Alternative plant habitats for common phytoseiid predators of the cassava green mite (Acari: Phytoseiidae,Tetranychidae) in northeast Brazil

A survey was conducted to identify possible alternative plant habitats of the most common phytoseiid predators associated with the cassava green mite, Mononychellus tanajoa (Bondar), in their native environment in northeast Brazil. Thirty-two phytoseiid species were collected, including Amblyseius aripo (DeLeon), Amblyseius idaeus (Denmark and Muma) and Amblyseius limonicus Garman and McGregor s.l., the three predators previously shown as the most common on cassava. In increasing order, A. idaeus, Phytoseius guianensis DeLeon and A. aripo were the most common phytoseiids collected on the plant habitats examined. A. limonicus s.l. was one of the least common phytoseiids on plants other than cassava. Altermative plant habitats seem important in harboring A. aripo and A. idaeus, but not A. limonicus s.l..     

Influence of humidity and water availability on the survival ofAmblyseius idaeus andA. anonymus (Acarina: Phytoseiidae)

Amblyseius idaeus (Denmark & Muma) andA. anonymus Chant & Baker are morphologically very similar species of phytoseiids inhabiting areas in South America that have very different levels of humidity. Above 60% RH, nearly all eggs of both species hatch successfully, but below 60% RH the egg hatching rate ofA. anonymus is very poor, whereas eggs ofA. idaeus hatch at humidities as low as 30% RH. The mobile stages ofA. idaeus are better able to survive in absence of food and water than those ofA. anonymus. Water availability promotes survival of both species, but its effect onA. idaeus exceeds that onA. anonymus. These differences in low-humidity tolerance and survival ability are consistent with the climatic origin of these phytoseiid species;Amblyseius ideaus has been reported from very dry and humid areas, andA. anonymus from humid areas only.In comparison with other phytoseiid species, the eggs ofA. idaeus have the highest tolerance to low humidities reported to date, and, among the phytoseiids that are shown to be capable of controllingTetranychus spp., this tolerance appears to be exceptionally high. The impact of this result on the scope of biological control ofTetranychus spp. is discussed.     

Reproductive success ofAmblyseius idaeus andA. anonymus on a diet of two-spotted spider mites

Amblyseius idaeus is a drought-resistant predatory mite of tetranychid spider mites. In this paper, life-history parameters ofA. idaeus and a morphologically similar species,A. anonymus, are measured and compared on a diet ofTetranychus urticae. Intrinsic rates of increase of both phytoseiids are similar, and are comparable to rates of phytoseiids that successfully controlTetranychus species. This makes both species promising candidates as biological control agents,A. idaeus especially under dry conditions.     

Regional persistence of locally unstable predator/prey populations

Phytoseiid mites are efficient predators capable of completely destroying colonies of spider mites. Thus, coexistence of phytoseiids and their tetranychid prey at a local scale (typically an individual plant) is not likely for more than a single predator/prey cycle. However, the species may coexist at a regional scale, i.e. in a complex environment consisting of many plants, provided local colonisations, extinctions and recolonisations occur asynchronously. This review investigates some of the factors responsible for establishing and maintaining spatial asynchrony between local populations of prey and predators, such as dispersal, environmental heterogeneity and demographic stochasticity. Existing predator/prey models are considered in order to find agreement between theory and empirical data. Based on our current knowledge of spatial processes and their importance for the overall dynamics and persistence of predator/prey interactions, some consequences and aspects for biological control of crop pests by means of natural enemies are outlined.     

Dynamics and potential impact of ‘generalist’ phytoseiids in agroecosystems and possibilities for establishment of exotic species

Generalists, i.e. those species which consume a wide variety of foods, including some of plant origin such as pollen, are the dominant members of the phytoseiid mite fauna in some perennial crops. Biological attributes commonly evident in generalist in contrast to specialist phytoseiids include a lower reproductive potential on mite prey, close association with certain plant species, population increases in the absence of mite prey, and intraplant distribution unrelated to that of spider mites. Some studies are reviewed that suggest the ability of certain generalist phytoseiids to maintain tetranychid populations at low densities. Possible biological factors influencing the establishment of exotic generalist phytoseiids are considered. Some of the attributes usually considered important in effective phytoseiid predators may be of minor importance in more stable situations at low population densities.     

Criteria for release of genetically-improved phytoseiids: an examination of the risks associated with release of biological control agents

Biological control of agricultural pests by phytoseiid predators has been achieved through classical introductions, conservation of indigenous and established foreign species, and augmentation of both introduced and indigenous species. Laboratory selection of phytoseiids has produced several strains that have been mass reared and released for pest management programs in glasshouses and agricultural cropping systems. Concerns over risks of classical biological control have developed recently. The development of recombinant DNA (rDNA) techniques for the genetic manipulation of crops and microorganisms also has inaugurated a debate on the safety of releasing transgenic organisms into the environment. This debate will extend to the release of phytoseiids that have been manipulated with rDNA techniques. Risks associated with releasing phytoseiids for augmentation or classical biological control programs are minimal and the benefits are great. Research initiated to answer questions about the risks of releasing transgenic phytoseiids into the environment provides opportunities to expand our understanding of the ecological impact of phytoseiids in agricultural and natural environments and could lead to improved pest management tactics.     

Egg hatching response to a range of air humidities for six species of predatory mites

The efficiency of biological control agents is often affected by local conditions such as low relative humidity (r.h.). This is especially true for mites of the family Phytoseiidae, and particularly for their egg stage. A key issue in biological control is thus to assess the drought tolerance of phytoseiid species that are (or that will be) commercialized. In this study, we evaluated the egg hatching response to a humidity range for six species of Phytoseiidae, by assessing the relative air humidity and vapour pressure deficit (VPD) at which 50% of the eggs hatch: r.h.₅₀ and VPD₅₀. Typhlodromus athiasae Porath & Swirskii and Phytoseiulus longipes Evans (Argentinean strain) were the most drought‐tolerant, with a r.h.₅₀ of 43% (VPD₅₀ = 18.2 hPa) and 43% (18.1 hPa), respectively. The most drought‐sensitive species were Amblyseius swirskii (Athias‐Henriot) (63%, 11.4 hPa), Phytoseiulus macropilis (Banks) (69%, 9.8 hPa), and Phytoseiulus persimilis Athias‐Henriot (70%, 9.4 hPa). Neoseiulus idaeus Denmark & Muma and P. longipes (Chilean strain) showed intermediate responses with 49% (16.1 hPa) and 50% (15.9 hPa), respectively. These results were similar to what had been obtained for the same species by other authors. However, unexpected inter‐ and intraspecific differences uncorrelated to climatic conditions of the area of origin were observed. The possibility to localize species and/or populations that could be efficient as biological control agents under dry conditions is discussed.     

Variation in the olfactory response of 13 populations of the predatory mite Amblyseius womersleyi to Tetranychus urticae-infested plant volatiles (Acari: Phytoseiidae, Tetranychidae)

We studied the response of the predatory mite Amblyseius womersleyi collected in 13 different sites in Japan toward Tetranychus urticae-infested kidney bean leaf volatiles in a Y-tube olfactometer. The predatory mites were collected from eight plant species infested by one of three tetranychid mite species. The predators' responses to the infested-leaf volatiles varied from 33% to 97% among the populations. The predators collected at 10 sites showed a significant preference for infested-leaf volatiles, whereas those collected at three tea plantations did not distinguish between the infested- and uninfested-leaf volatiles. We discussed the possible factors that affected the olfactory response of A. womersleyi towards the infested leaf volatiles.     

Abundance of phytoseiid mites on Vitis species: effects of leaf hairs,domatia, prey abundance and plant phylogeny

We observed the number of predatory mites (Phytoseiidae:Typhlodromus caudiglans) on the foliage of 20 North American species of grapes (Vitis spp) plus the domesticated EuropeanVitis vinifera, all grown in a common garden. We found relatively few phytophagous mites. The numbers of phytophagous mites were not correlated with the plant characteristics that we measured. We found approximately five times as many predatory mites as phytophagous mites and the numbers of these phytoseiid predators were not affected by the availability of prey. Similarly, numbers of phytoseiids were unaffected by plant gender and, hence, the availability of pollen, another source of food. The numbers of phytoseiids were not clustered according to the taxonomic grouping of the tested plant species. Leaf surface characteristics explained over 25% of the variance in the numbers of phytoseiids. Numbers of phytoseiids were positively associated with the density of vein hairs, the density of bristles in leaf axils, and the presence of leaf domatia. These results suggest that sheltered habitats rather than food availability may limit the numbers of phytoseiid mites on grapevines.     

Can climate change jeopardize predator control of invasive herbivore species? A case study in avocado agro-ecosystems in Spain

Climate change is one of the most important factors affecting the phenology, distribution, composition and diversity of organisms. In agricultural systems many pests and natural enemies are arthropods. As poikilotherm organisms, their body temperature is highly dependent on environmental conditions. Because higher trophic levels typically have lower tolerance to high temperatures than lower trophic levels, trends towards increasing local or regional temperatures may affect the strength of predator/prey interactions and disrupt pest control. Furthermore, increasing temperatures may create climate corridors that could facilitate the invasion and establishment of invasive species originating from warmer areas. In this study we examined the effect of environmental conditions on the dynamics of an agro-ecosystem community located in southern Spain, using field data on predator/prey dynamics and climate gathered during four consecutive years. The study system was composed of an ever-green tree species (avocado), an exotic tetranychid mite, and two native species of phytoseiid mites found in association with this new pest. We also present a climatological analysis of the temperature trend in the area of study during the last 28 years, as evidence of temperature warming occurring in the area. We found that the range of temperatures with positive per capita growth rates was much wider in prey than in predators, and that relative humidity contributed to explain the growth rate variation in predators, but not in prey. Predator and prey differences in thermal performance curves could explain why natural enemies did not respond numerically to the pest when environmental conditions were harsh.     

Aerial dispersal of phytoseiid mites (Acari: Phytoseiidae): estimating falling speed and dispersal distance of adult females

Aerial dispersal is important to immigration and redistribution of phytoseiid mites that often can provide biological control of spider mite pests. Falling speed of a mite and wind largely determine dispersal distance of such a passively blown organism. A diffusion model of wind-blown phytoseiids could provide insight into their dispersal. To this end, we measured body weights and falling speeds of adult females of 13 phytoseiid and one tetranychid mite species. These data were then incorporated into seed dispersal models (Greene and Johnson, Okubo and Levin) and results were compared to mite dispersal distances in wind tunnel, greenhouse and field. Weights of phytoseiid species ranged from 5.25 to 2l.7 μg; starved mites weighed less than fed mites. Geometric diameters ( d _g ) of idiosomas were correlated to weights. Falling speeds for phytoseiids were 0.39–0.73 m/s, and less than for T. urticae (0.79 m/s) in still air. In some species, active mites had slower falling speeds than inactive (anesthetized) mites indicating that behavior may influence falling. Starved mites had significantly slower falling speeds than fed mites and dispersed farther. Equation-based estimates of falling speed were close to measured ones (2–8% deviation) for some species. There were significant relationships between falling speed and body weight and morphological traits. Greene and Johnson's seed dispersal model provided better fits to dispersal of mites in the wind tunnel, greenhouse and field studies than Okubo and Levin's model. Limits of models in describing mite dispersal distance and applications to IPM are discussed.     

Factors affecting density-independent survival of an island population of tsetse flies in Zimbabwe

Analysis is presented of the factors affecting survival probability in populations of tsetse flies Glossina morsitans morsitans Westwood and G. pallidipes Austen (Diptera: Glossinidae) on Antelope Island, Lake Kariba, Zimbabwe. For mature male and female adult G. m. morsitans mean temperature (T _bar) accounted for 70 and 50%, respectively, of the variance in mark-recapture estimates of survival when the flies were not subjected to trapping. Saturation deficit (SD) only accounted for 36 and 33%, respectively. Maximum temperature (T_max) and SD accounted for 36–42% of the variance in male and female G. pallidipes. For the corresponding Moran curve estimates of the survival over all developmental stages, SD lagged by three weeks accounted for 61 and 41% of the variance for male and female G. m. morsitans, respectively, and 64 and 56% for G. pallidipes. The corresponding figures for plots against T_max were 44, 23, 23, and 21%, respectively. The same patterns were seen in the whole data set once allowance was made for the effect of trapping on survival and for an effect of season, correlated with an index of photosynthetic activity. For male G. m. morsitans there was a significant effect of saturation deficit, but not temperature, on immature survival. Decreased adult survival at high temperatures results from the need to feed more frequently and hence to take more risks per unit time. High saturation deficits result directly in reduced emergence of healthy flies from pupae.     

Living at the threshold: Where does the neotropical phytoseiid mite Typhlodromalus aripo survive the dry season?

The establishment of the neotropical predatory mite Typhlodromalus aripo in sub-Saharan Africa has resulted in broadly successful biological control of the cassava green mite Mononychellus tanajoa throughout the cassava belt of Africa. In some mid-altitude areas and drier lowland savannahs of sub-Saharan Africa, which are characterized by cool or hot long (≥5 months) dry seasons, the predator disappears from its habitat in the cassava apex during the dry season and reappears after the onset of rains. It is not known, however, where the predator remains during this time period. In this study, we conducted a field enclosure experiment of cassava plants with the objectives to determine if (a) T. aripo survives at very low densities in the apex, if (b) it survives in the soil or leaf litter below the cassava plant, and if (c) it recolonizes the cassava plant from the surrounding vegetation. Towards the end of the dry season, when the predators had disappeared from all cassava plants included in the experiment, five treatments were applied: (1) plants without enclosure; (2) plants with enclosure; (3) plants with enclosure, apices removed; (4) plants with enclosure, glue barrier around stem; and (5) plants kept free of T. aripo, without enclosure. Predator (re)appearance on cassava apices was monitored non-destructively at weekly intervals and was expressed as the proportion of plants with at least one apex with T. aripo per total number of plants of the treatment. The predators reappeared first on the plants of the treatments (1), (2), and (4). With a time lag of 7–8 weeks, the predators appeared also on the plants of the treatments (3) and (5). The time pattern of the predator’s (re)appearance in the cassava apex of the different treatments suggests that (a) T. aripo survives the dry season in very low densities in the cassava apex; this result is supported by an assessment of the efficiency of non-destructive visual in-field apex inspections which proved that about 10% of the cassava apices that had T. aripo were not recognized as such; (b) T. aripo does not survive in the soil or leaf litter, but we did document cases in a screenhouse experiment, where few individuals migrated down to the ground and walked over exposed soil until they reached the apex bouquet traps; additionally, microclimate measurements in various cassava plant strata proved that the cassava apex and the cassava stem base are the locations with the highest relative humidity during the dry season—which makes the stem base a potentially interesting refuge; (c) T. aripo does not survive in the surrounding vegetation, which is supported by a vegetation survey, where T. aripo was not found on any other plant species than cassava.     

The adaptive value of hatching towards the end of the night: lessons from eggs of the haematophagous bug Rhodnius prolixus

Abstract. The daily hatching rhythm of Rhodnius prolixus eggs is established under an LD 12 : 12 h photoperiod. The endogenous nature of this rhythm is demonstrated under continuous darkness. Hatching takes place during the last half of the night, when the maximum environmental relative humidity (RH) and minimum temperature (i.e. the combination that yields the lower water vapour saturation deficits) occur in wild habitats. This temporal window of approximately 7 h recurs at 24‐h intervals, producing a hatching rhythm in the population. The effects of the RH upon egg‐hatching are analysed. In agreement with previous studies, hatching success is strongly affected by environmental RH. Although 88% of eggs hatch at 75% RH, only 4% and 10% hatch at 0% or 100% RH, respectively. These results support the hypothesis that temporal synchronization is related to the avoidance of low environmental RHs, high environmental temperatures, or high water vapour saturation deficit during hatching, thus minimizing their deleterious effects. Given that eggs cannot choose optimum microclimatic conditions, selective pressures appear to have originated from an adaptive temporal rather than spatial hygropreference.     

Species diversity and seasonal dynamics of Acari on abandoned apple trees in southern Ontario,Canada

Foliage-inhabiting mites and associated insects were observed over a 3-year period on abandoned apple trees at two sites in southern Ontario. This study included species diversity and seasonal dynamics as well as the total habitat size and its seasonal fluctuations. Due to heavy feeding on the leaves in the early season by the fall cankerworm, at one observation site, the habitat available for foliage-inhabiting mites was regulated so that both the total number and the area of leaves on the tree did not increase significantly until early July. However, the influence of this on the population of the dominant mite species was minimal. The mite community was relatively stable over time. Four phytophagous species, representing a basic trophic level, were fed upon by a group of predacious mites. The phytophagous mites consisted of two rust-mite species (Diptacus gigantorhynchus sp. complex andEriophyes pyri sp. complex) and two tetranychid species (Bryobia rubrioculus andEotetranychus uncatus). On the basis of consistency in seasonal abundance,D. gigantorhynchus was the only prey species which could support the early-season population increase of the predacious mites. Ten species of predacious mites were recorded on the apple trees, nine phytoseiids and one stigmaeid; two separate species of these were common at each of two observation sites:Typhlodromus pomi and the stigmaeidZetzellia mali at one site; and two phytoseiid species,Phytoseius macropilis andAmblyseius finlandicus, at the other. There seemed to be a rather simple and stable prey/predator system at each site throughout the three seasons. Species dominance on these apple trees was investigated for the phytoseiids, especially from the perspective of the issue of one-species dominance.     

Survival of six species of African ticks in relation to saturation deficits

The survival of unfed males and females of six species of African ticks was monitored at five different saturation deficits at constant temperature (25°C). The survivorship curves for each species comprised a pre-mortality period, prior to when ticks started to die and a mortality period corresponding to a rapid increase in the mortality rate. Longevity was defined as pre-mortality plus mortality. A negative correlation between the longevity of the ticks and the saturation deficits was found with ticks surviving longer at lower deficits. The survival of males and females was similar. At low saturation deficits (2–4 mmHg) Amblyomma hebraeum survived the longest periods (74 weeks). Some correlation was found between the tick survival under dehydrating conditions and habitat associations. Rhipicephalus appendiculatus and Haemaphysalis leachii, the most mesic in distribution, had the shortest longevity (21 and 13 weeks, respectively) at high saturation deficits (7–21 mmHg). Hyalomma marginatum rufipes, the most xerophilic in distribution, had the longest survival (39.3±10.5 weeks) at high saturation deficits. Other factors apart from the adult survival should be taken into account when accounting for the tick distribution, in particular the tolerance of earlier developmental stages to desiccation.     

Genetics of Cortisone-Induced Cleft Palate in the Mouse—embryonic and Maternal Effects

Differences between mouse strains in frequency of embryonic, cortisone-induced cleft palate were examined. Probit analysis demonstrated a family of linear and parallel dose-response curves for different inbred and hybrid embryos. Since the differences between genotypes were not in the slopes of the response curves but rather in their location, it is proposed that the median effective dose (ED₅₀) of cortisone required to induce cleft palate (or the tolerance) provides a more appropriate definition of the response trait and its difference than a frequency statement. The tolerance of C57BL/6J is dominant to that of A/J. A maternal effect of A/J relative to C57BL/6J dams caused a two-fold reduction in the embryonic tolerance of cortisone. Cortisone-induced cleft palate and mortality were separate response traits.—In these and previous studies on cortisone- and other glucocorticoid-induced cleft palate in the mouse, the nature of the cleft-palate-response curve appeared to be the same for all glucocorticoids, and within-strain differences in tolerance could be used as measures of potency or bioassays for a particular effect of the glucocorticoids.     

Effect of the width of the herbicide strip on mite dynamics in apple orchards

Herbicide strips are used in apple orchards to promote tree growth and survival, to increase yield and to reduce the risk of rodent damage to tree bark. However, herbicide strips, particularly wider ones, may cause problems including soil erosion, reduced organic matter, leaching of nitrates into ground water and increased incidence of plant diseases and pests, including two-spotted spider mites, Tetranychus urticae Koch. In this 2 year study we monitored mite dynamics in apple trees and used sticky bands on tree trunks to determine rates of T. urticae immigration into Nova Spy apple trees in plots with wide (2 m) or narrow (0.5 m) herbicide strips. Use of wider herbicide strips promoted two risk factors that could trigger outbreaks of tetranychid mites. First, concentrations of leaf N in apple trees were higher and those of P and K were lower with the wide strips. Such changes in nutritional quality of leaves would increase the potential for more rapid population growth of T. urticae, and to a lesser extent, the European red mite, Panonychus ulmi (Koch). Second, there were higher rates of T. urticae immigration from the ground cover vegetation into the trees. In 2006, and for most of 2007, densities of T. urticae were higher with wide herbicide strips, whereas densities of P. ulmi were not enhanced. However, by late August to early September in 2007, densities of both tetranychids were lower with wide herbicide strips. This is because both risk factors were counterbalanced, and eventually negated, by the enhanced action of phytoseiid predators, mostly Typhlodromus pyri Scheuten. From July through September 2006, ratios of phytoseiids to tetranychids were always several-fold lower with wide herbicide strips but in 2007, from mid-July onwards, predator–prey ratios were usually several-fold higher with wide strips. However, this numerical response of phytoseiids to prey density can only occur where the pesticide program in orchards is not too harsh on phytoseiids. Hence the impact of width of herbicide strip is contingent on the composition and size of the phytoseiid complex and the impact of pesticides on predation.     

Prey-related odor preference of the predatory mites Typhlodromalus manihoti and Typhlodromalus aripo (Acari: Phytoseiidae)

Typhlodromalus manihoti and Typhlodromalus aripo are exotic predators of the cassava green mite Mononychellus tanajoa in Africa. In an earlier paper, we showed that the two predators were attracted to odors from M. tanajoa-infested cassava leaves. In addition to the key prey species, M. tanajoa, two alternative prey mite species, Oligonychus ossypii and Tetranychus urticae also occur in the cassava agroecosystem. Here, we used a Y-tube olfactometer to determine the attraction of the predators to odors from O. gossypii- or T. urticae-infested cassava leaves and their prey-related odor preference. T. aripo but not T. manihoti was slightly attracted to odors from O. gossypii-infested leaves. Both predator species showed a stronger response to odors from cassava leaves infested by M. tanajoa over odors from cassava leaves infested by O. gossypii. Neither predator species was attracted to odors from T. urticae-infested leaves and the predators preferred the odors from M. tanajoa-infested leaves over those from T. urticae-infested leaves. When O. gossypii was present together with M. tanajoa on the same leaves or on different sets of leaves offered together as an odor source the two predators were attracted. In contrast, after mixing non-attractive odors from T. urticae-infested leaves with attractive odors from M. tanajoa-infested leaves, neither T. aripo nor T. manihoti was attracted. Ecological advantages and disadvantages of the predators’ behavior and possible implications for biological control of M. tanajoa are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effects of prestarvation diet on starvation tolerance of the predatory mite Neoseiulus californicus (Acari: Phytoseiidae)

Understanding the factors affecting stress tolerance in phytoseiid mites is critical for their integration into biological control programs. In the present study, the effects of diet (varying in prey species, physiological status and phenotype) are examined on the future starvation tolerance of the predatory mite Neoseiulus californicus McGregor (Acari: Phytoseiidae). The predators are fed from egg to adulthood on diapausing or nondiapausing Tetranychus urticae, Tetranychus kanzawai (wild and albino strains) or the nondiapausing species Panonychus citri (wild and albino strains). Thereafter, 3‐day‐old mated adult females are held without food at 25 ± 1 °C and a relative humidity of 98 ± 2%. The survival of these starved females is observed daily until all females have died. The survival curves and mean survival times of N. californicus are found to vary among prey types and are significantly longer when the predator is fed with diapausing prey. This enhanced survival is consistent with high concentrations of glycogen and triacylglyceride in the body of the predator at the onset of starvation. The predators fed nondiapausing prey have shorter survival times, and the glycogen and triacylglyceride contents in their bodies are low or undetectable. The protein contents of the predator's body are similar after consuming different prey types, except for a high concentration when fed the albino strain of P. citri. Protein content is unlikely to play a direct role in starvation tolerance, although it may affect the response to varying glycogen and triacylglyceride levels. These findings indicate that nutritional value of prey has a strong impact on the starvation tolerance of N. californicus.