Bullerin OEPP/EPPO Bulletin 21, 719-722 (1991)

Influence of relative humidity on development and egg-laying in Panonychus citri under controlled conditions

by F. BEITIA and A. GARRIDO Instituto Valenciano de Investigaciones Agrarias, Apartado Oficial,

461 13-Moncada/Valencia (Spain)

The influence of relative humidity on several aspects of the biology of citrus red mite, Punonychus citri, has been analysed. Five constant RH values (45, 55, 65, 75, 85%), with a fixed constant temperature of 25*2"C, have been studied. An RH of 65% was optimal for mite development and egg-laying potential, providing a maximum rm value of 0.145.

introduction

Populations of Panonychus citri in citrus crops are influenced by several biotic and abiotic factors, which determine population fluctuations during the year. Climatic conditions, especially temperature and relative humidity (RH), have been studied in the laboratory to provide a solid basis to explain mite behaviour in the field.

It seems that RH has a direct effect on egg-laying (Boudreaux, 1958); however, most laboratory experiments with P. citri have been conducted with strictly controlled temperature, but with humidity values fluctuating within a wide range (Munger, 1963; Saito, 1979; Garrido et al., 1984; Jones & Morse, 1984). In our work, the influence of R H on immature and adult development, and female egg-laying, in P. citri has been analysed.

Materials and methods

Mites from a laboratory mass rearing were used. The climatic conditions in the rearing room were 25*2"C, 60f 10% R H and 16:8 (1ight:dark). Small plants of sour orange (Citrus auranrium) were grown under the same conditions as hosts for mite development. These small plants, with the first two leaves well developed, were individually placed in 100-ml plastic boxes containing a nutrient solution for their maintenance (Moutous & Fos, 1973). This method has already been used with satisfactory results (Garrido et al., 1984; Beitia & Garrido, 1985). For the experiments, a UNITEMP climatic chamber (for simulation of environmental conditions) was used; this controls temperature (+ 0.YC) and humidity (f 2%) and provides a constant photoperiod: 16: 8 (1ight:dark).

Experimental procedure

Relative humidities tested were: 45, 55, 65, 75, 85%, always at 25°C constant temperature. To study the development of the immature stages, 10 deutonymph females per small plant

were used. When they were in the teleiochrysalis stage, 10 males were introduced to ensure fertilization. Only 10 eggs per plant, laid in an 8-h period, were considered for study. Ten small plants were used, each considered as a replicate. The two mite nymph stages (protonymph and deutonymph) were considered as the nymphal stage.

To study egg-laying, 2 females per plant were employed, with 10 replicates. At the

719

720 F. Beitiu & A . Garrido

teleiochrysalis stage, 2 males were introduced for fertilization. The biological aspects monitored were: pre-laying, egg-laying and post-laying periods, and number of eggs per female.

In both experiments, three daily assessments were made. To calculate intrinsic rate of development (r,,,), Birch’s equation was used (Birch, 1948).

Statistical treatment

An analysis of variance was performed for all the values, with an LSD test to detect differences between means.

Results

The results of the development experiment are shown in Table 1. Significant differences were found between all relative humidities.

For egg-laying, significant differences were also found, basically in the number of eggs for each RH value (Table 2). The results of the rm calculation were:

45Yo4.040 55Yo4.092 65Yu4.145 75%,-0.127 85Yo4.090.

Table 1. Duration (days) of development stages in Panonychus citri at several constant relative humidities Duree en j des stades de developpement de P. cirri en fonction de I’humiditt relative

RH (‘!A) Egg Larva Nymph Adult

45 6.45k0.08a 2 . 2 3 f 0 . 1 2 ~ 4.31f0.17a 19.92f1.78a 55 6.32f0.13a 2.12*0.10a 4.26f0.14a 14.60f1.61 b 65 6.00+0.15 b 1.65f0.12 b 3.59k0.11 b 13.75k1.42b 75 6.45k0.14a 1.75k0.09 b 4.24*0.11 a 19.80*1.08 a 85 6.48k0.18a 1.84f0.20b 4.35fO.17a 21.05+1.31 a

In each column, all values followed by a different letter have significant differences (P<0.05) (LSD test). All values are the average of 10 replicates (xkSE).

Table 2. Duration (days) ofegg-laying period and number of eggs laid in Panonychus cifri, at several constant relative humidities Duree en j du stade d’oviposition et ponte chez P. cirri en fonction de I’humiditi. relative

Period RH (!A) Eggs/female Preovip. Oviposition Postovip.

45 53.60f1.80a 1.15f0.04a 17.17fO.lOa 1.60k0.07a 55 21.42*1.71 b 1.67k0.04b 9.58k0.17 b 3.35k0.05 b 65 25.00f1.61 c 1.58f0.07 b 10.24k0.12 b 1.93+0.04c 75 25 .70k1.42~ 1.82+0.05c 14 .13 f0 .13~ 3.85k0.04d 85 20.70k1.75 b 2.10k0.04d 16.65+0.ll a 2.55$-0.05e

Key as for Table I ,

Influence of RH on Panonychus citri 7 2 1

A significant regression between rm and RH was proved, with 65% optimal value for the development of mite populations.

Discussion

From these results, we can conclude that RH has little influence on the development of the immature stages of Panonychus citri, but it has a high incidence on adult life-span. Moreover, RH determines absolutely the reproductive capacity of females of P . citri. In general, high humidities have a negative effect on egg-laying (Boudreaux, 1958); we found that 75 and 85% RH values led to a decrease in egg-laying, in relation to 65%, which is the optimal RH. At 45% an increase in fecundity is detected, which is in agreement with Shinkaji (1959). Differences with the results of Saito (1979) on adult life-span and female fecundity can be attributed to the fact that, in Saito’s experiments, adults developed on the upper leaf side, while our work was carried out on both upper and lower leaf sides, as occurs in nature (Shinkaji, 1962).

On the other hand, Yasuda (1982) found faster development values and higher fecundity in his experiments at constant temperature, but with fluctuating RH (50-80%). These differences could also be induced by using different strains of the same species (Nickel, 1960; Saito, 1979).

Finally, the variation in rm values in relation to humidity must be considered. To determine rm values, the two most important parameters are development time and fecundity (Snell, 1978); therefore, the influence of RH is clearly understandable. A significant regression was found between rm values and RH. A 65% value is optimal for the development of P. citri, due to the high fecundity rate. A 45% RH provides the higher fecundity rate but it produces a very high mortality in all immature stages and determines a lower rm rate (0.040).

Acknowledgements

The authors would like to acknowledge A. Borras for her assistance in the translation of the text into English.

Influence de I’humidite relative sur le developpement et la ponte chez Panonychus citri en conditions contrdlees

L’analyse a tte faite de I’influence de I’humiditk relative sur divers aspects de la biologie de I’acarien des agrumes, Panonychus citri. L‘etude a porte sur cinq valeurs d’HR (45, 55, 65, 75, 85%), a une temperature constante de 25k2”C. Le developpement et la ponte potentiels de I’acarien etaient optimaux a une HR de 65%, avec une valeur rm maximale de 0,145.

Bnnfln~eoTnocnTenbnoR~namnoCT~napa3~nr~en C l l O C O 6 ~ O C T b O T K J l ~ b l B a H H F l R H ~ Panonychus citri B KonTponnpyeMbix ycnoenflx

BnmHxe OTHOCMTenbHOfi BJIaXHOCTPl H a HeKOTOpble aCneKTbl6~onorm KpaCHOrO qklTpyCOBOr0 KneqMKa Panonychus citri 6b1no nonueprHyTo aaanmy. nmb nocTomHbIx ee s ~ a s e ~ ~ ~ (45%, 55%, 65%, 75%, S5%), npki s a ~ a ~ ~ o f i nocTomHofi TeMnepaType 25 *TC nonseprnncb T~aTeJIbHOMy H3Y’leHPIK). OTHOCLITeJIbHaR BnaXHOCTb B 65% IIBMJIaCb OnTklManbHOfi AnR Pa3BIITklR KJIeuIMKa &I IIOTeHUMaJIa KJlaaKM I I M q npcl MaKCPiManbHOM 3HaqeHRM rm U 0,145.

References

BEITIA, F. & GARRIDO, A. (1985) [Study of the sterilization of Cales noucki How. (Hym.:Aphelinidae) by using pesticides.] Anales del lnstituto Nucional de Znuestiguciones Agrarius, Agricolu 28(3), 147-1 55 (in Spanish).

722 F. Beitia & A . Garrido

BIRCH, L.C. (1948) The intrinsic rate of natural increase of an insect population. Journal of Animal Ecology

BOUDREAUX, H.B. (1958) The effect of relative humidity on egg-laying, hatching and survival in various spider mites. Journal of Insect Physiology 2, 65-12.

GARRIDO, A., DEL BUSTO, T. & TARANCON, J. (1984) [Bioecology and control ofPanonychu.7 citri(McGregor) (Acarina: Tetranychidae)]. Levante Agricola nos 249-250, 26-43 (in Spanish),

JONES, V.P. & MORSE, J.G. (1984) A synthesis of temperature-dependent developmental studies with the citrus red mite, Panonychus citri (McGregor) (Acari: Tetranychidae). Florida Entomologist 67,213-221.

MOUTOUS, G. & Fos, A. (1973) Essais de rhizogenkse chez la feuille de vigne isoke. Revue de Zoologie Agricole et de Pathologie Vbgttale 72, 21-32.

MUNGER, F. (1963) Factors affecting growth and multiplication of the citrus red mite, Panonychus citri. Annals of the Entomological Society of America 56, 861-874.

NICKEL, J.L. (1960) Temperature and humidity relationships of Tetranychus desertorurn Banks with special reference to distribution. Hilgurdia 30, 41-100.

SAITO, Y. (1979) Comparative studies on life histories of three species of spider mites (Acarina: Tetranychidae). Applied Enromology and Zoology 14, 83-94.

SHINKAJI, N. (1959) [Experimental studies on the influence of temperature and relative humidity upon development of citrus red mite Panonychus (Metatetranychus) citri (McGregor). 2. On the influence of temperature and relative humidity on the post-embryonal development, adult life span and egg-laying potency of the citrus red mite.] Bulletin of Horticultural Division Tokoi-Kinki Agricultural Experimental Sfation 5, 129-142 (in Japanese).

SHINKAJI, N. (1962) Some investigations on the behavior of citrus red mite (Punonychus citri McGregor). Bulletin ofHorticultura1 Research Station of Okitsu (Ser. b) 1, 192-205.

SNELL, T.W. (1978) Fecundity, developmental time and population growth rate. Oecologia 32, 119-125. YASUVA, M. (1982) [Influence of temperature on some of the life cycle parameters of the citrus red mite,

Panonychus citri (McGregor) (Acarina: Tetranychidae).] Japanese Journal of Applied Entomology and Zoology 26, 52-57 (in Japanese).

17, 15-26.