Pollens of 19 plant species were used as diets in a laboratory study of the life history of Euseius aizawai (Ehara & Bhandhufalck) (Acari: Phytoseiidae). The mite ingested pollens of all tested plant species and produced eggs. On eight of them the predator completed all developmental stages, with a life cycle of 5.47±0.88 to 6.95±1.61 days. The longest life-span of adults was observed when feeding on pollens of Coriaria sinica, Zea mays, mixed species and Punica granatum. Larvae had a higher survival rate (70% to 82%) when reared with the pollens of Luffa cylindrica, Trachycarpus excelsa, punica granatum and Betula platyphylla. Longevity of adult females was longer with pollens of Coriaria sinica, Zea mays and mixed species. The oviposition period was from 15.05±4.05 to 25.55±15.25 days, the longest when feeding on pollens of Zea mays, mixed species and Coriaria sinica. Fecundity ranged from 23.50±13.50 to 54.50±19.50 per female and the highest was on pollens of mixed species, Luffa cylindrica and Punica granatum. The sex ratio (proportion of females) was highest when feeding on pollen of Luffa cylindrica. The most suitable range of humidities for egg hatching was 70% to 90%. The net reproduction rate of increase (R0) was the highest on the pollen of Luffa.cylindrica, and the intrinsic rate of natural increase (rm) and finite rate of increase (λ) were the highest on the pollen of Trachycarpus excelsa.
Introduction
The citrus red mite, Panonychus citri (McGregor), is a major pest of citrus in Guizhou Province, southwest China. Farmers have to spray more than eight times each year in order to control this mite, especially during the fruiting season, but the efficiency of chemical control is low due to the development of resistance to acaricides. In addition, the cost of labour for acaricide application is high. We therefore feel that it is necessary to search for a predator that is easy and cheap to produce for augmentative biocontrol.
Several predators of the family Phytoseiidae are known to attack the citrus red mite (Gerson 2003). In central Guizhou, Euseius aizawai (Ehara & Bhandhufalck) is an important natural enemy of P. citri in citrus orchards. This mite had been well conserved for controlling P. ctiri in citrus orchards in Guiyang, Guizhou, but little is known about its biology. In order to improve the use of this predator, we conducted observations and experiments with different plant pollens in the laboratory, and investigated the possibility for rearing this mite on the pollen of various plants for the augmentative control of the citrus red mite in citrus orchards.
This paper reports the biology and population life table parameters of E. aizawai in the laboratory, to provide a better basis for evaluating it as a predator of citrus red mite in citrus orchards in Guizhou, China.
Material and methods
Food sources
Panonychus citri was collected from a citrus orchard in Guizhou, China and was used for an experiment on prey choice in the laboratory.
Pollens were collected from the 19 plant species listed below:
Family
Species
1. Pinaceae
Pinus massoniana
2. Solanaceae
Datura stramonium
3. Graminaceae
Zea mays
4. Fagaceae
Quercus glauca
5. Punicaceae
Punica granatum
6. Malvaceae
Althaea rosea
7. Betulaceae
Betula platyphylla
8. Palmaceae
Trachycarpus excelsa
9. Cucurbitaceae
Cucurbita moschata
10. Cucurbitaceae
Luffa cylindrica
11. Coriariaceae
Coriaria sinica
12. Chenopodiaceae
Kochia scoparia
13. Caprifoliaceae
Lonicera japonica
14. Compositae
Chrysanthemum coronarium
15. Cruciferae
Brassica campestris
16. Leguminosae
Caesalpinia sepiaria
17. Oleaceae
Ligustrum lucidum
18. Papilionaceae
Pisum sativum
19. Euphorbiaceae
Ricinus communis
All pollens were collected during the flowering season and stored in a refrigerator at 0–5°C for use after being dried in sunlight.
Laboratory conditions and methods
The study was conducted in a rearing room at 26±0.5°C, 75%RH and photoperiod of 12L: 12D. The rearing unit for the laboratory culture was a piece of round sponge fully soaked with water in a petri dish (12 cm diameter). The sponge was covered by a black cloth which was then covered by a plastic membrane. A piece of paper was placed in the centre of the membrane for holding pollen grains, which were replaced every other day. Cotton fibres were placed on the membrane for mite oviposition.
Observations on predator feeding citrus red mites
Feeding on prey: Each treatment consisted of 30 eggs, larvae, nymphs and adults of P. citri, plus one adult female of E. aizawai, placed together on a membrane in a 12 cm diameter petri dish. Three replicaties were observed and the number of P. citri consumed were recorded 24 hours after the start of the experiment.
Feeding on pollens: Rearing units were similar to those used in the former test, except for the size of the petri dishes (9 cm in diameter). Pollens from the 19 plant species mentioned above were used for each treatment, which consisted of 3 replicaties. Per treatment, 30 predator nymphs 1 to 2 days old were used, the control being a dish only with pure water, without pollens. Each treatment was examined on the seventh, tenth and fourteenth day, and the observations ended when all mites in the controls died. Plant pollens that had increased the number of A .aizawai by more than 36% by the tenth day were chosen for further experiments.
Developmental rate, survival rate, female longevity and feundity of E.aizawai
Pollen grains of seven plant species and 1 of mixed species (of equal parts from Luffa cylindrical, Coriaria sinica, Betula platyphylla, Quercus glauca and Pinus massoniana) were used as foods. The rearing unit was in a 12 cm diameter petri dish that had 5 pieces of 3 cm diameter membrane. Each type of pollen, plus a predator larva that had hatched within 3 hours was added to every small cell. Each treatment was in 30 replicates which were observed until all mites died. Developmental durations and survival rates were observed every 6 hours under a microscope and recorded. When they became adults the females were paired with males, and as they began to lay eggs, cotton fibres were placed onto the membrance. Observations were carried out at 9:00 every day and the recorded eggs were transferred to be reared in another dish. Hatching rate was observed for these eggs, and their sex ratio was determined when all mites became adults.
The influence of humidity on hatching
Different saturated salt solutions were made up (Winston and Bates, 1960), in order to create seven different relative humidities (RH) in sealed desiccators. The temperature was maintained at 26±0.5°C. Eggs laid within 3 hours were transferred onto a slide and then put into the desiccators for examining the rate of hatching. At least 51 eggs were examined per treatment.
Life table analysis
Population parameters: A life table using time-specific survival rates (lx) and fecundity (mx ) for each 24 hours period was constructed for calculating the following life table parameters (Xia 1998; Liu et al. 2004; Ji et al. 2005):
Net reproduction rate of increase: Ro=Σlxmx
Mean generation time (in days): T=Σlxmx-x/Ro
Intrinsic rate of increase: rm =InRo/T
Finite rate of increase: λ=erm
Population doubling time (in days): p.d.t=ln2/ rm
Population trend index (I): A life table was constructed for the experimental population on the basis of the observations of the survival of all immature, egg / female, and the sex ratio. The population trend indices were also calculated. .
Results
General observations on feeding habits
Feeding on citrus red mites: E. aizawai attacked all stages of the citrus red mite. When given a mixture of 30 individuals of each stage, each predator female consumed 5.8, 7.7, 4.6 and 2.5 eggs, larvae, nymphs and adults of prey, respectively, within 24 hours.
Feeding on plant pollen: E. aizawai fed on pollens from all 19 plants that were tested. The mites developed and oviposited normally. Thes plant pollens could thus be used as food for mass-rearing the predatory mites, either in the laboratory or in the field.
Developmental duration of E. aizawai
The development of E. aizawai consists of egg, larva, protonymph, duetonymph and adults. Data on the duration of the various developmental stages are presented in Table 1. Euseius aizawai completed its development on 7 plant pollens and the pollen mixture, but the duration of each developmental stage was different amongst the eight treatments. Durations of the egg stage were much longer than those of the larval, protonymphal and duetonymphal stages. It took 5.47±0.88 to 6.95±1.61 days for this species to complete a life cycle on the various pollens (Table 1) and the life cycle was shortest on the pollen mixture, which was the best for rearing this predator in the laboratory.
TABLE 1.
Duration of the developmental stages of Euseius aizawai on the various pollens.
Survival rate of A.aizawai
The survival rate of the various developmental stages differed among different pollen treatments. Survial rates of egg, protonymph and duetonymph were higher than that of the larval stage, the highest survival rate reaching only 82.00% on L. cylindrca. Overall survial rate was highest on the pollen mixture (Table 2).
TABLE 2.
Survival rates of each developmental stage of Euseius aizawai on the various pollens.
Longevity and fecundity of adult female of E. aizawai
Mating and oviposition: Mating accurs as soon as the male and female A.aizawai emerge as adults. The male holds the female below her idiosoma during pairing. The eggs produced by fertilized females gave rise to both male and female offspring. Females prefer to lay eggs on the fibres of cotton placed on plastic arenas in the laboratory.
Longevity and fecundity: Longevity and fecundity differed on the different pollens (Table 3). The longevity of adult females ranged from 18.08±6.22 to 41.09±22.44 days, the shortest being on pollen of B. platyphylla and the longest on pollen of C. sinica. The pre-oviposition period ranged from 1.02±0.27 to 1.49±0.22 days, the shortest being recorded on pollen of Z. mays (Table 3). The length of the oviposition perriod was the most important factor determining the reproductive ability; it was only 15.05±4.05 on pollen of B. platyphylla, and longest, 25.55±15.25 days, on pollen of Z. mays. The fecundities of females reared on mixed pollens and on the pollen of L. cylindrica were significantly higher than those on the other pollens, with a daily production rate of 3.00±2.00 and 3.50±2.50 eggs/female/day, and a sex ratio of 1.81:1 and 2.25:1, respectively. Mixed pollens and pollen of L. cylindrica were thus considered as the most suitable food resource for the maximal fecundity of E. aizawai. Adult females can lay a large number of eggs during the oviposition period, which makes it possible for the mite to rapidly develop a large population.
TABLE 3.
Longevity and fecundity of females of Euseius aizawai on the various pollens.
The effect of humidity on hatching
The egg hatching rates of E. aizawai under various RH levels are presented in Table 4. The hatching rate increased with the increase in relative humidity; when RH rates were 70%–90%, the hatching rate ranged from 91.43% to 100.00%. Therefore 70%–90% RH rates were considered as optimal for egg hatching. When the RH reached 100%, hatching rate decreased drastically. On the other hand, the hatching rate was 26.76 % at relative humidity of 40%.
TABLE 4.
Effect of relative humidity on the egg hatching rate of Euseius aizawai feeding on L. cylindrica.
Population life table parameters of E. aizawai
Population parameters were calculated and are listed in Table 5, which indicates that different plant pollens exerted great influence on the population parameters of E. aizawai. Both the net reproductive rate (R0) and the intrinsic rate of natural increase (rm) were, respectively, more than one and zero, which showed that these pollens can promote the population increase of this mite. But the net reproductive rates on mixed pollens and pollens of L. cylindrica, T. excelsa and P. granatum were greater than on other pollens, which agreed with results of previous analyses. The mean longevity of a generation on mixed pollens and pollen of C. sinica, Z .mays, L. cylindrica and P. granatum was longer than those of the other pollens. Both the intrinsic rate of natural increase (rm ) and the finite rate of increase (λ) on pollens of T. excelsa, P. platyphylla, L. cylindrica, C. sinica, P. granatum and mixed pollens were higher than on those of the other pollens, and population doubling time (p.d.t) with these plant pollens was shorter.
TABLE 5.
Population life table parameters of Euseius aizawai on the various pollens.
Discussion
Many species of Euseius are generalist predators that perform well on pollens (Type VI predatory according to McMurtry & Croft 1997). The use of pollen as a diet of these mites has greatly facilitated their mass-rearing. In Guizhou, Euseius nicholsi (Ehara & Lee), which was a biocontrol agent of Eotetranychus kankitus Ehara, which was successfully reared on pollen of several plant species (Zhi et al., 1998).
Euseius aizawai is a natural enemy preying on pest mites on leaves of citrus and can control the citrus red mite infesting citrus orchards in central Guizhou. In this study, we showed that this predatory mite fed on pollens of all 19 tested plant species. Thus, these plant pollens may be used as food for mass-rearing the predatory mite, either in the laboratory or in the field. Euseius aizawai reared on plant pollens in the laboratory may be released for controlling citrus red mites in citrus orchards. The flowering plants growing by the side of citrus orchards should be protected and may provide pollens for predatory mites when the spider mite numbers are low in the field.
Although E. aizawai reproduced on all pollens, its developmental duration, survival rate, life cycle, longevity and feundity differed with different plant pollens. The intrinsic of natural increase (rm) on pollen of T. excelsa was the highest indicating that the pollen of T. excelsa is the best for laboratory mass-rearing of this predatory mite.
Acknwledgements
We should like to express our sincere appreciation to Professor Lairong Liang who identified the predator mite species. Financial support for this project was provided by Natural Sciences Foundation of Guizhou Province, China.
