils on a per weevil basis. Males of both species contained less than 10% of the activity of the mature females. Extracts of female black vine weevils, a nonbruchid species, did not stimulate callus formation. Based on partitioning and TLC analysis, the biologically active constitutent(s) was stable and nonpolar. Thus, theNp allele probably conditions sensitivity to a nonpolar component of pea weevil oviposition as a mechanism of resistance to the weevil.     

Effect of various essential oils on Callosobruchus maculatus (F.) (Coleoptera: Bruchidae)

Essential oils were extracted from four West African plant species [Tagetes minuta (Family Compositae), Hyptis suaveolens (Family Labiatae), white basil Ocimum canum (Family Labiatae), and sweet basil O. basilicum (Family Labiatae)] by steam distillation. The oil of the pepper Piper guineense (Family Piperaceae), was extracted from the fruits by hydro distillation and ethanol extraction. Mixed essential oil and total ethanol extract was used. Kaolin powder (clay) was mixed (aromatized) with these different oils. Cowpea weevils were reared on chickpeas and newly emerged males and females were deposited on uninfested seeds. Bioassays, i.e. fumigation with pure essential oils and aromatized kaolin powders, were carried out on adults and eggs. Twenty four hours after fumigation, 99 and 0% adult mortality were observed, respectively, as the result of treatments with Ocimum basilicum and the control. The application of powders aromatized with the same oils to weevil pairs resulted in a complete lack of oviposition, whereas 31, 56 and 76 eggs were laid in the controls after 24, 48 and 72 h, respectively. Application of aromatized powders did not have a significant effect on egg hatching (50 out of 110 with O. canum, 100 out of 115 with O. basilicum and 100 out of 130 in the control sample) but did have a significant impact on adult emergence: 0% for the two treatments compared with 100% in the controls. Our results suggest that plants of the genus Ocimum can be used as an alternative to synthetic insecticides.     

Oviposition deterrent from eggs ofCallosobruchus maculatus: Spacing mechanism or artifact?

Females ofCallosobruchus maculatus (F.) avoid ovipositing on host seeds already bearing conspecific eggs, and thus distribute eggs evenly among seeds. This behavior was presumed to be mediated by an ether-soluble oviposition marker that is deposited with the egg and can be extracted from egg-laden artificial hosts (glass beads). Ablation experiments revealed that the true factors promoting an even dispersion of eggs were perceived by the maxillary and labial palpi. In contrast, receptors on the antennae were largely responsible for avoidance of seeds treated with oviposition marker. Taken together, these results suggest that a careful distinction should be drawn between factors that promote spacing of eggs under natural conditions and general oviposition deterrents that may be isolated from both sexes.