Failure of the mite-pathogenic fungus Neozygites tanajoae and the predatory mite Neoseiulus idaeus to control a population of the cassava green mite, Mononychellus tanajoa

Monitoring of a population of the phytophagous cassava green mite, Mononychellus tanajoa (Bondar), and its natural enemies was undertaken in central Bahia, Brazil, in mid-1996. In spite of the presence of extremely high densities of the predatory phytoseiid mite Neoseiulus idaeus Denmark & Muma, the phytophagous mite population reached such high densities itself that there was total overexploitation of the cassava plants, leading to total leaf loss. Meanwhile, the mite-pathogenic fungus Neozygites tanajoae Delalibera, Humber & Hajek did not affect the M. tanajoa population in its growth phase as there was no inoculum present, even though we predict from a simple regression model that there was the potential for epizootics at that time. Soon after the M. tanajoa population crashed due to defoliation, there could have been an epizootic but there were simply no mite hosts to infect. These data demonstrate the ineffectiveness of one natural enemy (the predator) in terms of prey population regulation and demonstrate the importance of timing in the possible effectiveness of the other (the pathogen). For the pathogen, this probably explains its sporadic effect on host populations as previously reported. We conclude that the fungus is likely to be most useful as an adjunct to biological control with predatory mites other than N. idaeus.     

Long-term preservation of Neozygites tanajoae (Entomophthorales: Neozygitaceae) in cadavers of Mononychellus tanajoae (Acari: Tetranychidae)

To determine the effect of storage on fungal survival, mummified cadavers of the cassava green mite pathogen, Neozygites tanajoae were placed at different conditions of temperature and relative humidity. The best condition for long-term preservation was ?10°C. At this condition, the fungus retained viability for 10 years when the experiment was terminated, with a decrease in sporulation with time. Cadavers placed at 4°C and 5% RH sporulated for 2 years, while the fungus survived for only 7 days at 25°C and 50% RH.     

Effect of temperature,humidity and photoperiod on mortality of Mononychellus tanajoa (Acari: Tetranychidae) infected by Neozygites cf. floridana (Zygomycetes: Entomophthorales)

The effect of temperature, humidity and photoperiod on the development of Neozygites cf. floridana (Weiser and Muma) in the cassava green mite, Mononychellus tanajoa (Bondar) was studied in the laboratory. Dead infected mites began to appear 2.5 days after inoculation. At 33 and 28°C peak mortalities were higher and occurred earlier (after 2.5 days), than at 23 and 18°C. Mean LT₅₀ (time for half the infected mites to die) decreased with increasing temperature as follows: 3.9, 3.0, 2.9 and 2.5 days at 18, 23, 28 and 33°C, respectively. When placed under conditions of high relative humidity for a period of 24 h, the percentage of dead infected mites from which the fungus sporulated was highest at 28°C (51.4%) and lowest at 33°C (6.5%). The development of the fungus inside the mite was not significantly affected by ambient humidity or photoperiod. No significant interactions between tested factors were found.     

Importance of ambient saturation deficits in an epizootic of the fungus Neozygites floridana in cassava green mites (Mononychellus tanajoa)

The mite-pathogenic fungus Neozygites floridana Fisher (Entomophthorales: Neozygitaceae) is considered to have potential for the biological control of the cassava green mite, Mononychellus tanajoa (Bondar). However, its activity is sporadic and laboratory data suggest a strong dependence on night-time saturation deficits for transmission. We report on an epizootic of this fungus in a mite population in northeastern Brazil. During the epizootic, host populations appeared to be limited by a combination of the pathogen and a predatory mite Neoseiulus idaeus (Acari: Phytoseiidae). When temperatures increased, the epizootic finished and the host population began to grow. Abiotic conditions could not explain the variation in host mortality following pickup of infective propagules in this epizootic. However, night-time saturation did help to explain the variation in transmission from infective cadavers to newly killed hosts. This supports laboratory observations that horizontal transmission between hosts is determined mainly by saturation deficits, while the process of infection is little affected by abiotic conditions. A further field observation was the near-absence of resting spores in dead mites (ca. 0.1% of cadavers), suggesting that the pathogen population was unsuccessful in producing inoculum to infect future M. tanajoa populations. The implications are that this pathogen will only be effective as a biological control agent in periods of high relative humidity, and establishment in new areas may be limited by resting spore formation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Relative contribution of biotic and abiotic factors to the population density of the cassava green mite, Mononychellus tanajoa (Acari: Tetranychidae)

The cassava green mite, Mononychellus tanajoa, is a key pest of cassava, Manihot esculenta Crantz (Euphorbiaceae), and it may be kept in check by naturally occurring predatory mites of the family Phytoseiidae. In addition to predatory mites, abiotic factors may also contribute to regulate pest mite populations in the field. Here, we evaluated the population densities of both M. tanajoa and the generalist predatory mite Euseius ho DeLeon (Acari: Phytoseiidae) over the cultivation cycle (11 months) of cassava in four study sites located around the city of Miranda do Norte, Maranhão, Brazil. The abiotic variables rainfall, temperature and relative humidity were also recorded throughout the cultivation cycle of cassava. We determined the relative importance of biotic (density of E. ho) and abiotic (rainfall, temperature and relative humidity) factors to the density of M. tanajoa. The density of M. tanajoa increased whereas the density of E. ho remained constant throughout time. A hierarchical partitioning analysis revealed that most of the variance for the density of M. tanajoa was explained by rainfall and relative humidity followed by E. ho density and temperature. We conclude that abiotic factors, especially rainfall, were the main mechanisms driving M. tanajoa densities.     

The role of resting spores in the survival of the mite-pathogenic fungus Neozygites floridana from Mononychellus tanajoa during dry periods in Brazil

Survival of pathogens during long periods of unfavorable conditions can be critical to their ecology and to their use in biological control. In northeastern Brazil, the mite pathogen Neozygites floridana must survive hot and dry conditions between wet seasons when it infects the cassava green mite Mononychellus tanajoa. We report on large numbers of mite cadavers bearing resting spores towards the end of epizootics in mid-1995. High within-leaf variability indicated that local factors may be important in determining resting spore formation. These spores remain in the host cadaver on a leaf until the cadaver breaks up, whereupon the spores fall freely to the soil, there to remain dormant. Laboratory simulation of field conditions led to ca. 25% of mycosed individuals bearing resting spores. Mummies (without resting spores) kept in hot and dry conditions showed little or no viability within 2 months, implying no role for survival over extended dry periods. It is proposed that resting spores form the principal means by which this pathogen survives the dry season in the study area. This has implications for its introduction to new areas in classical biological control.     

Host specificity of the cassava green mite pathogen Neozygites floridana

Tests were conducted on the hostspecificity of a Brazilian isolate of thefungus Neozygites floridana, a potentialbiological control agent for the cassava greenmite, Mononychellus tanajoa, in Africa.Five insect and two mite species, mostly fromthe cassava agroecosystem, were evaluated forsusceptibility to N. floridana, namelyEuseius concordis, E. citrifolius, Phenacoccus herreni, Stethorus sp., Aleurothrixus aepim, Apoanagyrusdiversicornis, and Bombyx mori.Individuals of each species were exposed tocapilliconidia (the infective stage of thefungus). None of the tested individuals wasfound with hyphal bodies (the vegetative stageof the fungus), whereas 73 to 94% of thecassava green mites in the controls becameinfected. Non-germinated capilliconidia were,however, found attached to several individualsin most species. N. floridana appears tobe safe for exportation. Further evaluation ofits performance against M. tanajoa inAfrica is therefore desirable.     

Functional responses of Iphiseius degenerans and Neoseiulus teke (Acari: Phytoseiidae) to changes in the density of the cassava green mite, Mononychellus tanajoa (Acari: Tetranychidae)

The functional responses of protonymph and adult female Iphiseius degenerans and Neoseiulus teke to increasing density of three stages of their prey, the cassava green mite (CGM), Mononychellus tanajoa, were studied on excised cassava leaf discs under laboratory conditions. The responses obtained were predominantly sigmoid type III curves with the highest plateau when both stages of I. degenerans and N. teke were preying on CGM eggs. In all cases, the predation rate of the former species exceeded that of the latter. The empirical data were fitted by four different models. From the models, the attack coefficient (a) and handling time (T _h) were estimated. For a given predator stage (protonymph or adult female), the predator's attack coefficient declines and handling time increases as the prey gets larger. For a given prey stage, the predator's attack coefficient increases and handling time decreases as the predator stage becomes larger.     

Reproductive responses of Iphiseius degenerans and Neoseiulus teke (Acari: Phytoseiidae) to changes in the density of the cassava green mite, Mononychellus tanajoa (Acari: Tetranychidae)

We assessed the reproductive responses of adult female Iphiseius degenerans and Neoseiulus teke to increasing density of three stages of their prey, Mononychellus tanajoa, on cassava leaf discs under laboratory conditions. The oviposition rates increased with number of prey consumed per predator per day with a maximum of approximately two eggs per day for I. degenerans and four eggs per day for N. teke. The oviposition rate of N. teke was higher when consuming eggs than other prey stages. Neoseiulus teke was more efficient than I. degenerans in converting consumed prey into egg production. The data were adequately described by simple mathematical models.     

Sampling strategies for assessing the overall density of the cassava green mite (Mononychellus tanajoa)

Six different sampling methods to estimate the density of the cassava green mite, Mononychellus tanajoa, are categorized according to whether leaves or leaflets are used as secondary sampling units and whether the number of leaves on the sampled plants are enumerated, estimated from an independent plant sample, or not censused at all. In the last case, sampling can provide information only on the average number of mites per leaf and its variance, while information on stratum sizes is necessary to estimate the mean number of mites per plant as well. It is shown that leaflet-sampling is as reliable as leaf-sampling for the same number of sampling units. When stratum sizes are estimated from a separate plant sample, sampling time may also be reduced, but the estimated mean density and its variance may be biased if mite density and plant size are correlated. Sampling data show that the within-plant variance contributes relatively little to the overall variance of the population density estimates. It points at a sampling strategy in which the number of primary units (plants) is as large as possible at the expense of secondary units (leaflets) per plant. Mean-variance relationships may be applied to estimate sample variances and can be used even when only one leaflet is taken per plant per stratum. An unequal allocation of primary units among strata can increase precision, but the gain is small compared with an equal allocation. Leaf area can be predicted from the length of the longest leaflet and the number of leaflets.     

The effect of host plants on Tetranychus evansi, Tetranychus urticae (Acari: Tetranychidae) and on their fungal pathogen Neozygites floridana (Entomophthorales: Neozygitaceae)

In a series of tritrophic-level interaction experiments, the effect of selected host plants of the spider mites, Tetranychus evansi and Tetranychus urticae, on Neozygites floridana was studied by evaluating the attachment of capilliconidia, presence of hyphal bodies in the infected mites, mortality from fungal infection, mummification and sporulation from fungus-killed mite cadavers. Host plants tested for T. evansi were tomato, cherry tomato, eggplant, nightshade, and pepper while host plants tested for T. urticae were strawberry, jack bean, cotton and Gerbera. Oviposition rate of the mites on each plant was determined to infer host plant suitability while host-switching determined antibiosis effect on fungal activity. T. evansi had a high oviposition on eggplant, tomato and nightshade but not on cherry tomato and pepper. T. urticae on jack bean resulted in a higher oviposition than on strawberry, cotton and Gerbera. Attachment of capilliconidia to the T. evansi body, presence of hyphal bodies in infected T. evansi and mortality from fungal infection were significantly higher on pepper, nightshade and tomato. The highest level of T. evansi mummification was observed on tomato. T. evansi cadavers from tomato and eggplant produced more primary conidia than those from cherry tomato, nightshade and pepper. Switching N. floridana infected T. evansi from one of five Solanaceous host plants to tomato had no prominent effect on N. floridana performance. For T. urticae, strawberry and jack bean provided the best N. floridana performance when considering all measured parameters. Strawberry also had the highest primary conidia production. This study shows that performance of N. floridana can vary with host plants and may be an important factor for the development of N. floridana epizootics.     

Sampling strategies for assessing the intra-plant density of the cassava green mite (Mononychellus tanajoa): A case-study

Sampling data from a single cassava plant were used for testing various sampling methods. It was found that the degree of leaf damage was poorly correlated with the number of cassava green mites (CGM) present on a leaf. Sampling of specific mite stages should take into consideration the effect of leaf damage on the age distribution of the mites. Simple random sampling resulted in a high between-leaf variation caused by a vertical gradient in spider-mite abundance. The between-leaf variation could be reduced by stratified sampling. The optimum number of strata was three. Subsampling of leaflets combined with stratification provided the most precise estimates for a given sampling effort.Various regression methods for estimating the area of a cassava leaf from some simple measurements were compared. Combining the length of the longest leaflet with the number of leaflets gave the best prediction, explaining 95.8% of the total variation in observed leaf areas.     

A rapid and non-destructive method to assess leaf injury caused by the cassava green mite,Mononychellus tanajoa (Bondar) (Acarina: Tetranychidae)

A relative scale of Leaf Damage Indices (LDI) from 0 to 5 describes the visible injury to leaves of cassava, Manihot esculenta Crantz caused by the cassava green mite, Mononychellus tanajoa (Bondar). As the scale is ordinal and thus not quantitative, the observed LDIs are converted individually to relative loss of chlorophyll on a ratio scale before an average injury is determined. This calibration is required because the ordinal and the ratio scales are not linearly related. A calibration curve was established on the basis of laboratory experiments to determine the chlorophyll content, c, of leaves representing various leaf damage indices. Several monotonously decreasing functions were fitted to the experimental data yielding the following relation , where c _o is the chlorophyll content of unijured leaf tissue and a is a constant describing the steepness of the curvilinear relation. This means that LDIs could be converted to relative loss of chlorophyll, d, where . The photosynthetically active leaf area of plants can be estimated by combining the relative loss of chlorophyll with leaf area assessments and adds the effects of defoliation and suspended growth to the chlorophyll depletion. The difference in photosynthetically active area that arises between uninjured and injured plants over a period of time provides a measure of spider mite injury that can be related to growth and yield. The method integrates the injury inflicted over a period of time, allows successive observations of the same plants, and is rapid and reasonably precise considering the time savings.     

Effect of cassava exudate and prey densities on the survival and reproduction of Typhlodromalus limonicus (Garman & McGregor) s.l. (Acari: Phytoseiidae), a predator of the cassava green mite,Mononychellus tanajoa (Bondar) (Acari: Tetranychidae)

The effects of cassava exudate and prey densities on reproduction and survival of the predatory mite, Typhlodromalus limonicus (Garman & McGregor) (Acari: Phytoseiidae), were investigated in the laboratory. Females were provided either cassava exudate ad lib. daily, low or high numbers of the cassava green mite prey, Mononychellus tanajoa (Bondar) (Acari: Tetranychidae) daily, or exudate for 5 or 10 days before switching to a low or high prey diet. Females fed only exudate laid no eggs. Females fed exudate before prey experienced a significant decrease (30%) in the number of eggs laid compared to females fed high numbers of prey daily. The reduction in fecundity was the result of prolonged preoviposition periods (2.0 days on prey daily vs 4.0 days on exudate before prey) and reduced number of eggs laid per female per day (1.7 eggs per female per day on prey daily vs 0.4 eggs per female per day on exudate before prey). Females fed only exudate had a greater survival rate and longevity than females fed prey daily or females fed exudate before a diet of prey. These results suggest that T. limonicus can survice for a limited period on cassava exudate during periods of low prey availability, but requires prey to complete oögenesis and propagate the population.     

Overwintering and prevalence of Neozygites floridana (Zygomycetes: Neozygitaceae) in hibernating females of Tetranychus urticae (Acari: Tetranychidae) under cold climatic conditions in strawberries

To evaluate overwintering strategies of the fungus Neozygites floridana, an important natural enemy of Tetranychus urticae, hibernating T. urticae females were investigated for the presence of fungal structures throughout one winter (October 12, 2006 to February 19, 2007) in field-grown strawberries in a cold climate in Norway (min. ambient temp -15.3 degrees C). Neozygites floridana was present as hyphal bodies inside live, hibernating females in T. urticae populations throughout the sampling period. The lowest percentages of hibernating females with hyphal bodies were found at the two first dates of sampling at 5.5 and 0% on October 12 and 19, respectively. The prevalence then increased and peaked at 54.4% on January 14. Resting spores (immature) were also found in live hibernating females at some dates, but at lower prevalence than for hyphal bodies and predominantly only until November 8. Prevalence of resting spores in live hibernating females ranged from 2.5 to 13.8%. Total number of T. urticae was also recorded, and most mites of all four categories (nymphs, males, non-hibernating and hibernating females) were found at the first sampling date. At this date non-hibernating females were the most abundant. A sharp decrease in non-hibernating females, nymphs and males was, however, seen from mid-October to mid-November; also numbers of hibernating females decreased, but not as fast. The relative abundance of hibernating females compared to non-hibernating females increased from 32.2% at the first collection (October 12) to 97.7% at the last collection (February 2). This study confirms that N. floridana survives the winter as a semi-latent hyphal body infection, protected inside live hibernating females. It is therefore ready to develop and sporulate as soon as climatic conditions permit, resulting in early season infection of T. urticae.     

Survival of Neozygites cf. floridana (Zygomycetes: Entomophthorales) in mummified cassava green mites and the viability of its primary conidia

The survival of Neozygites cf. floridana (Weiser and Muma) as dry hyphal bodies in mummified cassava green mites, Mononychellus tanajoa (Bondar), at 5.0% RH in the dark was affected by storage temperature. Survival of the fungus in mummies kept at 24±1.0°C could be demonstrated for 6–7 months. When stored at 4°C, the fungus sporulated from 90% of the mummies liberating an average of 186.9 primary conidia per mummy even after a storage period of 16 months, when the experiment was terminated. The temperature, humidity and light condition significantly affected the viability of primary conidia. The percent viability across all factors dropped from 98.4% after 0 h (beginning of the experiment) to 23.4% after a 1 h exposure to the conditions tested. Lower temperatures maintained higher viabilities with 86.3% of the conidia surviving after 18 h at 18°C, whereas almost all conidia died after 12 h at 33°C. Conidia survived less than 1 h when exposed to SDs (saturation deficit) of 2.0 mm Hg or higher at any tested temperature.     

The dynamics of the cassava green mite Mononychellus tanajoa in a seasonally dry area in Kenya

The population dynamics of the cassava green mite Mononychellus tanajoa was studied on cassava during 35 weeks (early March to first of November 1989) in an experimental field near Lake Victoria in Western Kenya. The mite population peaked at the onset of the long dry season with 1,100 mites/leaf, declined sharply to a level of about 300 individuals/leaf, not to increase again until the next rainy season commenced. An indigenous phytoseiid predator Iphiseius degenerans was abundant during the dry spell with a maximum about 9 predators/leaf.A nonlinear regression analysis revealed that food depletion in combination with I. degenerans predation limited the population growth of the mites, whereas rain intensity had no effect. The predator exhibited no aggregative response to high densities of M. tanajoa and stayed mainly in the lower part of the canopy while the spider mites preferred the top, indicating that I. degenerans is a generalist predator without capacity to control M. tanajoa alone. However, in combination with another density dependent factor, such as food depletion, the predator may have prevented the spider mites from causing complete defoliation during the dry season.     

Specificity of a strain ofNeozygites sp. (Zygomycetes: Entomophthorales) toMononychellus tanajoa (Acari: Tetranychidae)

Neozygites sp. is commonly found infecting the cassava green mite,Mononychellus tanajoa (Bondar), in parts of northeast Brazil. The introduction of this fungus into other regions requires the knowledge of its specificity, especially in relation to natural enemies of different cassava pests. Laboratory tests indicated the development of germination tubes ofNeozygites in some females ofTetranychus bastosi Tuttle et al. andT. urticae Koch, with subsequent formation of a reduced number of hyphal bodies in someT. bastosi. No females of the phytoseiid predatorsAmblyseius idaeus (Denmark & Muma) andAmblyseius limonicus Garman & McGregor s.l. were infected byNeozygites sp.     

The cassava green mite in Africa: One or two species? (Acari: Tetranychidae)

Despite a certain amount of confusion that has prevailed for several years with respect to theMononychellus complex (cassava green mite=CGM; Acari: Tetranychidae) which attacks cassava plantations in Africa, strong arguments indicate that a single taxonomic unit corresponding to the speciesMononychellus progresivus Doreste, 1981, was introduced in 1971 from South America.     

Field Evaluation of Brazilian Isolates of Neozygites floridana (Entomophthorales: Neozygitaceae) for the Microbial Control of Cassava Green Mite in Benin,West Africa

Two Brazilian isolates and one Benin (indigenous) isolate of Neozygites floridana were released against the cassava green mite, Mononychellus tanajoa , in January 1999 in the Adjohoun district, Ouémé administrative region, Republic of Benin. Post-release monitoring conducted 8, 14, 22 and 36 weeks later showed very low mean infection rates on M. tanajoa by isolate (0.03-0.4%). However, 48 weeks after releases, mean infection rates increased noticeably to between 2.3 and 18.7%, and higher infection rates were observed for the Brazilian isolates compared with the indigenous one. The highest infection rate for the indigenous isolate was 4.5% while it reached over 30% for the Brazilian isolates (36.5 and 34.0%). Observations made to study dispersal from inoculated plants showed the absence of infected mites at 4 m from the inoculated plants in all fields 8 weeks after the releases, while they were already present on those at 2 m away. From the next monitoring, 14 weeks after the releases, infection was found at all three sampling positions (inoculated plants and plants at 2 and 4 m away). Only four mites with resting spores were found in over 460 000 mites examined. The highest infection levels were observed in December during 'harmattan' a period characterized by hot days and cool nights with high relative humidity.