Water stress augments silicon-mediated resistance of susceptible sugarcane cultivars to the stalk borer Eldana saccharina (Lepidoptera: Pyralidae)

Silicon (Si) can improve resistance of plants to insect attack and may also enhance tolerance of water stress. This study tested if Si-mediated host plant resistance to insect attack was augmented by water stress. Four sugarcane cultivars, two resistant (N21, N33) and two susceptible (N26, N11) to Eldana saccharina Walker were grown in a pot trial in Si-deficient river sand, with (Si+) and without (Si-) calcium silicate. To induce water stress, irrigation to half the trial was reduced after 8.5 months. The trial was artificially infested with E. saccharina eggs after water reduction and harvested 66 days later. Silicon treated, stressed and non-stressed plants of the same cultivar did not differ appreciably in Si content. Decreases in numbers of borers recovered and stalk damage were not associated with comparable increases in rind hardness in Si+ cane, particularly in water-stressed susceptible cultivars. Overall, Si+ plants displayed increased resistance to E. saccharina attack compared with Si- plants. Borer recoveries were significantly lower in stressed Si+ cane compared with either stressed Si- or non-stressed Si- and Si+ cane. Generally, fewer borers were recovered from resistant cultivars than susceptible cultivars. Stalk damage was significantly lower in Si+ cane than in Si- cane, for N21, N11 and N26. Stalk damage was significantly less in Si+ combined susceptible cultivars than in Si- combined susceptible cultivars under non-stressed and especially stressed conditions. In general, the reduction in borer numbers and stalk damage in Si+ plants was greater for water-stressed cane than non-stressed cane, particularly for susceptible sugarcane cultivars. The hypothesis that Si affords greater protection against E. saccharina borer attack in water-stressed sugarcane than in non-stressed cane and that this benefit is greatly enhanced in susceptible cultivars is supported. A possible active role for soluble Si in defence against E. saccharina is proposed.     

Silicon impedes stalk penetration by the borer Eldana saccharina in sugarcane

Many plants grown in soils amended with silicon (Si) display increased levels of resistance to attack by insect herbivores. This study aimed to determine if Si treatment impeded Eldana saccharina Walker (Lepidoptera: Pyralidae) stalk penetration and subsequent damage, as well as borer mass gain, on the node and internode of a susceptible (N11) and a resistant (N33) sugarcane cultivar. Sugarcane [Saccharum spp. (Poaceae)] cultivars were grown in a pot trial in Si‐deficient river sand, with (Si+) and without (Si–) calcium silicate. Sugarcane was infested with 2‐week‐old E. saccharina larvae and harvested at four times, 24, 48, 72, and 96 h after infestation. Silicon‐treated plants showed significant increases in Si content compared to controls, and the external rind was significantly harder for Si+ cane than Si– cane. Silicon treatment significantly decreased borer penetration, stalk damage, and larval mass gain. The results are consistent with the hypothesis that Si contributes to sugarcane stalk borer resistance by impeding larval penetration. Silicon appears to contribute to the suppression of E. saccharina directly through reduced larval growth and feeding damage to the crop, and indirectly by delaying stalk penetration, resulting most likely in increased exposure time of young larvae to natural enemies, adverse climatic factors, or control measures that target young larvae (e.g., insecticides).     

Interactions between Fusarium species from sugarcane and the stalk borer, Eldana saccharina (Lepidoptera: Pyralidae)

In this study, beneficial relationships between Fusarium species in sugarcane and the stalk borer, Eldana saccharina (Lepidoptera: Pyralidae) were demonstrated in vitro . In addition, Fusarium species with antagonistic properties were found to inhabit sugarcane. Attenuated Fusarium isolates from sugarcane stalks collected across the South African sugar industry were incorporated into diet formulated to mass rear E. saccharina . Of the over 200 Fusarium isolates obtained, 10 that significantly improved the survival and development of E. saccharina larvae and considered to be beneficial were selected for further study, as were 10 that were detrimental or antagonistic to the stalk borer. Twelve of the selected isolates were identified as F. sacchari by direct sequencing of translation elongation factor-1α fragments, eight of which resulted in reduced numbers of surviving larvae and significantly lower larval masses. F. proliferatum isolates resulted in increased survival and mass of E. saccharina and three of the five isolates assigned to the F. pseudonygamai group enhanced E. saccharina development. Results from olfactory choice experiments suggested the production of attractive and repellent metabolites by certain isolates. Findings from this study may offer additional or alternative strategies for managing E. saccharina infestation of sugarcane.     

Bt-maize as a potential trap crop for management of Eldana saccharina Walker (Lep., Pyralidae) in sugarcane

Abstract:  Notwithstanding the introduction of several pest management tactics, the stalk borer Eldana saccharina Walker (Lep., Pyralidae) remains the most serious pest in South African sugarcane. A novel tactic for managing this pest in sugarcane would be the use of a dead-end trap crop that attracts moths for oviposition and curtails subsequent larval development, thereby reducing pest population size. Glasshouse bioassays, in which moths chose to oviposit on maize producing Bacillus thuringiensis Cry1Ab toxin ( Bt -maize), non- Bt -maize or sugarcane of two cultivars (borer-resistant and -susceptible), showed that E. saccharina laid significantly more eggs and egg batches per dry leaf and unit mass of dry leaf on maize ( Bt or non- Bt ) than on either of the cane cultivars. When moths had a choice of ovipositing on 2-, 3-, 4- or 5-month-old maize ( Bt and non- Bt ), dry leaf number and mass of dry leaf material was significantly correlated with number of eggs and egg batches, indicating that older plants, which carried larger amounts of dry leaf matter, were more attractive for oviposition. Finally, glasshouse assays in which hatching larvae fed on 2.5-, 3.5- and 4.5-month-old Bt and non- Bt -maize plants, showed that the Cry1Ab toxin was effective in killing E. saccharina larvae in all Bt -maize plant growth stages, confirming that Bt -maize fulfilled the third requirement (curtailing larval development) of a dead-end trap crop for this pest. We argue that Bt -maize warrants further testing in the field as a trap crop, both alone and as a component of a 'push–pull' or habitat management system for E. saccharina in sugarcane.     

Silicon-mediated resistance of sugarcane to Eldana saccharina Walker (Lepidoptera: Pyralidae): effects of silicon source and cultivar

Abstract:  The effects of four silicon sources – a USA calcium silicate, a local (South African) calcium silicate, Slagment^® and fly ash – on the resistance of sugarcane cultivars (two resistant and two susceptible) to Eldana saccharina Walker (Lepidoptera: Pyralidae) were studied in a potted sugarcane trial. Silicon sources were applied at 5000 or 10 000 kg/ha for the calcium silicates and Slagment; fly ash was applied at 15 000 or 30 000 kg/ha. The greatest increase in plant silicon content (particularly in stalks) was recorded for plants treated with local calcium silicate. Silicon uptake did not vary significantly between the susceptible and resistant cultivars, although the resistant cultivars had inherently higher silicon content than the susceptible ones. Treatment with silicon significantly reduced borer damage and borer performance at the higher treatment level. In general, borer damage and performance decreased with increasing rates of applied silicon and both variables were inversely related with per cent stalk silicon. On average, the higher silicon rate reduced damage by 34% in the susceptible cultivars and by 26% in the resistant cultivars, supporting the argument that susceptible cultivars benefit more from silicon treatments than resistant ones. We propose that calcium silicate amendments could be employed in the integrated, area-wide management of E. saccharina and in the management of soil acidity, both of which are widespread problems in the South African sugar industry.     

Status of Eldana saccharina (Lepidoptera: Pyralidae), its host plants and natural enemies in Ethiopia

Surveys for sugarcane stem borers were undertaken in Ethiopia to determine the prevalence and distribution of these and their natural enemies in crops and indigenous host plants. Eldana saccharina Walker was not recovered from sugarcane, but was present in three indigenous wetland sedges, Cyperus papyrus, C. fastigiatus and C. dives in the southern, central and northern part of the country. The latter indigenous host plant was present in waterways adjacent to sugarcane on the commercial sugar estates. The tachinids Schembria eldanae Barraclough and Actia sp. were common parasitoids of E. saccharina larvae in these indigenous sedges. The braconid Dolichogenidea sp. was recovered from E. saccharina larvae in C. dives. Pathogens comprising Beauveria bassiana, Bacillus thuringiensis and Entomophthora sp., were found to be important mortality factors of E. saccharina larvae in the indigenous sedges. The occurrence of E. saccharina in Ethiopia is reported for the first time, and the host plant preferences of the borer and its indigenous natural enemies found during the surveys are recorded. In addition, its potential threat to sugarcane production in Ethiopia is discussed.     

Soil silicon amendments increase resistance of sugarcane to stalk borer Eldana saccharina Walker (Lepidoptera: Pyralidae) under field conditions

Background and aims

Soil amendment with silicon (Si) can significantly increase resistance of susceptible sugarcane cultivars grown in pots to stalk borer Eldana saccharina (Lepidoptera: Pyralidae). This study tested the hypothesis that a single application of silicate can increase resistance to E. saccharina and increase yield in field-grown sugarcane.

Methods

Two Si materials (Calmasil® and Slagment® at 4 and 8 t/ha) were applied at planting to a field trial extending over three successive crops and incorporating three sugarcane cultivars varying in borer susceptibility.

Results

Both materials, especially Slagment, significantly increased soil, leaf and stalk Si content, but leaf Si levels seldom exceeded 0.5 %. Silicon treatment significantly reduced percent stalks bored in all three crops and stalk length bored in the second ratoon crop, but did not affect borer numbers per 100 stalks (E/100) or increase cane or sucrose yield. Borer damage and E/100 were significantly and consistently reduced in the resistant cultivar.

Conclusions

We argue that if leaf Si% in field sugarcane can be elevated to or exceed 0.8 %, using materials that release Si slowly, substantial reductions in stalk damage and sucrose loss could be achieved in susceptible cultivars in low-Si soils.     

Study on some bioecological aspects of Eldana saccharina Walker (Lep., Pyralidae) on Zea mays L. and alternative host plants

Abstract:  In an attempt to set up habitat management strategy against Eldana saccharina Walker, an economically important graminaceous stem and cob borer, two laboratory experiments were carried out. The first experiment was designed to assess oviposition preference of E. saccharina for growth stages, plant parts and freshness of different host plants and between host plants species. Zea mays L., Sorghum arundinaceum (Desv.) Stapf, Andropogon gayanus Kunth, Panicum maximum Jacq., Pennisetum polystachion (Linn.) and Cyperus papyrus L. were assessed. Results showed that E. saccharina preferred wild grasses to maize for oviposition. More than 94% eggs were laid on wild grasses. Of all tested grasses, P. polystachion was the most attractive with more than 30% of collected eggs. For all plant species, post-tasseling stage was more attractive than the tasseling and pre-tasseling ones. More than 90% of total eggs were laid on dry leaves and sheaths and less than 10% on fresh leaves. Different characteristics relating to population dynamics of E. saccharina were assessed in the second experiment. Larval survival rate was 10% on Z. mays , 0.50% on S. arundinaceum and around 0% on the other wild grasses. Larval development length was longer on S. arundinaceum than on Z. mays . Females reared on Z. mays were three times more fecund than those reared on S. arundinaceum . Complete life table of females from larvae reared on Z. mays was studied.     

Calcium silicate enhances resistance of sugarcane to the African stalk borer Eldana saccharina Walker (Lepidoptera: Pyralidae)

Abstract

• 1 The stalk borer Eldana saccharina is the most destructive pest in sugarcane in South Africa. This study investigated: (1) the potential of applied silicon in enhancing plant resistance of sugarcane to E. saccharina, using calcium silicate as a carrier; (2) whether there was any interaction between cane variety (and stalk borer resistance) and silicon treatment.
• 2 Six commercial varieties of sugarcane were treated in a pot‐plant trial with two levels (5000 and 10 000 kg/ha) of calcium silicate. After artificial infestation with E. saccharina, response to the treatments was assessed in terms of borer numbers and mass, and stalk damage.
• 3 Calcium silicate significantly enhanced resistance at the higher rate compared with the control. Borer mass was reduced by 19.8% and stalk length bored by 24.4%. Lower treatment values were intermediate between those of the higher treatment and the control.
• 4 The interaction between variety and Si treatment was not significant when varieties were examined individually in the analysis. However, the interaction was significant for borer mass when varieties were grouped according to their resistance characteristics. Susceptible varieties might benefit more from treatment with silicon than resistant ones, as resistant varieties showed no significant effect of silicon.
• 5 All varieties had increased silicon content due to the treatments, but differed appreciably in stalk silicon content at the different treatment levels. Similarly, within varieties, stalk silicon content did not correspond consistently with borer response patterns and silicate application rates.

     

Biocontrol of the sugarcane borer Eldana saccharina by expression of the Bacillus thuringiensis cry1Ac7 and Serratia marcescens chiA genes in sugarcane-associated bacteria

The cry1Ac7 gene of Bacillus thuringiensis strain 234, showing activity against the sugarcane borer Eldana saccharina, was cloned under the control of the tac promoter. The fusion was introduced into the broad-host-range plasmid pKT240 and the integration vector pJFF350 and without the tac promoter into the broad-host-range plasmids pML122 and pKmM0. These plasmids were introduced into a Pseudomonas fluorescens strain isolated from the phylloplane of sugarcane and the endophytic bacterium Herbaspirillum seropedicae found in sugarcane. The ptac-cry1Ac7 construct was introduced into the chromosome of P. fluorescens using the integration vector pJFF350 carrying the artificial interposon Omegon-Km. Western blot analysis showed that the expression levels of the integrated cry1Ac7 gene were much higher under the control of the tac promoter than under the control of its endogenous promoter. It was also determined that multicopy expression in P. fluorescens and H. seropedicae of ptac-cry1Ac7 carried on pKT240 caused plasmid instability with no detectable protein expression. In H. seropedicae, more Cry1Ac7 toxin was produced when the gene was cloned under the control of the Nm(r) promoter on pML122 than in the opposite orientation and bioassays showed that the former resulted in higher mortality of E. saccharina larvae than the latter. P. fluorescens 14::ptac-tox resulted in higher mortality of larvae than did P. fluorescens 14::tox. An increased toxic effect was observed when P. fluorescens 14::ptac-tox was combined with P. fluorescens carrying the Serratia marcescens chitinase gene chiA, under the control of the tac promoter, integrated into the chromosome.     

Spatial dynamics of lepidopterous pests on Zea mays (L.) and Pennisetum purpureum (Moench) in the forest zone of Cameroon and their implications for sampling schemes

The most common lepidopterous borers attacking maize and/or the wild host Pennisetum purpureum in the forest zone of Cameroon are the noctuids Busseola fusca (Fuller), Sesamia calamistis (Hampson) and Poeonoma serrata (Hampson), the pyralids Eldana saccharina (Walker) and Mussidia nigrivenella (Ragonot), and Cryptophlebia leucotreta (Tortricidae). The within-plant distribution on maize and elephant grass was studied for the predominant species B. fusca , and on maize only for E. saccharina to determine the basic sampling unit. On both plant species, B. fusca showed a strong oviposition preference for young plant parts. By contrast, E. saccharina larvae and pupae on maize were only found on older plant parts indicating that it does not oviposit on young plants. No part of the plant strata appeared to be a stable sampling unit and it is recommended to carry out whole plant or whole tiller sampling for maize and grass, respectively. For the development of sampling plans, dispersion was described for all species using Taylor's power law and a non-linear model which gives the relationship between the proportion of infested plants [ P ( I )] and mean density ( m ). Busseola fusca egg batches as well as diapausing larvae and pupae on maize showed a random distribution whereas all the other cases were aggregated, with B. fusca egg batches on elephant grass exhibiting the lowest and M. nigrivenella on maize the highest aggregation. Optimal sample size/mean density curves were developed for groups of insects with low and high aggregation.     

Recombinant Gluconacetobacter diazotrophicus containing Cry1Ac gene codes for 130-kDa toxin protein

Recombinant Gluconacetobacter diazotrophicus containing Cry1Ac gene from Bacillus thuringiensis var. kurstaki borne on pKT230, shuttle vector, was generated. PCR amplification of Cry1Ac gene present in recombinant G. diazotrophicus yielded a 278-bp DNA product. The nitrogenase assay has revealed that the recombinant G. diazotrophicus in sugarcane stem produced similar levels of nitrogenase compared to wild-type G. diazotrophicus. The presence of 130-kDa protein in apoplastic fluid from sugarcane stem harvested from pots inoculated with recombinant G. diazotrophicus shows that the translocated G. diazotrophicus produces 130-kDa protein which is recognized by the hyperimmune antiserum raised against 130-kDa protein. The first instar Eldana saccharina neonate larvae that fed on artificial medium containing recombinant G. diazotrophicus died within 72 h after incubation.     

Host plant‐related variation in thermal tolerance of Eldana saccharina

Understanding tolerance of thermal extremes by pest insects is essential for developing integrated management strategies, as tolerance traits can provide insights into constraints on activity and survival. A major question in thermal biology is whether thermal limits vary systematically with microclimate variation, or whether other biotic or abiotic factors can influence these limits in a predictable manner. Here, we report the results of experiments determining thermal limits to activity and survival at extreme temperatures in the stalk borer Eldana saccharina Walker (Lepidoptera: Pyralidae), collected from either Saccharum spp. hybrids (sugarcane) (Poaceae) or Cyperus papyrus L. (Cyperaceae) and then reared under standard conditions in the laboratory for 1–2 generations. Chill‐coma temperature (CT_min), critical thermal maximum (CT_max), lower lethal temperatures (LLT), and freezing temperature between E. saccharina collected from the two host plants were compared. CT_min and CT_max of E. saccharina moths collected from sugarcane were significantly lower than those from C. papyrus (CT_min = 2.8 ± 0.4 vs. 3.9 ± 0.4 °C; CT_max = 44.6 ± 0.1 vs. 44.9 ± 0.2 °C). By contrast, LLT of moths and freezing temperatures of pupae did not vary with host plant [LLT for 50% (LT₅₀) of the moth population, when collected from sugarcane: ?3.2 ± 0.5 °C, from C. papyrus: ?3.9 ± 0.8 °C]. Freezing temperatures of pupae collected from C. papyrus were ?18.0 ± 1.0 °C and of those from sugarcane ?17.5 ± 1.8 °C. The E. saccharina which experienced the lowest minimum temperature (in C. papyrus) did not have the lowest CT_min, although the highest estimate of CT_max was found in E. saccharina collected from C. papyrus and this was also the microsite which reported the highest maximum temperatures. These results therefore suggest that host plant may strongly mediate lower critical thermal limits, but not necessarily LLT or freezing temperatures. These results have significant implications for ongoing pest management and thermal biology of these and other insects.     

Limiting the success of stem borers (Prodoxus quinquepunctellus) in yuccas: indirect effects of ants, aphids, and fruit position

Abstract.  1. Although the moth–yucca mutualism is often studied as a pairwise interaction, yucca plants are also the sole host for a variety of other visitors. One of these additional visitors is a stem-boring moth, Prodoxus quinquepunctellus.
2. In this study, it is shown how the reproductive success of Prodoxus indirectly depends on the interactions between yuccas and their pollinators ( Tegeticula , Prodoxidae) as well as the indirect effects of ants and aphids.
3. Aggressive wood ants foraging on yuccas will attack adult Prodoxus moths while attempting to oviposit. This reduces the number of eggs laid in yucca stalks, leading to fewer larvae feeding in the stalks.
4. Once in the stalk, the survival of Prodoxus eggs/larvae depends upon the rate at which the flowering stalks dry out during fruit maturation. Portions of the stalk above the highest fruit dry out quickly and survivorship approaches zero in these dry sections, while larvae in green sections of the flowering stalk have significantly higher survival rates. The presence of aphids feeding on the stalk slows down the rate of stalk drying and could lead to increased survival of Prodoxus larvae.
5. Overall, ants have strong indirect effects on P. quinquepunctellus by controlling how many eggs are laid in the stalk and by influencing the distribution of aphids. However, it is primarily the presence and position of the fruit that can affect larval survivorship, and fruit position is a function of pollinator visits and resource limitation. These complex interactions illustrate the importance of studying the yucca–moth mutualism in a community context.     

水稻施用硅肥对稻纵卷叶螟幼虫保护酶和解毒酶活性的影响

【目的】研究施硅水稻对稻纵卷叶螟Cnaphalocrocis medinalis(Guenée)幼虫保护酶(过氧化氢酶CAT、过氧化物酶POD、超氧化物歧化酶SOD)和解毒酶(谷胱甘肽-S-转移酶GST、乙酰胆碱酯酶ACh E)活性的影响,为探明硅增强水稻抗稻纵卷叶螟的机理和稻纵卷叶螟的综合治理提供依据。【方法】采用感虫水稻品种TN1,设置2种施硅水平,即0.32 g Si/kg土壤(Si+)和不施硅(Si-),测定取食水稻24、48、72、96 h后3龄幼虫体内CAT、POD、SOD、GST、ACh E活性的动态变化。【结果】幼虫取食Si+水稻前期(24~48 h),CAT、POD、SOD活性呈上升趋势,并在48 h时达到最高值;取食后期(48~96 h),CAT、POD、SOD活性下降并在96 h时显著低于Si-处理。在Si-处理中,幼虫CAT和POD活性"先降低、再升高",SOD活性呈一直增大的趋势。取食Si+水稻幼虫GST活性始终显著高于Si-处理,而ACh E活性均低于Si-水稻,并在72 h和96 h时显著低于Si-水稻。处理间幼虫存活率存在显著差异,Si+水稻上1龄和3龄幼虫的存活率显著低于Si-水稻。这些结果说明,硅处理可能使稻纵卷叶螟幼虫产生应激反应引起保护酶活性在短时间内增大,但取食一段时间后保护酶活性下降。ACh E活性受到抑制,会引起昆虫神经传导的异常反应,造成昆虫死亡。【结论】硅可能通过参与植物的生理代谢过程,诱导植物增加次生代谢物的合成和积累,引起稻纵卷叶螟保护酶和解毒酶的活性变化,可能会影响稻纵卷叶螟的存活。     

Population genetics of Eldana saccharina Walker (Lepidoptera: Pyralidae) and the implications for management using biocontrol

The African sugarcane stalk borer, Eldona saccharina Walker (Lepidoptera: Pyralidae), is widely distributed throughout sub-Saharan Africa and is an important insect pest of maize and sugarcane. The insect shows significant variation in behaviour, host plant and natural enemy guild in different regions. Several attempts to redistribute the natural enemies of E. saccharina from West Africa to South Africa were unsuccessful. The significant behavioural, host plant and natural enemy variations as well as failures of biocontrol attempts evoked a hypothesis of genetic diversification. To evaluate this hypothesis a molecular analysis was conducted on geographically isolated populations of E. saccharina from East, North, South and West Africa, using the cytochrome c oxidase subunit I (COI) region of the mitochondrial genome. The results revealed that E. saccharina populations are separated into four major units corresponding to the West Africa, Rift Valley, South/East Africa and southern African populations. Mitochondrial DNA divergence among the four populations ranged from 1% to 4.98%. To examine the impact of the observed genetic variation on the fertility of inter-population crosses, a mating experiment was conducted between the Rift valley and South African population to produce an F1 generation, and these were backcrossed with the South African parent population. Fertility of eggs produced by the F1/parent population cross was significantly reduced when compared to fertility of the "true" South African line, and the F1/F1 cross. The contributions of the observed genetic differences and inter population incompatibility for the failure of previous biocontrol attempts are discussed and recommendations on future biocontrol practices are given.     

硅对菜心炭疽病发生、菜薹形成及硅吸收沉积的影响

采用不同浓度硅(Si)对菜心进行处理, 研究了Si对菜心炭疽病发生、菜薹形成及Si吸收沉积的影响.结果表明：Si对菜心炭疽病的发生有明显的调控作用,并存在基因型差异;感病品种2.5 mmol·L^-1和抗病品种0.5 mmol·L^-1Si处理对炭疽病菌的抗性最强,病情指数最低,菜薹产量最高.Si对菜心品质也有明显影响,不同浓度Si处理可促进感病品种的叶绿素、粗纤维、Vc的合成,并诱导可溶性糖的生成,且叶绿素、粗纤维含量随Si处理浓度的提高而增加;抗病品种的叶绿素含量随Si处理浓度的增加明显增加,而Vc含量下降,Si对抗病品种的粗纤维、可溶性糖含量影响不显著;Si对两个品种菜薹的粗蛋白和可溶性蛋白质含量影响均不显著.施Si明显提高了菜心叶片Si的积累,且叶片的Si含量随着Si处理浓度的增加而显著提高;叶片组织沉积的Si粒大小不一,Si在叶片表皮组织呈不均匀分布.表明Si在菜心叶片中的积累可提高植株的抗病能力,但其积累量与植株的抗病能力不呈直线相关关系.     

Active defence of herbivorous hosts against parasitism: Adult parasitoid mortality risk involved in attacking a concealed stemboring host

Phytophagous insects have several defence strategies to defend themselves against attack by parasitic wasps. Larval lepidopteran hosts can defend themselves actively to prevent oviposition by the parasitoid. Among the aggressive kinds of behaviour exhibited by hosts against parasitoids are violent wriggling, biting and spitting. The behaviour of the braconid parasitoid Cotesia sesamiae attacking stemboring larvae inside their feeding tunnel in the plant stem was investigated in maize and sugarcane stem pieces and transparent artificial tunnels. Attacking a defending stemborer host inside the confined space of a tunnel was shown to be risky for the female parasitoid. A considerable proportion (25%) of female wasps were killed in their attempt to attack the spitting and biting host. No difference was found in the behaviour of C. sesamiae attacking the suitable host Sesamia calamistis or the unsuitable host Eldana saccharina. The consequences of this high mortality risk involved in each host attack is discussed in relation to the ecology of the parasitoid.     

Bio-assay study of response by a parasitoid to frass and feeding substrates of its host, the stalk borer Eldana saccharina

In Petri dish and olfactometer tests Goniozus natalensis Gordh (Hymenoptera: Bethylidae) exhibited a host-searching response upon contact with, and at a short distance from, frass of its natural host, Eldana saccharina Walker (Lepidoptera: Pyralidae), whichever the substrate on which E. saccharina had been feeding. The substrates were two host plants, sugarcane or Cyperus papyrus L.; diet media based respectively on sugarcane, papyrus, or cellulose; or a synthetic diet medium, containing no plant material. Host-searching behaviour increased in intensity with age and after mating of G. natalensis females. Parasitoids responded similarly to frass produced by male and female E. saccharina larvae. The substrates themselves elicited little response in G. natalensis. Frass of another borer, Sesamia calamistis Hamps. (Lepidoptera: Noctuidae), fed on sugarcane-based medium, also elicited little response in G. natalensis. The results suggest that frass of E. saccharina larvae contains a material, evidently volatile, to which their parasitoid G. natalensis responds, irrespective of the substrate on which the E. saccharina subsist.     

Grape variety affects female but also male reproductive success in wild European grapevine moths

Abstract.  1. For insect herbivores the quality of the larval host plant is a key determinant of their fitness. Only little attention, however, has been given to the effects of plants on mating success of males and its consequence for the reproductive output of their mates. In addition, almost all the studies that have investigated the influence of host plants on herbivore fitness components have been done in the laboratory, and less is known of these effects in natural conditions.
2. Using the phytophagous European grapevine moth ( Lobesia botrana Den. & Schiff., Lepidoptera: Tortricidae), we tested the influence of grape cultivars as larval food on the probability of acquiring a mate for both sexes, and on the reproductive output of females and males.
3. Results from this study stress the importance of larval host plants on the reproductive success of both sexes. Larval diet differentially affected mating success and reproductive output of male and female moths. Fecundity, egg size, and egg hatchability were significantly different when larvae were fed on particular grape cultivars.
4. A given cultivar that is of poor quality for females is generally also of poor quality for males. A cultivar, however, could be suitable for females but not for males and vice-versa. Apparently, the nutrients required for adult reproduction are not necessarily the same for males and females.
5. The important conclusion from this study is that evaluating the differential effect of host-plant species on traits associated with reproductive success of herbivores requires that the effects on both sexes be taken into account.