Effects of seed treatments and storage duration on Myzus persicae (Hemiptera: Aphididae) and amaranth fresh leaf yield.

The green peach aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae), is a key insect pest of amaranth in East Africa. Pest management has been restricted to indiscriminate application of insecticides to foliage. Applying systemic insecticides to seeds has been shown to manage aphid infestations in other crop systems. We evaluated two commercially available seed treatments in East Africa, Apron Star (thiamethoxam 20 g/kg + metalaxyl-M 20 g/kg + difenoconazole 2 g/kg) and Menceron (imidacloprid 233 g/L + pencycuron 50 g/L + thiram 107 g/L) for their efficacy against M. persicae and impact on fresh leaf yield with two Amaranthus species, Amaranthus blitum (2 selections), Amaranthus hybridus (4 selections) and untreated control. Two storage periods (24 h and 3 months) with seed treatments were used. Each amaranth selection was treated individually with Apron and Monceren or untreated, and seeds were planted either 24 h or 3 months after treatment. Significant reduction in live aphids was observed with A. blitum and A. hybridus selections grown with seed treatment, at 6, 8 and 10 d after infestation (DAI) when compared with seeds grown without seed treatment. Untreated seeds of A. hybridus (selection 5) had significantly higher number of live aphids up to 243, greater percentage of damaged leaves and leaf damage score up to 84% and 64% respectively when compared with treated seeds of specific amaranth at 10 DAI. No significant difference was noted between seed treatment and storage time. Amaranth seeds treated with Monceren offered more protection against infestations of Myzus persicae than amaranth seeds treated with Apron under high tunnel experiments.

Identification of Repellents from Four Non-Host Asteraceae Plants for the Root Knot Nematode, Meloidogyne incognita.

Olfactory cues guide plant parasitic nematodes (PPNs) to their host plants. We tested the hypothesis that non-host plant root volatiles repel PPNs. To achieve this, we compared the olfactory responses of infective juveniles (J2s) of the PPN Meloidogyne incognita to four non-host Asteraceae plants, namely, black-jack (Bidens pilosa), pyrethrum (Chrysanthemum cinerariifolium), marigold (Tagetes minuta), and sweet wormwood (Artemisia annua), traditionally used in sub-Saharan Africa for the management of PPNs. Chemical analysis by coupled gas chromatography-mass spectrometry (GC/MS) combined with random forest analysis, followed by behavioral assays, identified the repellents in the root volatiles of B. pilosa, T. minuta, and A. annua as (E)-ß-farnesene and 1,8-cineole, whereas camphor was attractive. In contrast, random forest analysis predicted repellents for C. cinerariifolium and A. annua as ß-patchoulene and isopropyl hexadecanoate. Our results suggested that terpenoids generally account for the repellency of non-host Asteraceae plants used in PPN management.

Resistance of Amaranthus Spp. to the Green Peach Aphid (Hemiptera: Aphididae).

The green peach aphid [Myzus persicae (Sulzer)] is an important pest of amaranth grown for leaf consumption (i.e., leafy amaranth) in the tropics. Aphids reduce the amount of fresh leaf yield of amaranth and the value of leafy amaranth as aphid-infested leaves are not marketable. Our objective was to evaluate Amaranthus species selected by a breeding program in East Africa to develop cultivars for leaf consumption with resistance to M. persicae. We focused on antibiosis to determine whether varieties of Amaranthus spp. could be grown without producing an aphid population. Artificial infestations of aphids were placed on multiple selections of three species of Amaranthus: two selections of A. blitum, four selections of A. hybridus and one selection of A. hypochondriacus. Aphid populations were assessed over a 5-wk period. Evaluations of vegetative yield, leaf damage symptoms, and specific leaf area (SLA) were made of the seven selections at the end of this experiment. Aphid populations assessed 49 d after planting differed significantly (P ≤ 0.001) among the amaranth species and within selections of the same species. The selections of A. blitum had the lowest aphid populations, and A. hybridus had the highest populations. Selections of A. hybridus produced the most marketable leaves (i.e., aphid free). The fresh weight of A. blitum were the lowest of the seven selections, whereas A. hybridus had the greatest fresh leaf weight. Implications of these finding for further promotion of amaranth breeding are discussed related to pest management for leaf production.

Elicitation of Differential Responses in the Root-Knot Nematode Meloidogyne incognita to Tomato Root Exudate Cytokinin, Flavonoids, and Alkaloids.

Root exudates of plants mediate interactions with a variety of organisms in the rhizosphere, including root-knot nematodes (RKNs, Meloidogyne spp.) We investigated the responses of the motile stage second-stage juveniles (J2s) of Meloidogyne incognita to non-volatile components identified in the root exudate of tomato. Using stylet thrusting, chemotaxis assays, and chemical analysis, we identified specific metabolites in the root exudate that attract and repel J2s. Liquid chromatography quadrupole time-of-flight mass spectrometry analysis of bioactive fractions obtained from the root exudate revealed a high diversity of compounds, of which five were identified as the phytohormone zeatin (cytokinin), the flavonoids quercetin and luteolin, and alkaloids solasodine and tomatidine. In stylet thrusting and chemotaxis assays, the five compounds elicited concentration-dependent responses in J2s relative to 2% dimethyl sulfoxide (negative control) and methyl salicylate (positive control). These results indicate that J2 herbivory is influenced by root exudate chemistry and concentrations of specific compounds, which may have potential applications in RKN management.

Management of Cyst and Root Knot Nematodes: A Chemical Ecology Perspective.

Plant parasitic nematode infection of crops can be highly detrimental to agricultural production. Since the discovery that plant roots release chemicals that attract the infective stage of plant parasitic nematodes some 80 years ago, significant progress in identifying the signaling molecules has occurred only relatively recently. Here, we review the literature on chemical ecological studies of two major plant parasitic nematode groups: root knot nematodes in the genus Meloidogyne and cyst nematodes in the genus Globodera because of the negative impact their parasitism has on farming systems in Africa. We then highlight perspectives for future directions for their management.

Cucumber and Tomato Volatiles: Influence on Attraction in the Melon Fly Zeugodacus cucurbitate (Diptera: Tephritidae).

The main hosts of the melon fly Zeugodacus cucurbitate are cultivated and wild cucurbitaceous plants. In eastern Africa, the melon fly is a major pest of the Solanaceae plant Solanum lycopersicum (tomato). We hypothesized that shared species-specific volatiles may play a role in host attraction. We tested this hypothesis by comparing the olfactory responses of the melon fly to Cucumis sativus (cucumber) (Cucurbitaceae) and tomato plant odors in behavioral and electrophysiological assays, followed by chemical analysis to identify the key compounds mediating the interactions. Our results identified 13 shared components between cucumber and tomato plant odors. A synthetic blend of seven of the shared components dominated by monoterpenes at concentrations mimicking the volatile bouquet of cucumber and tomato attracted both sexes of the melon fly. Our results suggest that the presence and quantity of specific compounds in host odors are the main predictors for host recognition in Z. cucurbitate.

Temperature-dependent models of development and survival of an insect pest of African tropical highlands, the coffee antestia bug Antestiopsis thunbergii (Hemiptera: Pentatomidae).

The antestia bug Antestiopsis thunbergii (Hemiptera: Pentatomidae) is a major pest of Arabica coffee in African tropical highlands. It feeds on coffee plant vegetative parts and berries leading to a direct reduction in coffee yield and quality. This study aimed to determine A. thunbergii thermal requirements, and to obtain new information on the pest demography as influenced by temperature. Temperature-dependent models were developed using the Insect Life Cycle Modelling software (ILCYM) through a complete life table study at seven constant temperatures in the range 18-32°C. Non-linear functions were fitted to A. thunbergii development, mortality, fecundity and senescence. Model parameters and demographic variables obtained from the models were given for each temperature and development stage. Life table parameters were estimated for nine constant temperatures, from 18°C to 26°C, using stochastic simulations. The minimum temperature threshold (Tmin) and the thermal constant (k) for the development from egg to adult were estimated from a linear function at 12.1°C and 666.67° days, respectively. The maximum temperature threshold (Tmax) was estimated at 33.9°C from a Logan model. The optimum temperature for immature stages' survival was estimated to be between 22.4 and 24.7°C. The maximum fecundity was 147.7 eggs female-1 at 21.2°C. Simulated A. thunbergii life table parameters were affected by temperature, and the maximum value of intrinsic rate of increase (rm) was 0.029 at 22°C and 23°C. In general, the life cycle data, models and demographic parameters we obtained were in line with previous reports for antestia bugs or other stink bug species. The relationships between the pest thermal requirements and ecological preferences in highland coffee were discussed. Our results will contribute to risk prediction under climate change for this important coffee pest.

Parasitic nematode Meloidogyne incognita interactions with different Capsicum annum cultivars reveal the chemical constituents modulating root herbivory.

Plant volatile signatures are often used as cues by herbivores to locate their preferred hosts. Here, we report on the volatile organic compounds used by the subterranean root-knot nematode (RKN) Meloidogyne incognita for host location. We compared responses of infective second stage juveniles (J2s) to root volatiles of three cultivars and one accession of the solanaceous plant, Capsicum annum against moist sand in dual choice assays. J2s were more attracted to the three cultivars than to the accession, relative to controls. GC/MS analysis of the volatiles identified common constituents in each plant, five of which were identified as α-pinene, limonene, 2-methoxy-3-(1-methylpropyl)-pyrazine, methyl salicylate and tridecane. We additionally identified thymol as being specific to the accession. In dose-response assays, a blend of the five components elicited positive chemotaxis (71-88%), whereas individual components elicited varying responses; Methyl salicylate (MeSA) elicited the highest positive chemotaxis (70-80%), α-pinene, limonene and tridecane were intermediate (54-60%), and 2-methoxy-3-(1-methylpropyl)-pyrazine the lowest (49-55%). In contrast, thymol alone or thymol combined with either the preferred natural plant root volatiles or the five-component synthetic blend induced negative chemotaxis. Our results provide insights into RKN-host plant interactions, creating new opportunities for plant breeding programmes towards management of RKNs.

Steroidal glycoalkaloids: chemical defence of edible African nightshades against the tomato red spider mite, Tetranychus evansi (Acari: Tetranychidae).

BACKGROUND: Tetranychus evansi is an invasive pest of solanaceous crops in Africa, and in the field it differentially attacks edible African nightshades. The chemical basis for the differential attack on these plant species is largely unknown. Using bioassays and chemical analysis, we investigated the differential bioactivity of leaf extracts of three edible African nightshade species, Solanum sarrachoides, S. scabrum and S. villosum, on adult T. evansi females. RESULTS: Only the bioactivity of the leaf extract of S. sarrachoides (LC50 7.44 mg mL(-1)) and that of its most polar fraction (LC50 5.44 mg mL(-1)) paralleled that of the positive control, neem oil (LC50 1.89 mg mL(-1)), across all doses tested. Liquid chromatography-quadruple time of flight-mass spectrometry identified a mixture of steroidal glycoalkaloids (SGAs), including α-solasonine, α-solamargine and derivatives of tomatine and demissine, which were neither detected in the crude extract nor in any of the fractions obtained from S. scabrum and S. villosum. CONCLUSION: Our results suggest that the presence of SGAs may play a key role in the differential defence of edible African nightshades against attack by T. evansi. These findings may add to the plethora of environmentally friendly tools from natural plant products for management of T. evansi.