Life-cycle Parameters of Aphelenchus avenae. on Botrytis cinerea and Fusarium oxysporum

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1 Life-cycle Parameters of Aphelenchus avenae on Botrytis cinerea and Fusarium oxysporum Dong-Ro CHOI * and Nobuyoshi IsHIBASHI * * Fecundity of the fungivorous nematode, Aphelenchus avenae cinerea and Fusarium oxysporum at 25 Ž was compared., cultured on Botrytis A. avenae laid an average of 230 eggs on B. cinerea and 89 on F. oxysporum with oviposition periods of 25.8 and 22.3 days, respectively. Peak oviposition generally occurred within 4-6 days with B. cinerea and days with F. oxysporum after maturation. The average life span of females was 32.3 days with B. cinerea and 29.6 days with F. oxysporum. Larval mortality was 15 and 20 percent with B. cinerea and F. oxysporum, respectively. Survivorship of females on both fungi declined in a similar manner. Intrinsic rate of natural increase (rm) of A. avenae was estimated to be 0.79 and 0.50 with B. cinerea and F. oxysporum, respectively. Jpn. J. Nematol. 21: 1-5 (1991). Key words: Aphelenchus avenae, Botrytis cinerea, fecundity, fungivorous nematode intrinsic rate of natural increase., The fungivorous nematode, Aphelenchus avenae, is a cosmopolitan species and can be reared in vitro on a broad range of fungi. At least 92 species from 50 fungus genera have been recorded as food sources of A. avenae9). TOWNSHEND8) found that A. avenae fed and reproduced on 54 out of 59 fungus species, however, its reproductive rate varied according to fungus species. CHOI and ISHIBASHI3) reported that multiplication varied among five isolates of A. avenae collected in Kyushu, Japan. Based on the feeding habit, many investigations have been made on A. avenae as a biological control agent for soil-borne disease1, 2, 3). For use as a biological control agent, mass production must be established. Basic parameters for reproduction are required for large scale rearing. The life cycle of this nematode has been intensively investigated4), however, no reproduction parameters are available at present. This paper reports the reproduction parameters of A. avenae on fungal mats of Botrytis cinerea and Fusarium oxysporum. MATERIALS AND METHODS Aphelenchus avenae from an upland rice field at Kumamoto was maintained at 25 Ž on Botrytis cinerea growing on potato dextrose agar (PDA). One gravid female was introduced into a drop of distilled water on a glass slide and a single egg was obtained after 2 hr. The newly hatched juvenile from this egg was placed on a fungal mat (7 mm d.) of B. cinerea or F. oxysporum f. sp. lagenariae growing on 1/5 strength PDA in an acryl ring (14 mm d., 5 mm h.) fixed to a glass slide. The ring was covered with a cover slip to prevent desiccation, and the nematode was incubated at 25 Ž in the dark with 35 replications. Observations were made 6 times a day from the 4th day after incubation until the commencement of egg-laying. The first egg-laying time was recorded and the fungal mat was renewed every 2 days after the start of egg-laying by transferring the adult female to a new mat. Old mats on which eggs were deposited were left for *Agricultural Sciences Institute, RDA, Suwon, Korea. * *Department of Applied Biological Sciences, Faculty of Agriculture, Saga University, Saga 840, Japan.

2 Vol.21 Japanese Journal of Nematology December, days at 25 Ž and the hatched juveniles were counted. The death of the adult was individually recorded to complete a data set on life expectancy and mortality for each host fungus. The reproductive potential of A. avenae on a given fungus was obtained by calculating the average number of offsprings produced per female entering the population (Ro; the net reproductive rate) and the instantaneous growth rate of the population (r; the intrinsic rate of increase), according to the LOTKA equation10); ƒ 0 Lxmx exp(-rmx) = 1. R0 is calculated from life tables as the sum to the maximum age class reached in the population (t) of the number of female offspring produced per female during each time interval (mx) times the probability of surviving from birth to that age (lx); Ro=ƒ t0lxmx.r was estimated as the capacity for increase (r,), and calculated from Ro and the cohort generation time (Tc) Tc=(1/ Ro) ƒ t0 lxmx, where lx is age-specific survival and mx is age-specific fecundity. rc= (loge Ro) I Tc rc was used to give an initial estimation of rm from the LoTKA equation for the minimum developmental time from egg to egg production. RESULTS All eggs hatched within hours after eggs were deposited. The average generation time from egg to egg production was 6.5 days with B. cinerea and 7.3 days with F. oxysporum. The daily oviposition per female of A. avenae is shown in Fig. 1. The maximum reproductive period after maturation was 4-6 days with B. cinerea and days with F. oxysporum. In these periods, a female produced 15.5 eggs a day on average for B. cinerea and 5.7 eggs for F. oxysporum. Thereafter, the daily egg-production gradually declined until oviposition ceased. A female produced 8.9 eggs on B. cinerea and 4.0 eggs on F. oxysporum a day and their average oviposition periods were 25.8 and 22.3 days, respectively. The average number of eggs laid by a single female was and 88.7 eggs on B. cinerea and F. oxysporum, respectively (Fig. 2). Figure 3 shows the survivorship of A. avenae cultured on B. cinerea and on F. oxysporum. Egg mortality under these experimental conditions was not observed. Larval mortality was 15 % with B. cinerea and 20 % with F. oxysporum. Survivorship curves of adults showed a similar pattern for both fungi. Life- Fig. 1. Mean daily oviposition per female of Aphelenchus avenae (Kumamoto isolate) on the fungal mats of Botrytis cinerea and Fusarium oxysporum at 25 Ž.

3 Fig. 2. Cumulative egg-production of Aphelenchus avenae (Kumamoto isolate) per female on Botrytis cinerea and Fusarium oxysporum at 25 Ž. Fungi as food were renewed every 2 days. Fig. 3. Survivorship curves of Aphelenchus avenae (Kumamoto isolate) cultured on Botrytis cinerea and Fusarium oxysporum from maturation onward at 25 Ž.

4 Vol.21 Japanese Journal of Nematology December, 1991 cycle parameters of A. avenae on these fungi are summarized in Table 1. Life span of A. avenae was not significantly different between the two host fungi at the 5% level by the t-test. Lifecycle parameters were larger on B. cinerea than on F. oxysporum (p= 0.05), except for reproductive period. The results of these computations are given in Table 2. The relationship between fungus species and "rm" was significantly different; 0.79 for B. cinerea and 0.50 for F. oxysporum. It is theoretically possible that the population of A. avenae (Kumamoto isolate), with unlimited space and resources, could multiply from a single female to 140,000 on B. cinerea and 1,800 on F. oxysporum 15 days after maturation. DISCUSSION The present study attempted determining the effect of two food sources on the life-cycle parameters of a fungivorous nematode, Aphelenchus avenae. PILLAI and TAYLOR7) reported that host preference, host suitability and morphometrics of A. avenae considerably varied according to fungus species. Fecundities of Bursaphelenchus fungivorous5) and B. xylophilus6) were investigated by transferring the females to a new fungus culture at regular intervals until nematodes died. We took the same procedure for determining the life-cycle parameters. In the present experiment, the population parameters of A. avenae on fungal mats of Botrytis cinerea were more reproducible than parameters on Fusarium oxysporum. Botrytis cinerea was associated with lower larval mortality, shorter generation time, and higher fecundity. Nematodes were placed on the fungal mats so attraction would have little influence on reproduction. If by-products were affecting the nematode it would affect total life span and larval mortality but these are not significantly different. Suitability of the food source might also affect size of the nematode but this was not measured. A. avenae was cultured on B. cinerea prior to use in this experiment which may have created a bias in this experiment because the nematodes had been pre-selected to grow well on this fungus and not on F. oxysporum. Table 1. Life-cycle parameters of Aphelenchus avenae on fungal mats of Botrytis cinerea and Fusarium oxysporum at 25 Ž. *S indicates significant differences between two means by the t -test (p = 0.05). NS indicates no significant difference. Table 2. Population parameters of Aphelenchus avenae on Botrytis cinerea and Fusarium oxysporum. Ro: net reproductive rate. Tm: minimum generation time. rm: intrinsic rate of increase. Tc: cohort generation time. rc: capacity for increase.

5 LITERATURE CITED 1) BARKER, K. R. (1964) On the disease reduction and reproduction of the nematode Aphelenchus avenae on isolates of Rhizoctonia solani. Pl. Dis. Reptr. 48, ) BARNES, G. L., RUSSEL, C. C., FOSTER, W. D. & McNEW, R. W. (1981) Aphelenchus avenae, a potential biological control agent for root rot fungi. Pl. Dis. 65, ) CHOI, D. R., ISHIBASHI, N. & TANAKA, K. (1988) Possible integrated control of soil insect pests, soilborne disease, and plant nematodes by mixed application of fungivorous and entomogenous nematodes. Bull. Fac. Agr., Saga Univ., 65, ) FISHER, J. M. (1969) Investigation on fecundity of Aphelenchus avenae. Nematologica 15, ) FRANKLIN, M. T. & HOOPER, D. J. (1962) Bursaphelenchus fungivorous n. sp. (Nematoda: Aphelenchoidea) from rotting Gardenia buds infected with Botrytis cinerea Per. ex Fr. Nematologica 8, ) MAMIYA, Y. & FURUKAWA, M. (1977) Fecundity and reproductive rate of Bursaphelenchus lignicolus. Jpn. J. Nematol. 7, ) PILLAI, J. K. & TAYLOR, D. P. (1967) Influence of fungi on host preference, host suitability, and morphometrics of five mycophagous nematodes. Nematologica 13, ) TOWNSHEND, J. L. (1964) Fungus hosts of Aphelenchus avenae BASTIAN, 1865 and Bursaphelenchus fungivorous FRANKLIN & HOOPER, 1962 and their attractiveness to these nematode species. Can. J. Microbiol. 10, ) WALKER, G. E. (1984) Ecology of mycophagous nematode Aphelenchus avenae in wheat-field and pine-forest soils. Plant and Soil 78, ) WHARTON, D. A. (1986) A functional biology of nematodes. Croom Helm, London & Sydney, Accepted for publication : July 26, 1991.