Βιο-οικολογική μελέτη του αρπακτικού ακάρεως Euseius stipulates (Athias-Henriot) (Acari: Phytoseiidae) σε συνθήκες εργαστηρίου
Authors: Μπούρας Στυλιανός 
Published: August 2021

Euseius stipulatus (Athias-Henriot) is a predatory mite widespread in the Mediterranean region. It has been recorded in Algeria, Greece, Turkey, Italy, Spain and former Yugoslavia. In Greece, this predator has been recorded in all geographic regions with citrus orchards. It is considered to be important for the biological control of spider mites in citrus orchards. The present thesis deals with certain aspects of the biology and ecology of E. stipulatus, such as the effect of different pollen and temperatures on development, survival and reproduction of the predator, the mite's predation preference between conspecific and heterospecific eggs or larvae, the nutritional benefits of the predator when it resorts to cannibalism and intraguild predation, the females' ability to discriminate between conspecific and heterospecific eggs and between own and conspecific eggs during oviposition and lastly, the toxicity of four pesticides to E. stipulatus. All experiments were carried out in the Department of Agricultural Zoology & Entomology of the Agricultural University of Athens. Development, survival and reproduction of this phytoseiid mite feeding on seven commercially obtained pollen were studied under constant laboratory conditions (20 ± 1°C, RH 65 ± 5%, photoperiod 16L:8D h). Mites were kept individually at rearing units with ample quantity of almond (Prunus amygdalus Batch), apple (Malus domestica Borkh), apricot (Prunus armeniaca L.), cherry (Prunus avium L.), pear (Pyrus communis L.), plum (Prunus domestica L.) and walnut (Juglans regia L.) pollen as food source. Developmental time from egg to adult varied between the several pollen tested from 8.38 ± 0.08 to 9.58 ± 0.11 days for females and from 8.23 ± 0.12 and 9.07 ± 0.12 days for males. Female longevity varied from 11.53 ± 1.22 to 51.38 ± 2.45 days, while fecundity ranged from 22.84 ± 2.30 to 43.61 ± 3.78 eggs/female. The predator was unable to reproduce when feeding on walnut pollen. Data were submitted to life table analysis and values of the intrinsic rate of increase were derived, ranging from 0.079 to 0.146 (day⁻¹). The cumulative Weibull function that was used to describe the age specific survival of females produced excellent fits to the survival data. Results show that almond, plum, cherry and apricot pollen possess higher nutritional value for E. stipulatus than pear and apple pollen and thus may contribute in sustaining and increasing the predator population in field conditions. Walnut pollen can be utilized by the predator only to survive during short periods of time when principal or alternative food sources are scarce. Based on previous results, development, survival and reproduction of E. stipulatus feeding on almond pollen were studied at seven constant temperatures (15, 20, 25, 27.5, 30, 32 and 34 ± 1 °C) at laboratory conditions (RH 65 ± 5% and a photoperiod of 16L:8D h). Within the temperature range tested, developmental period from egg to adult varied from 5.53 ± 0.09 to 19.33 ± 0.19 and 5.37 ± 0.10 to 17.96 ± 0.25 days for females and males, respectively. Developmental times of the males were generally shorter than those of the females. The developmental data obtained, were used for the estimation of the thermal requirements of this predator by a linear and a nonlinear model. The lower developmental threshold of the egg to adult period was calculated by the linear function at 7.47 ± 1.95 and 7.24 ± 1.71 °C for females and males, respectively. Total number of degree-days needed for the completion of this period was 120.15 ± 13.83 and 118.69 ± 11.82 for females and males, respectively. The optimum and maximum temperature for development were estimated by the Logan function at 30.02 and 37.06 °C for females and 30.02 and 37.05 °C for males. Female longevity decreased from 73.94 ± 4.19 days at 15 °C to 8.61 ± 0.52 days at 34 °C. The highest number of eggs laid was 52.97 ± 5.15 at 30 °C and the lowest was 36.90 ± 2.47 at 15 °C. The two higher temperatures tested (32 and 34 °C) had an adverse effect on development and population increase as developmental times were elongated and no oviposition took place. The Weibull distribution and the cumulative Weibull function described daily oviposition and survival data very good. The highest value of the intrinsic rate of increase was recorded at 30 °C (0.245), while the lowest was recorded at 15 °C (0.073) (days⁻¹). Mean generation time ranged from 44.96 days at 15 °C to 15.00 at 30 °C. The predation preference of E. stipulatus between eggs or larvae of E. stipulatus and Iphiseius degenerans Berlese, the predator's development and reproduction when feeding on conspecifics or heterospecifics and the females' ability to discriminate between conspecific and heterospecific eggs and between own and conspecific eggs during oviposition were studied at laboratory conditions. When females were offered simultaneously eggs or larvae of both species, a significant preference of E. stipulatus to feed on individuals of I. degenerans was recorded. Higher predation rates were exhibited when females were offered larvae as prey, while the predation rates of eggs were extremely low. On both sexes, developmental times of protonymph and deutonymph were significantly longer when the predator fed on conspecifics. As far as immature development is concerned, the nutritional benefits of E. stipulatus when preying on I. degenerans were similar to those that it obtained when feeding on almond pollen. However, mean egg production was significantly smaller. Choice experiments revealed the predator's preference to oviposit near conspecific eggs rather than near eggs of I. degenerans. Additionally, females preferred to oviposit near a cluster of related eggs and displaced more frequently heterospecific or unrelated eggs than conspecific. These responses show that E. stipulatus has the ability to discriminate between conspecifics and heterospecifics and when offered both at the same time prefers to prey on the latter. The nutritional benefits obtained by this behaviour enable the predator to complete its development and reproduce at times when some other, more preferable food is available or scarce. The females' ability to discriminate not only between conspecific and heterospecific eggs during oviposition but also between conspecific eggs with different degree of relatedness enables them to oviposit near a cluster of related eggs and avoid cannibalism by non-kin conspecifics. The toxicity of four pesticides on E. stipulatus was evaluated at laboratory conditions (20 ± 1°C, RH 65 ± 5%, photoperiod 16L:8D h). Products tested were clofentezine, fluvalinate, dicofol and pirimicarb. Their application was made on citrus leaf discs until run-off with a hand-held sprayer held approximately 23 cm from the leaf surface. Larvae of E. stipulatus were transferred collectively on leaf discs and data on their development, survival and reproduction during first five days of adulthood were recorded. Both initial and residual toxicity of the pesticides were evaluated. According to the IOBC classification scheme, clofentezine and pirimicarb were found to be harmless (E = 8.87 and 29.68% respectively), while fluvalinate was harmful (E = 100.00%) to the predator. Pesticides were also tested under the EPPO classification scheme which showed that further field experiments are needed for the classification of dicofol (E = 98.90%). Both clofentezine and pirimicarb may be safely incorporated in integrated pest management programs, without any concerns towards E. stipulatus. Fluvalinate retains its high toxicity even ten days post its application. The high initial toxicity of dicofol gradually wears off and ten days after its application, an E. stipulatus release may follow without any further effect to the predator.