Different bioassays for investigating orientation responses of the banana weevil, Cosmopolites sordidus, show additive effects of host plant volatiles and a synthetic male-produced aggregation pheromone

Abstract Three different bioassay methods to investigate the orientation behaviour of the banana weevil, Cosmopolites sordidus (Germar) (Coleoptera: Curculionidae), to host plant volatiles and a synthetic pheromone (cosmolure+) were compared. A locomotion compensator was used to separately record walking tracks in response to three odour sources. The data show that C. sordidus uses odour‐conditioned anemotaxis in its orientation to the odour sources tested. Of the two olfactometers tested, a dual port olfactometer using a continuous airflow showed stronger discrimination by C. sordidus to the different odours compared with a double pitfall olfactometer. The results of all three bioassays indicate that C. sordidus responds in an additive way to the combination of fermentation plant volatiles and the synthetic pheromone.     

Banana defence responses to Cosmopolites sordidus feeding and methyl jasmonate application

Each year 25–75% of banana and plantain yields are lost because of rhizome damages caused by banana weevil (Cosmopolites sordidus) in growing regions of sub‐Saharan Africa. However, the specific plant defence response of the rhizome tissue in relation to the C. sordidus attack is unknown. Consequently, in this study, we evaluated whether plant defence substances in the rhizome are correlated with the degree of larval damage and whether applications of methyl jasmonate (MJ) elicit a greater induction of the plant defence potential against C. sordidus. Moreover, we attempted to reveal cellular modifications in response to the root feeding herbivore through histochemical staining. The banana cultivars “Km5” and “Mbwazirume” with tolerance and susceptibility to C. sordidus, respectively, were used in a pot experiment to evaluate percent rhizome damage, leaf chlorophyll content, total phenolic content (TPC), antioxidant capacity and cell morphology in response to C. sordidus attack and/or MJ applications compared to untreated control plants. We found that C. sordidus‐induced rhizome damage was 30% in the susceptible cultivar but less than 5% in the tolerant cultivar. The percent rhizome damage was not related to leaf chlorophyll content but showed a significant negative linear relationship to both TPC and antioxidant capacity. Larvae feeding induced a considerably greater increase of polyphenolic defence compounds in Km5 than in Mbwazirume; however, this response was opposite in the MJ treatment, suggesting that the phytohormone induced the susceptible plant to invest more into the synthesis of defence chemicals that in turn lead to reduced C. sordidus damage. Tissue staining demonstrated a greater deposition of lignin and suberin in C. sordidus challenged rhizome, presumably to seal off healthy tissue with a physical barrier from continued pest attack. It is concluded that MJ induces polyphenolics in susceptible Mbwazirume banana that reduced C. sordidus damage.     

Acoustic recording as a non‐invasive method to detect larval infestation of Cosmopolites sordidus

The banana weevil, Cosmopolites sordidus (Germar) (Coleoptera: Curculionidae), is a major pest of banana cropping systems, as larvae bore into the corm of banana plants and damage the insertion points of primary roots, leading to plant snapping and toppling. Plant infestation by the banana weevil is assessed only via invasive techniques, such as plant cutting that highlights larval tunnels. We present here a non‐invasive technique based on a bioacoustic sensor with a band‐pass filter to detect larval activity inside banana corm. On the basis of a test under controlled conditions, we observed that the range of larval activity was between 1.4 and 2.6 kHz. Pulses lasted ca. 168 ms and were separated by intervals of 300 ms. The technique enables detection of more than 90% of infested suckers. The use of this technique to understand larval development of banana weevil is discussed.     

The challenge of biological control of Cosmopolites sordidus Germar (Col. Curculionidae): A review

The banana weevil (also known as banana root borer) Cosmopolites sordidus Germar (Coleoptera Curculionidae) is the major pest of banana and plantain. Because banana ranks 2nd in fruit production worldwide, this pest has substantial social and economic importance. The biological control of the C. sordidus remains challenging because of its behaviour and resistance to parasitism and predation. The last review concerning the biological control of C. sordidus was published two decades ago, and relevant knowledge and methods have developed in the interim. The present paper provides an update of that knowledge and summarizes past and current challenges as well as providing perspectives on achieving sustainable control of C. sordidus. We first discuss studies on the classical biological control of C. sordidus, underlining the limits of classical biological control methods such as the importation of predators, parasitoids or pathogens. Next, we consider conservation biological control of C. sordidus, with a focus on ants. We also highlight an ‘arthropod bias’ that has led to a lack of information on the role of vertebrates in the regulation of C. sordidus.     

The use of aggregation pheromone to enhance dissemination of Beauveria bassiana for the control of the banana weevil in Uganda

Candidate strains of Beauveria bassiana were identified for use in integrated pest management of the banana weevil Cosmopolites sordidus. Horizontal field transmission of B. bassiana between banana weevils using different delivery systems, including aggregation pheromones, was investigated. We observed that infected weevils could transmit the fungal pathogen to healthy individuals. Most dead weevils (52%) due to B. bassiana infection were found at the base of banana plants in the leaf sheath or in the soil near banana plants. Significantly more weevils died from the pathogen in plots where B. bassiana was applied in combination with the pheromone than where it was applied alone. Our data demonstrate that C. sordidus aggregation pheromone can be a valuable tool to enhance the dissemination of B. bassiana for the control of C. sordidus.     

Deleterious effects of plant cystatins against the banana weevil Cosmopolites sordidus

The general potential of plant cystatins for the development of insect‐resistant transgenic plants still remains to be established given the natural ability of several insects to compensate for the loss of digestive cysteine protease activities. Here we assessed the potential of cystatins for the development of banana lines resistant to the banana weevil Cosmopolites sordidus, a major pest of banana and plantain in Africa. Protease inhibitory assays were conducted with protein and methylcoumarin (MCA) peptide substrates to measure the inhibitory efficiency of different cystatins in vitro, followed by a diet assay with cystatin‐infiltrated banana stem disks to monitor the impact of two plant cystatins, oryzacystatin I (OC‐I, or OsCYS1) and papaya cystatin (CpCYS1), on the overall growth rate of weevil larvae. As observed earlier for other Coleoptera, banana weevils produce a variety of proteases for dietary protein digestion, including in particular Z‐Phe‐Arg‐MCA‐hydrolyzing (cathepsin L–like) and Z‐Arg‐Arg‐MCA‐hydrolyzing (cathepsin B–like) proteases active in mildly acidic conditions. Both enzyme populations were sensitive to the cysteine protease inhibitor E‐64 and to different plant cystatins including OsCYS1. In line with the broad inhibitory effects of cystatins, OsCYS1 and CpCYS1 caused an important growth delay in young larvae developing for 10 days in cystatin‐infiltrated banana stem disks. These promising results, which illustrate the susceptibility of C. sordidus to plant cystatins, are discussed in the light of recent hypotheses suggesting a key role for cathepsin B–like enzymes as a determinant for resistance or susceptibility to plant cystatins in Coleoptera. © 2009 Wiley Periodicals, Inc.     

Combination of the fungus Beauveria bassiana and pheromone in an attract‐and‐kill strategy against the banana weevil,Cosmopolites sordidus

An attract‐and‐kill approach based on pellets from soybean or palm stearin fats blended with the entomopathogenic fungus Beauveria bassiana (Bals.) Vuill. sensu lato and the aggregation pheromone sordidin (Cosmolure^®) was tested against the banana weevil, Cosmopolites sordidus (Germar) (Coleoptera: Curculionidae). The viability of B. bassiana conidia, blended with hydrogenated oil and exposed for up to 150 min to heating at 50 °C, was not affected and the aggregation pheromone did not undergo any decomposition. Conidial viability in pellets decreased by 50% after an average of 15.1 and 9.1 days at 25 and 40 °C, respectively, when packaged in polypropylene bags. Active packaging (hermetic bag + O₂/moisture‐absorbing sachet) increased the shelf lives almost 10 and 6 times at 25 and 40 °C, respectively. In olfactometer bioassays, fat pellets amended with pheromone (sordidin, 1% wt/vol) were highly attractive to C. sordidus adults for up to 15 days, after which the pheromone release rate had decreased by about 90% and pellets were no longer attractive. Pellets with pheromone and conidia were as attractive to C. sordidus as banana rhizomes, and considerably more attractive than pieces of pseudostem. In no‐choice experiments conducted in boxes, survival of insects exposed to fungus‐impregnated pellets was affected by fat type (soybean fat vs. palm stearin) and bioassay temperature (25 vs. 30 °C), with results favoring soybean fat pellets at the higher temperature (96.9% of mortality after 18 days and ST₅₀ of 7.7 days). However, mortality levels were low (21.7% for soybean fat pellets) or very low (1–5% for palm stearin pellets) in choice experiments carried out at 25 °C when fungus‐impregnated pellets were applied before or after exposure of pseudostem residues to insects, respectively. The potential of this delivery system to manage C. sordidus populations and other insect pests (including those with cryptic habits) is discussed.     

Dual inoculation of Fusarium oxysporum endophytes in banana: effect on plant colonization,growth and control of the root burrowing nematode and the banana weevil

The burrowing nematode (Radopholus similis (Cobb) Thorne) and the banana weevil (Cosmopolites sordidus Germar, Coleoptera: Curculionidae) are major pests of banana (Musa spp.) in the Lake Victoria basin region of Uganda. Among biological options to control the two pests is the use of non-pathogenic Fusarium oxysporum Schltdl.: Fries endophytes of banana. We investigated the ability of endophytic F. oxysporum isolates Emb2.4o and V5w2 to control the banana weevil and the burrowing nematode, alone and in combination. Plant colonization by the endophytes was determined by inoculating their chemical-resistant mutants separately and in combination, onto banana roots. Plant growth promotion was determined by measuring plant height, girth, number of live roots and fresh root weight at harvest, and control of the nematode and weevil was determined by challenging endophyte-inoculated plants with the pests 8 weeks after endophyte inoculation. Endophytic root colonization was highest in plants inoculated with both endophytes, compared with those inoculated with only one of the endophytes. Root colonization was better for isolate V5w2 than Emb2.4o. Dually inoculated plants showed a significant increase in height, girth, fresh root weight and number of functional roots following nematode challenge. Nematode numbers in roots were reduced 12 weeks after challenge of 8-week-old endophyte-inoculated plants. Significant reductions in weevil damage were observed in the rhizome periphery, inner and outer rhizomes, compared with endophyte non-inoculated controls. We conclude that dual inoculation of bananas with endophytic isolates Emb2.4o and V5w2 increases root colonization by the endophytes, reduces nematode numbers and weevil damage, and enhances plant growth in the presence of nematode infestation.     

Long-term impacts of elevated carbon dioxide and transgenic Bt cotton on performance and feeding of three generations of cotton bollworm

A field screening trial undertaken in Uganda showed that a number of Musa L. (Musaceae) cultivars and hybrids displayed high levels of resistance to banana weevil, Cosmopolites sordidus (Germar) (Coleoptera: Curculionidae), while most highland banana cultivars tended to be susceptible to weevil attack. In this study, research was undertaken to determine modalities of resistance to banana weevil. Laboratory studies suggested that all cultivars were attractive to the weevil and that females freely oviposited on all cultivars. While some differences were found between cultivars in attractivity and egg numbers, these were not related to subsequent damage. Therefore, antixenosis does not appear to play an important role in host‐plant resistance to banana weevil. Larval survivorship rates in living corms were, for the most part, low on resistant cultivars, suggesting that antibiosis mechanisms offer the primary avenues of resistance. In the laboratory, development was slower on some resistant cultivars although survivorship rates on excised corm material were not as well related to levels of resistance as that on living material. Sap appeared to play a minor role in reducing egg eclosion rates on some resistant cultivars. Methanol extracts from Kayinja, a resistant cultivar, inhibited larval development on corms of susceptible cultivars in the laboratory.     

Local and neighboring patch conditions alter sex‐specific movement in banana weevils

Understanding the mechanisms underlying the movements and spread of a species over time and space is a major concern of ecology. Here, we assessed the effects of an individual's sex and the density and sex ratio of conspecifics in the local and neighboring environment on the movement probability of the banana weevil, Cosmopolites sordidus. In a “two patches” experiment, we used radiofrequency identification tags to study the C. sordidus movement response to patch conditions. We showed that local and neighboring densities of conspecifics affect the movement rates of individuals but that the density‐dependent effect can be either positive or negative depending on the relative densities of conspecifics in local and neighboring patches. We demonstrated that sex ratio also influences the movement of C. sordidus, that is, the weevil exhibits nonfixed sex‐biased movement strategies. Sex‐biased movement may be the consequence of intrasexual competition for resources (i.e., oviposition sites) in females and for mates in males. We also detected a high individual variability in the propensity to move. Finally, we discuss the role of demographic stochasticity, sex‐biased movement, and individual heterogeneity in movement on the colonization process.     

Effect of fallows on population dynamics of Cosmopolites sordidus: toward integrated management of banana fields with pheromone mass trapping

• 1 The banana weevil Cosmopolites sordidus Germar (Coleoptera: Curculionidae) is the most serious pest of banana and plantain in most production areas, including the West Indies.
• 2 During a period of 2 years, we assessed the efficacy of a pheromone mass‐trapping control strategy of C. sordidus in field conditions in Guadeloupe at different cropping stages, both in sanitation fallows and in different ratoon banana crops.
• 3 In the fallows, catches peaked 3 months after beginning trapping and then decreased to zero after 9 months. By contrast, for the new plantations, the catches of C. sordidus increased after the 11th month and, in the older banana fields, there was no decrease in C. sordidus catches. The C. sordidus catches increased in the neighbouring banana plots, whereas they decreased in the fallows, and these catches decreased with the distance from fallow.
• 4 In conclusion, mass trapping with synergized pheromone traps within fallows should allow better sanitation of banana plantations. Yet, within the farms, fallows must not be located next to new plantations to avoid massive damage to the young plants. More generally, the present study emphasizes that the control of this insect should be managed at the farm scale and not at the field scale, with special attention being paid to the location of fallows.

     

Field validation of senesced banana leaf extracts for trapping banana weevils on smallholder banana/plantain farms

Palm wine alcohol extract of senesced banana leaf material, Musa spp., was tested for its efficacy in open field trapping of the banana weevil, Cosmopolites sordidus in Ghana from June to August 2015. Modified pitfall and bottle traps were baited with either individual treatments, that is palm alcohol extract, C. sordidus aggregation pheromone or pseudostem or with combinations of extract plus aggregation pheromone or extract plus pseudostem. The combination of extract plus aggregation pheromone was able to lure more weevils into traps compared with the respective individual lures. There was a 2.1-fold increase in mean catch per week when the palm alcohol extract was used in combination with pheromone compared with using pheromone alone, and a corresponding 2.6-fold increase when the extract was used with pseudostem in traps. There was no statistically significant interaction between the palm alcohol extract (presence or absence) and treatment (pheromone or pseudostem), but the best combination for maximal catches of adult banana weevils was a combination of palm alcohol extract with aggregation pheromone. Management of banana weevils with attractive banana leaf extract has important practical applications in parts of the world where other management options are too expensive or commercial treatments are in short supply, but where leaf material is cheap and readily available for local use by smallholder farmers.     

Experimental evaluation of the impacts of two ant species on banana weevil in Uganda

The banana weevil, Cosmopolites sordidus (Germar), is an important pest of bananas. Predatory ants are increasingly being viewed as possible biological control agents of this pest because they are capable of entering banana plants and soil in search of prey. We studied ant predation on banana weevil in Uganda in crop residues and live plants in both laboratory and field experiments. Field studies with live plants used chemical ant exclusion in some plots and ant enhancement via colony transfer in others to measure effects of Pheidole sp. 2 and Odontomachus troglodytes Santschi on plant damage and densities of immature banana weevils.In crop residues, an important pest breeding site, twice as many larvae were removed from ant-enhanced plots as in control plots. In young (2 month) potted suckers held in shade houses, ant ability to reduce densities of banana weevil life stages varied with the weevil inoculation rate. At the lowest density (2 female weevils per pot), densities of eggs, larvae, and pupae were reduced by ants. At higher rates there was no effect. In older suckers (5–11 months) grown in larger containers, banana weevil densities were not affected by ants, but damage levels were reduced. In a field trial lasting a full crop cycle (30 months), we found that the ants tested reduced the density of banana weevil eggs in suckers during the crop, but did not affect larval densities in the sampled suckers. However, most larvae occur in the main banana plants, rather than associated suckers. Nevertheless, levels of damage in mature plants at harvest did not differ between Amdro-treated and ant-enhanced plots, suggesting the ant species studied were not able to provide economic control of banana weevil under our test conditions.     

Partial characterization of the proteolytic enzymes in the gut of the banana weevil, Cosmopolites sordidus, and effects of soybean Kunitz trypsin inhibitor on larval performance

The proteolytic enzymes in the gut of the banana weevil, Cosmopolites sordidus (Germar) (Coleoptera: Curculionidae), have been characterized. Both larvae and adults rely on a complex proteolytic system based on at least cathepsin D‐, cathepsin B‐, trypsin‐, chymotrypsin‐, leucine aminopeptidase‐, carboxypeptidase A‐, and carboxypeptidase B‐like activities. All endoproteolytic activities were higher in the anterior section of the gut, whereas the exopeptidases were evenly distributed in the anterior and middle sections, and almost no activity was detected in the posterior section. Gelatin‐containing gels confirmed the spatial organization of the proteolytic digestive process. According to this proteolytic profile, the STI (soybean Kunitz trypsin inhibitor) was tested in vivo to establish its potential as a resistance factor against C. sordidus. Newly hatched larvae fed on diets containing 0.2% (w/w) STI experience lower survival rates and display significant reductions in larval growth. Biochemical analysis carried out on guts of larvae reared on STI‐treated diet showed a reduction of trypsin‐like activity compared to that from larvae fed on control diet. This decrease was compensated with an induction of cathepsin B, whereas cathepsin D, chymotrypsin, and leucine aminopeptidase were not affected. These results are discussed as a basis for selecting appropriate inhibitors to obtain transgenic banana and plantain plants with enhanced resistance to this pest.     

Olfactory cues used in host‐habitat location and host location by the parasitoid Cotesia urabae

There is a growing body of evidence that many hymenopteran parasitoids make use of olfaction as the primary mechanism to detect and locate hosts. In this study, a series of bioassays was conducted to investigate the orientation behaviour of the gum leaf skeletonizer larval parasitoid Cotesia urabae Austin & Allen (Hymenoptera: Braconidae) in both Y‐tube and four‐arm olfactometers. In a Y‐tube olfactometer, male C. urabae were attracted only to virgin conspecific females. Host‐plant leaves, damaged leaves, host larvae, and host larvae feeding on leaves were highly attractive to female C. urabae, whereas host frass and conspecific males were not. The multiple‐comparison bioassay conducted in a four‐arm olfactometer clearly indicates that C. urabae females were significantly more attracted to the host Uraba lugens Walker (Lepidoptera: Nolidae) larvae feeding on Eucalyptus fastigata H Deane & Maiden (Myrtaceae) leaves than to any other of the odour sources tested. The results of this study show that C. urabae individuals responded to chemical cues specific to the host plant and target host insect, and support hypotheses that unreliable cues are not utilized for host location by specific natural enemies.     

Efficacy of cultural control measures against the banana weevil, Cosmopolites sordidus (Germar), in South Africa

The banana weevil, Cosmopolites sordidus, is the most important insect pest of banana and plantain in the world. Cultural control methods were investigated over 2 years in southern KwaZulu‐Natal, South Africa. Harvesting at ground level and dissection of remnants (treatment 1), and covering the base of the mat (entire plant consisting of several meristems) with soil and moving debris to the inter‐row (treatment 2), were compared to a positive control that involved treatment of plants with a registered pesticide (treatment 3), and a negative control that involved harvesting at 150 cm from the collar with no soil or sanitation amendments (treatment 4). Yield, weevil damage and pseudostem girth of plants were measured from August to November annually, while adult beetle densities were assessed over 4 weeks in October/November and April. Nematode samples were taken and analysed in October/November every year. Damage parameters included the coefficient of infestation, the percentage coefficient of infestation (PCI) at two intervals, the summed PCI value, the percentage cross‐sectional damage of the central cylinder and cortex, and the mean cross‐sectional damage percentage. A randomized block design with three replicates was used in the trial. The parameters were similar before the onset of the trial. Fruit yield and plant girth, corrected by nematode densities, were not significantly different in any treatment, nor were the nematodes controlled. Soil cover and recession of remnants was the only effective treatment, significantly reducing the CI, but not the adult density or the other damage parameters. Soil cover showed promise as a cultural control method because it only needs to be applied seasonally and reduced the percentage cross‐sectional damage of the central cylinder, the damage parameter most closely related to yield, by 14%.     

7个香蕉品种对枯萎病的抗性评价及3种评价方法的对比

[目的] 评价香蕉自主选育品种对枯萎病病原菌尖孢镰刀菌古巴专化型（Fusarium oxysporum f. sp. cubense）热带4号小种（tropical race 4, Foc TR4）的抗性。[方法] 以浸根法和2种灌注法将Foc TR4接种于不同抗性的香蕉品种上,分析接种后香蕉的发病情况,比较筛选这3种评价方法,并评价7个香蕉品种的苗期和田间抗性水平。[结果] 采用浓度为2×10⁵ 个·mL^-1的PDB培养基Foc TR4孢子悬浮液灌注法进行香蕉苗期抗性评价更为高效可行;综合苗期和田间抗性评价结果,7个香蕉品种的抗性由高到低为：南天黄>红研3号>红研5号>红研2号>滇蕉1号>巴西蕉>红研1号。[结论] 以南天黄作为高抗品种对照,以巴西蕉作为高感品种对照条件下,红研3号和红研5号抗性为中等抗性偏强,红研2号达到中抗水平,滇蕉1号为感病,红研1号为高度感病。     

Improved in vitro bioassay for Musa acuminata cv. Pisang ambon kuning (AAA group) based on quantitative analysis of necrosis area and biomass changes during Foc4 infection

Abstract

Efforts in improving banana plants that are resistant to the Fusarium wilt-causing Fusarium oxysporum f.sp. cubense tropical race 4 (Foc4) are indispensable. In this study, we developed rapid, space-efficient in vitro bioassay for assessing banana plant resistance to Foc4 using 35?×?150?mm glass test tubes, followed by quantitative and objective analysis of necrosis area and biomass changes as represented by fresh weight changes. Disease resistance screening was conducted based on the necrosis area as quantified using ImageJ software and on biomass gain during in vitro bioassay. In vitro banana plantlets showed age-related resistance during the development of necrosis (p?=?.034, Kruskal–Wallis test in root and shoot system and p?=?.027, one-way ANOVA in shoot system only), in which plantlets that were infected at the youngest age (24 weeks’ post-initiation) showed the largest necrosis area (up to 46.6%). In addition, plant fresh weight gain in this group (0.233?±?0.041?g) was higher compared to the gains in older plantlets (0.079?±?0.117 and 0.009?±?0.069?g, infected at 28 and 38?weeks’ post-initiation, respectively). Overall, for consistent and reliable result, the age of banana plantlet should be taken into consideration in interpreting the result of this in vitro bioassay.