Evaluation of isolates of Trichoderma,Pseudomonas and Bacillus species as treatment for the control of post-harvest fungal rot disease of yam (Dioscorea spp.)

The efficacy of four biological control agents (BCAs): Trichoderma asperellum strain NGT158, T. longibrachiatum strain NGT167, Bacillus subtilis and Pseudomonas fluorescens for the management of post-harvest tuber rot among four yam species, Dioscorea rotundata, D. cayenensis, D. alata, and D. dumetorum was evaluated. Rotted yam tubers were collected across three agroecological zones in Nigeria to isolate six infecting fungal pathogens: Aspergillus niger, Lasiodiplodia theobromae, Rhizoctonia solani, Penicillium oxalicum, Fusarium oxysporum and Sclerotium rolfsii. The BCAs were isolated by serial dilution and rot inhibition of treated tubers was evaluated using destructive sampling method in vivo after six months of storage. Bacillus subtilis was generally most effective, especially when applied 24 h before the inoculation of test pathogens across the four yam species, with percent inhibition that ranged between 47.8 and 81.2%. However, the four BCAs showed good potential in the control of the fungal pathogens causing post-harvest yam rot.     

Severity of spoilage storage rots of white yam (Dioscorea rotundata Poir.)

Severity of storage rots in different sections of white yam tubers (Dioscorea rotundata Poir.) was investigated. Yam samples with rots were collected from a yam barn and from selected markets in Accra, Ghana, to identify the most predominant pathogens associated with the rots. Nine fungal spoilage microorganisms, including Aspergillus flavus, Aspergillus niger, Botryodiplodia theobromae, Fusarium culmorum, Fusarium oxysporium, Fusarium sp., Penicillium brevi‐compactum, Penicillium sp. and Rhizopus stolonifer and a bacterium species Erwinia carotovora were identified. The mean incidence of occurrence of the organisms on rotten tissues ranged from 1.2% to 28.5%. Of the 10 microorganisms isolated, B. theobromae, F. oxysporium and R. stolonifer were the most frequently encountered spoilage microorganisms in the markets. E. carotovora, Fusarium solani and Penicillium sp. were relatively sparse (incidence not exceeding 3%) compared to the other yam spoilage microorganisms. The surface area and weight of necrotic tissue induced by the spoilage fungi in the various zones of the tubers over a 28‐day storage period were assessed. All the spoilage microorganisms produced rots in the yams, although to different degrees. The severity of the rots increased in weight and area over the period when the tubers were in store but were normally not significantly different in the zones of tubers. There was, however, a linear progression of rots in the various zones of the yam tubers. Although there was generally no significant (P ≥ 0.05) difference in the severity of rots induced by the different microorganisms in the tubers, R. stolonifer commonly induced more rot in the zones of the tubers compared to B. theobromae and F. oxysporium.     

Evaluation of neem (Azadirachta indica) seed and ginger (Zingiber officinale) as potential control agents of yam (Dioscorea rotundata Poir.) tuber rot fungi

Yam is an important crop which serves as a source of income to small-holder farmers as well as a foreign exchange earner. Among the constraints faced by yam farmers are pests and diseases, especially deterioration caused by microbes during storage. Since over dependence on pesticides is being discouraged, neem seed and ginger extracts were evaluated as potential control agents against rot-causing fungi. The study was conducted in the Spanish laboratory, at the Faculty of Agriculture, University for Development Studies, Nyankpala Campus. Isolations were made from rotted yam tubers sampled from the Tamale Central market with potato dextrose agar (PDA) as the growth medium. Growth inhibition of the isolates was determined by growing pure cultures on PDA plates amended with 2?ml each of ethanol and aqueous extracts of neem seed and ginger as well as carbendazim. A pathogenicity test proved that Aspergillus flavus, A. niger, A. ochraceus and Penicillium digitatum isolated from the rotted tubers were responsible for the rot. A. niger had a significantly higher (p?<?0.05) occurrence (40%) than the others. Growth inhibitions by carbendazim and ethanol extracts of neem seed and ginger were comparable but significantly higher (p?<?0.05) than the aqueous extracts of neem seed and ginger as well as the control. However, the aqueous extracts of neem seed and ginger had a significantly higher (p?<?0.05) inhibition than the control. For instance, growth inhibition of A. niger by carbendazim, ethanol neem seed and ginger extracts were 70.5, 69.6 and 65.5%, respectively, while inhibition by aqueous neem seed and ginger extracts were 33.9 and 24.8%, respectively. Since aqueous extracts of neem seed and ginger significantly inhibited (p?<?0.05) growth of the rot-causing fungi, they can be used as surface protectants of stored tubers.     

Field Studies on Cross-protection against Japanese yam mosaic virus in Chinese yam (Dioscorea opposita) with an Attenuated Strain of the Virus

An attenuated strain of Japanese yam mosaic virus (JYMV), designated T‐3, was evaluated for its cross‐protection efficacy against virulent (native) strains of JYMV in Chinese yam (Dioscorea opposita) grown in farmers’ fields in Japan. Native strains of JYMV were detected by a polymerase chain reaction‐based assay in all the Chinese yam plants grown from virus‐free tubers in the first growing season in the fields. In contrast, the virus was detected in only one of fifty plants grown from tubers preinoculated with T‐3 during the experiments for 6 years, suggesting that T‐3 consistently cross‐protected against native JYMV in Chinese yam in the field. Chinese yam plants preinoculated with T‐3 produced significantly greater yield of tubers per plant compared with non‐inoculated plants.     

Pathogenic and genetic variability among Colletotrichum gloeosporioides isolates from different yam hosts in the agroecological zones in Nigeria

Anthracnose, caused by Colletotrichum gloeosporioides Penz., is the most severe foliar disease of water yam (Dioscorea alata) worldwide. Population genetic analyses can yield useful insights into the evolutionary potential of C. gloeosporioides and thus lead to the development of appropriate disease management strategies. The genetic structure of C. gloeosporioides populations from yam and non‐yam hosts in three agroecological zones of Nigeria was investigated. Microsatellite‐primed polymerase chain reaction (MP‐PCR), virulence phenotyping using five putative D. alata differentials, cross‐inoculation tests, and the presence/absence of a Glomerella teleomorph in yam fields were used to infer the evolutionary potential of C. gloeosporioides on yam. We observed high genotypic diversity (GD = 0.99 to 1.00) for populations from all hosts and agroecological zones, with multiple pathogen genotypes in individual anthracnose lesions. Genetic differentiation was low among pathogen populations from different hosts (G_ST = 0.10, θ = 0.034), and agroecological zones (G_ST = 0.04, θ = 0.018), indicating limited host differentiation and significant gene flow. No evidence was found for the existence of C. gloeosporioides f. sp. alatae reported in previous studies. The fungus was recovered from several non‐yam host species commonly found in yam fields but non‐yam isolates caused only mild to moderate symptoms on yam. Eighteen C. gloeosporioides virulence phenotypes were identified among 217 isolates but there was a weak correlation (r = 0.02, P = 0.40) between virulence phenotype and MP‐PCR haplotype. Consistent with the above findings, we observed for the first time the Glomerella teleomorph on anthracnose‐infected yam plants in Nigeria, indicating that sexual recombination might play an important role in anthracnose epidemics on yam. The implications of these findings for C. gloeosporioides evolutionary potential and anthracnose resistance breeding are discussed.     

Studies on Biological Control of Postharvest Rot in Yams (Dioscorea spp.) using Trichoderma viride

From in vitro and in vivo screening tests for antagonism by isolates of Trichoderma against postharvest pathogens of yams (Dioscorea spp.), an isolate of Trichoderma viride Pers. ex S.F. Gray was selected as the most promising candidate for the biocontrol of postharvest rot of yams. Inoculation of white yam (Dioscorea rotundata Poir.) with conidiaspores of T. viride and subsequent storage of the tubers under the ambient environment conditions of a traditional yam barn, resulted in a drastic reduction in the frequency of occurrence of the normal tuber surface mycoflora over a 4‐month (December‐April) storage period. Trichoderma viride on the other hand, maintained a high frequency of occurrence during the same period. Furthermore, whereas up to 52.0%) rot was found among groups of tubers that were artificially inoculated with the postharvest pathogens of yams, Aspergillus niger Van Tiegh., Botryodiplodia theobromae Pat. or Penicillium oxalicum Currie and Thom, and also the group that was not inoculated with any organism (control), among groups of tubers that were inoculated with T. viride the rot was either totally suppressed or only a low percentage of rot occurred. The significance of the findings is discussed in relation to yam storage especially by farmers with limited resources.     

Vegetative Compatibility Among Isolates of Colletotrichum gloeosporioides from Yam (Dioscorea spp.) in Nigeria

Isolates of Colletotrichum gloeosporioides obtained from yam‐based cropping systems in Nigeria, previously characterized on the basis of morphology, virulence and rDNA internal transcribed spacer (ITS) sequence variation were further compared for vegetative compatibility (VC). Chlorate‐resistant nitrate non‐utilizing (nit) mutants were generated from the isolates and used in complementation (heterokaryon) tests. Tests of VC between complementary mutants from different isolates indicated the presence of several genotypes within a single field, suggesting limited clonal spread. In some cases, isolates obtained from the same lesion were observed to belong to different vegetative compatibility groups (VCGs). No compatibility was observed between isolates of the highly virulent slow‐growing grey (SGG), the moderately virulent fast‐growing salmon (FGS) and the avirulent/weakly virulent fast‐growing grey (FGG) strains. Forty‐one C. gloeosporioides isolates belonged to 28 VCGs, giving a genotype diversity estimate of 0.68. This diversity confirmed the high variability of the pathogen population as revealed by previous characterization studies, however, a correlation between VCGs and isolate groupings based on morphology and virulence was not found. The finding that an isolate from weed was compatible with yam isolates indicated that transfer of important traits, such as virulence, may take place between isolates from yam and non‐yam hosts. The VCG diversity revealed by this study suggests that in addition to asexual reproduction, sexual reproduction may play an important role in the epidemiology of anthracnose on yam.     

Delimitation of physiological regions in yam tubers (Dioscorea sp.) and distribution pattern of saccharide degrading enzymes,cell sap pH and protein in these regions

Physiological regions of yam tubers were morphologically defined in different specie into ‘Head’, ‘Middle’ and ‘Tail’, while the limits of these regions were studied using phosphorylase activity. Variation in enzyme activity, pH and protein concentration was found in different regions of the tubers. Old yam tubers had significantly higher activities of saccharide degrading enzymes, hexokinase, phosphorylase, glucose-6-phosphate dehydrogenase, phosphofructokinase and pyruvate kinase, than the new tubers. However, activity of phosphofructokinase in newD. rotundata was higher than that of old tuber. The high activity of phosphorylase in different regions of all the yam tubers examined indicates a very important role of this enzyme in starch degradation inDioscorea species. The measured pH and protein concentration were also higher in old yam tubers. Except for phosphorylase, these enzymes had alkaline pH optima.     

Mycoflora of tuber surface of white yam (Dioscorea rotundata poir) and postharvest control of pathogens with Bacillus subtilis

Bacillus subtilis (Enrenberg) Cohn was investigated for its antagonistic properties against surface mycoflora of yam (Dioscorea rotundata Poir) tubers in storage. Yam tubers inoculated with a spore suspension of B. subtilis in potato dextrose broth using a knapsack sprayer showed a drastic reduction in the range and number of mycoflora, including pathogens of the tuber surface in contrast to the control tubers, during the five-month storage period in a traditional yam barn. However, B. subtilis maintained a high frequency of occurrence during the same period. Botryodiploidia theobromae Pat, Fusarium moniliforme Wollen and Reink., Penicillium sclerotigenum Yamamoto, and Rhizoctonia sp. were displaced completely on the treated tubers. The antagonism of B. subtilis was so effective that the normal tuber surface mycoflora was greatly reduced throughout the storage period of five months by a simple initial application of the antagonist.This revised version was published online in October 2005 with corrections to the Cover Date.     

Biological and molecular characteristics of Beauveria bassiana isolates from California Lygus hesperus (Hemiptera: Miridae) populations

Lygus hesperus is an important pest of many crops grown in the Western US. In addition, other species of Lygus cause damage in other parts of the world. To date, no selective pesticide exists for the control of Lygus spp. and broad spectrum pesticides that also kill natural enemies may lead to secondary pests. Entomopathogenic fungi may offer an alternative to chemical pesticides. Isolates of Beauveria bassiana collected from San Joaquin Valley of California (SJV) L. hesperus populations were screened for their ability to grow at high temperatures and for their ability to infect and kill L. hesperus adults and nymphs under laboratory conditions. No isolate grew at 37 or 35 °C but most isolates were able to grow at 32 °C. In addition, one L. hesperus isolate was more efficacious at higher doses than the commercial isolate. Microsatellite markers were used to determine that selected isolates could be distinguished from other isolates. Preliminary information suggested 82 SJV isolates of B. bassiana were closely related to each other but distantly related to the commercial isolate.     

The RustPuccinia abruptavar.partheniicola,a Potential Biocontrol Agent of Parthenium Weed: Environmental Requirements for Disease Progress

The rust fungusPuccinia abruptavar.partheniicola,a potential biological control agent of parthenium weed (Parthenium hysterophorus), was evaluated under controlled environmental conditions. A range of spore germination temperatures as well as dew period durations and temperatures were investigated to determine some of the environmental requirements for disease establishment and disease progress. Plants were inoculated with urediniospores and exposed to dew periods between 3 to 12 h at temperatures of 10, 15, or 20°C. For disease expression, the inoculated plants were then grown in a glasshouse at one of two temperature regimes (30/26°C or 18/13°C; day/night). Urediniospores germinated best at 12 ± 1°C, with lower germination rates at 5°C or above 20°C. No infection occurred when the plants were exposed to dew periods of ≤3 h, regardless of the incubation temperature. The disease progressed most rapidly when plants were inoculated and incubated for a dew period of at least 12 h at a temperature of 15 ± 1°C. The disease progressed most slowly following inoculation at dew periods of 6 h or less. Disease progress was more rapid when the plants were exposed to a cool-temperature regime (18/13°C) than when exposed to a warm-temperature regime (30/26°C). This suggests that good infection of parthenium weed could be obtained when the urediniospores arrive on the plants during the afternoon in the cooler months of the central Queensland autumn when relatively long dew periods are expected.     

Biological control of postharvest fungal rot of yam (<Emphasis Type="Italic">Dioscorea</Emphasis> spp.) with<Emphasis Type="Italic">Bacillus subtilis</Emphasis>

The potential of isolates of Bacillus subtilis from yam farm soil to control rot of yam in storage barns was investigated. Yam tubers inoculated in vivo with B. subtilis showed no rot while those inoculated with Aspergillus niger, Botryodiploidia theobromae or Penicillium oxalicum showed considerable rot. The set of yams in which B. subtilis and the fungi were simultaneously inoculated produced rot whereas those in which B. subtilis was inoculated a day before the fungi was inoculated were totally reduced or free of rot. Many fewer fungi were isolated from the surface of tubers treated with B. subtilis than from the untreated (control) and there was high recovery of B. subtilis (99–100%) throughout the period of storage. Rot build up was faster in uninoculated control tubers or those inoculated with a spoilage fungus, while those treated with the antagonist were totally reduced or free of rot. The culture filtrate of B. subtilis prevented spore germination in some spoilage fungi. The importance of this study in relation to farmers in developing countries is discussed.     

Identification and potential use of RAPD markers linked to Yam mosaic virus resistance in white yam (Dioscorea rotundatd)

Resistance to Yam mosaic virus (YMV) in tetraploid white yam (Dioscorea rotundatd) is inherited differentially as a dominant and recessive character. Elite D. rotundata breeding lines with durable resistance to YMV can be developed by pyramiding major dominant and recessive genes using marker‐assisted selection (MAS). The tetraploid breeding line, TDr 89/01444, is a source of dominant genetic resistance to yam mosaic disease. Bulked segregant analysis was used to search for random amplified polymorphic DNA (RAPD) markers linked to YMV resistance in F₁ progeny derived from a cross between TDr 89/01444 and the susceptible female parent, TDr 87/00571. The F₁ progeny segregated 1:1 (resistantsusceptible) when inoculated with a Nigerian isolate of YMV, confirming that resistance to YMV in TDr 89/01444 was dominantly inherited. A single locus that contributes to YMV resistance in TDr 89/01444 was identified and tentatively named Ymv‐1. Two RAPD markers closely linked in coupling phase with Ymv‐1 were identified, both of which were mapped on the same linkage group: OPW18₈₅₀ (3.0 centiMorgans [cM]) and OPX15₈₅₀ (2.0 cM). Both markers successfully identified Ymv‐1 in resistant genotypes among 12 D. rotundata varieties and in resistant F1 individuals from the cross TDr 93–1 × TDr 877 00211, indicating their potential for use in marker‐assisted selection. OPW18₈₅₀ and OPX15₈₅₀ are the first DNA markers for YMV resistance and represent a starting point in the use of molecular markers to assist breeding for resistance to YMV.     

Greenhouse screening ofTrichoderma isolates for control ofCurvularia leaf spot of yam

TenTrichoderma isolates were studied under greenhouse conditions as potential agents for biocontrol of yam leaf spot, caused byCurvularia eragrostidis. TheTrichoderma isolates demonstrated high efficiency of disease control. Application of the antagonists on the same day asC. eragrostidis inoculation showed the best results. One isolate achieved 75 percent disease severity reduction. The survival of theTrichoderma isolates decreased after its application on the phylloplane with values of 57 and 41 percent at the 49th day, in the absence and presence of the phytopathogen respectively. The antagonism persisted well againstC. eragrostidis, the TN1 isolate praises 89 percent.     

Epiphytic bacteria antagonistic to Curvularia leaf spot of yam

Curvularia eragrostidis yam leaf spot is a serious concern among the northeast Brazilian yam growing areas. In order to study its biocontrol, bacterial isolates from the yam phylloplane were tested against the pathogen. They were evaluated with respect to the following parameters: (1) inhibition of C. eragrostidis mycelial growth by using paired culture and cellophane membrane methods, (2) inhibition of conidium germination by using a paired suspension test, (3) reduction of disease severity and, (4) persistence of antagonistic action, on plants under greenhouse conditions. From a total of 162 bacterial isolates, 39 showed antagonism to the pathogen in paired culture. The bacteria produced extracellular, nonvolatile, and diffusible metabolites in the membrane cellophane test. Seventeen isolates resulted in more than 75% inhibition of C. eragrostidis mycelial growth. Among them, IF-26 showed the greatest antagonism. The isolates IF-82, IF-88, and IF-109 inhibited pathogen conidial germination, with average inhibition levels of 99.2, 98.2 and 96.2%, respectively. Under greenhouse conditions the antagonists were applied at three different time intervals relative to C. eragrostidis inoculation: 3 days before, at the same time, and 3 days after. IF-82 and IF-88 applied at the same time as pathogen inoculation both reduced disease severity 75%. IF-82 showed the best persistence of antagonistic action, with an average of 96.3%. IF-82, identified as Bacillus subtilis, was the best biocontrol agent for the yam leaf spot disease in this study.Correspondence to: Sami J. Michereff     

Strategies to combat the problem of yam anthracnose disease: Status and prospects

Yam (Dioscorea spp.) anthracnose, caused by Colletotrichum alatae, is the most devastating fungal disease of yam in West Africa, leading to 50%–90% of tuber yield losses in severe cases. In some instances, plants die without producing any tubers or each shoot may produce several small tubers before it dies if the disease strikes early. C. alatae affects all parts of the yam plant at all stages of development, including leaves, stems, tubers, and seeds of yams, and it is highly prevalent in the yam belt region and other yam-producing countries in the world. Traditional methods adopted by farmers to control the disease have not been very successful. Fungicides have also failed to provide long-lasting control. Although conventional breeding and genomics-assisted breeding have been used to develop some level of resistance to anthracnose in Dioscorea alata, the appearance of new and more virulent strains makes the development of improved varieties with broad-spectrum and durable resistance critical. These shortcomings, coupled with interspecific incompatibility, dioecy, polyploidy, poor flowering, and the long breeding cycle of the crop, have prompted researchers to explore biotechnological techniques to complement conventional breeding to speed up crop improvement. Modern biotechnological tools have the potential of producing fungus-resistant cultivars, thereby bypassing the natural bottlenecks of traditional breeding. This article reviews the existing biotechnological strategies and proposes several approaches that could be adopted to develop anthracnose-resistant yam varieties for improved food security in West Africa.     

Inheritance of resistance to Yam mosaic virus, genus Potyvirus, in white yam (Dioscorea rotundata)

Yam mosaic virus (YMV) causes the most-widespread and economically important viral disease affecting white yam (Dioscorea rotundata) in West Africa. The genetic basis of resistance in white yam to a Nigerian isolate of YMV was investigated in three tetraploid D. rotundata genotypes: TDr 93–1, TDr 93–2 and TDr 89/01444. F₁ progeny were produced using TDr 87/00571 and TDr 87/00211 as the susceptible parents. Segregation ratios indicated that a single dominant gene in a simplex condition governs the resistance in TDr 89/01444, while the resistance in TDr 93–2 is associated with the presence of a major recessive gene in duplex configuration. Segregation of progeny of the cross TDr 93–1×TDr 87/00211 fitted a genetic ratio of 2.48:1 resistant:susceptible, which can be expected when two simplex heterozygotes are crossed, indicating the possible modifying effect of the susceptible parent. A triple antibody immunosorbent assay (TAS-ELISA) was used for virus detection in inoculated plants. Slight mosaic symptoms appeared on most resistant individuals, while asymptomatic resistant genotypes with high ELISA (A₄₀₅) values were observed in all crosses. Such a heterogeneous response suggests the influence of additional modifier genes that segregate in the progeny. The finding that resistance can be inherited as a dominant or recessive character has important implications for YMV resistance breeding. Received: 15 August 2000 / Accepted: 12 April 2001     

Comparison of temperature effects on the in vitro growth and disease development in potato tubers inoculated with bacteria Pectobacterium atrosepticum,P carotovorum subsp. carotovorum and Dickeya solani

The relationship between the rate of in vitro growth of bacterial isolates of Pectobacterium atrosepticum, P. carotovorum subsp. carotovorum and Dickeya solani and their pathogenicity was investigated in tubers of two potato cultivars at four temperatures ranging from 18°C to 30°C. The rate of in vitro growth was highly positively correlated with the number of rotted tubers (r ranged from 0.91 to 0.93) and with the weight of macerated potato tissue, which, however, was only found for P. carotovorum and D. solani (r = 0.76; r = 0.91, respectively) and not for P. atrosepticum. The weight of macerated tissue increased with the temperature, but significant differences between species of bacteria were observed only at 26°C and above, at which temperatures D. solani was the most aggressive, followed by P. carotovorum and P. atrosepticum. Almost all potato tubers inoculated with bacteria showed symptoms of soft rot at 26°C and 30°C, but the number of rotting tubers at lower temperatures (22°C and 18°C) decreased significantly. The lowest disease incidence, 11% of tubers with symptoms, was observed for the D. solani and cultivar Sonda at 18°C, what was also confirmed in a separate experiment with tubers from four potato cultivars inoculated with the highly aggressive isolate of D. solani. At temperatures from 18°C to 30°C, the differences in disease severity between potato cultivars with various resistance to bacteria increased in line with temperature, while the differences in disease incidence decreased.     

Rapid assessment of resistance of tissue-cultured water yam (Dioscorea alata) and white guinea yam (Dioscorea rotundata) to anthracnose (Colletotrichum gloeosporioides Penz.)

Yam anthracnose is caused by the pathogen Colletotrichum gloeosporioides Penz. and has been identified as the most important biotic constraint to yam production worldwide. Rapid assessment of the disease is vital to its effective diagnosis and management. In this study, tissue-cultured yam plantlets of five lines of Dioscorea alata and nine of D. rotundata were rapidly assessed for their reactions to two isolates of yam anthracnose. The plantlets, obtained from meristem of the nodal cuttings, were grown for 8?weeks on Murashige and Skoog (MS) basal medium, acclimatised for 3?weeks, hardened for an additional 3?weeks, arranged in screen house in completely randomised design and sprayed with spore inocula prepared from 7?day-old culture of the two strains of Colletotrichum gloeosporioidies Penz. The relative resistance of the different Dioscorea spp. was evaluated using three disease indices – severity at seventh day after inoculation, SD7; area under disease progress curve, AUDPC; and disease severity rate, Rd. A modified rank-sum classification method put TDa 1425 and TDr 2040, with rank sum of 2.0 each, as resistant. TDr 2121, TDr 2287 and TDr 2048 were susceptible with rank sum of 27.50, 25.50 and 24.50, respectively. Dioscorea alata TDa 1425 and Dioscorea rotundata TDr 2040 were recommended in areas endemic with yam anthracnose, and also as parent lines while breeding for resistance to anthracnose.     

Biological characteristics and thermal requirements of a Brazilian strain of the parasitoid Trichogramma pretiosum reared on eggs of Pseudoplusia includens and Anticarsia gemmatalis

A new strain of the parasitoid Trichogramma pretiosum, was collected in Rio Verde County, State of Goiás, Central Brazil, and designated as T. pretiosum RV. This strain was then found to be the most effective one among several different strains of T. pretiosum tested in a parasitoid selection assay. Therefore, its biological characteristics and thermal requirements were studied, aiming at allowing its multiplication under controlled environmental conditions in the laboratory. The parasitoid was reared on eggs of Pseudoplusia includens and Anticarsia gemmatalis at different constant temperatures within an 18–32 °C temperature range. The number of annual generations of the parasitoid was also estimated at those temperatures. Results have shown that T. pretiosum RV developmental time, from egg to adult, was influenced by all temperatures tested within the range, varying from 6.8 to 20.3 days and 6.0 to 17.0 days on eggs of P. includens and A. gemmatalis, respectively. The emergence of T. pretiosum RV from eggs of A. gemmatalis was higher than 94% at all temperatures tested. When this variable was evaluated on eggs of P. includens, however, the figures were higher than that within the 18–30 °C range (more than 98%), and were also statistically higher than the emergence observed at 32 °C (90.2%). The sex ratio of the parasitoids emerged from eggs of A. gemmatalis decreased from 0.55 to 0.29 at 18–32 °C, respectively. However, for those emerged from eggs of P. includens, the sex ratio was similar (0.73, 0.72 and 0.71) at 20, 28 and 32 °C, respectively. The lower temperature threshold (Tb) and thermal constant (K) were 10.65 °C and 151.25 degree-days when the parasitoid was reared on eggs of P. includens; and 11.64 °C and 127.60 degree-days when reared on eggs of A. gemmatalis. The number of generations per month increased from 1.45 to 4.23 and from 1.49 to 4.79 when the parasitoid was reared on eggs of P. includens and A. gemmatalis, respectively, following the increases in the temperature.