A sampling plan was developed for the study of the population dynamics of Aphis gossypii, Glover on cotton. The distribution of the population of A. gossypii within the plant is not randomly distributed in relation to height. At the beginning (end of May‐June) and at the end (October) of the season, the upper part of the plant is more heavily infested than the lower but this relationship changes in mid‐summer (July‐August). Therefore, the samples for estimating mean population density should be taken proportionally in respect to the height of the plant. The optimum size of the sampling unit was found to be 10 leaves but at the beginning of the season, when the number of leaves per plant is low, a smaller sampling unit could be adopted or even the whole plant can be considered as the sampling unit. The number of samples required to attain a certain precision (i.e. a 10% standard error of the mean) was found to be a strong function of density, fewer samples are required at the higher densities. This is the result of the relationship between the mean and the variance of the population density as it is expressed by the value of the coefficient b in the expression of Taylor's power law. At the low densities, the adoption of the above level of precision leads to a very high number of samples and, therefore, a lower level of precision should be accepted.

The duration of the immature stages of Cacopsylla pyri L[ was studied under _eld conditions by arti_cially infesting pear branches on several dates during the year[ The duration of the egg stage decreased from winter to summer\ as the season progresses and temperature rises\ and slightly increased in September[ It ranged from 16[3 to 5[6 days[ The same trend was observed in the duration of the _rst three larval stages "L 0Ð2 # which ranged from 07[7 to 09[2 days[ For eggs deposited during the period FebruaryÐAugust the duration of the last two larval stages "L 3Ð4 # ranged from 06[4 to 01[0 days[ However\ the duration of L 3Ð4 developed from eggs deposited in September and which give rise to winter! form adults were the longest observed[ The rate of egg development was related to average ambient temperature with a highly signi_cant linear relationship[ This relationship indicates that the egg stage requires a constant number of 047[8 "SD 4[9# of day!degrees above an average temperature of 1[20>C to complete development[ The rate of development both of L 0Ð2 and L 3Ð4 were related to average ambient temperature with curvilinear relationships[ These relationships indicate a proportional increase in the developmental rate as temperature rises between 09Ð11>C[ At the higher average temperatures that occurred in the summer experiments "13Ð16>C# the acceleration of development of L 0Ð2 is reduced and the developmental rate of L 3Ð4 decreases[ The developmental rates of L 3Ð4 developed from eggs deposited in September did not follow the established relationship with temperature and they were lower than those in the other periods of the year with the same average temperature[ U[ S[

N u t i o na 1 A g ricul t u ral R e sea rc h Fo u nda t i on. Plan 1 P r o t cc I ion I n s t i t u t c o f Vo 1 o s . G rcece Abstract: During the years 1991 '92, the dynamics of the adult population of C'ucopsyllri p j r i L. mas studied in Greece (region of Magnesia). At regular intervals during the year, estimates were obtained of the size of the population of males and females, percentage gravid females, sex ratio, number of eggs per gravid female and daily rate of oviposition. The adult population of pear psylla was sampled in such a way that estimates can be directly related quantitatively with simultaneous estimates of the immature stages. During most of the year females were more abundant than males except during the period of high adult emergence and the period of diapause when sex ratio remained close to 1 : I .During the infestation period (February October) a large proportion of the females contained mature eggs except during the period of high emergence of new females when, briefly. the proportion of mature females in the total female population was low. The mean number of mature eggs per gravid female changed considerably during the same period (February-October). It was very high during late winter, spring and autumn and low during summer suggesting that ovarian development is depressed during summer. which may be due to high temperatures. Consequently, the daily rate of oviposition was high during spring and autumn and low during summer. During late winter (February-March) the daily rate of oviposition was low, although the females contained a high number of mature eggs suggesting that ovarian development and rate of oviposition are not equally affected by weather conditions. U S.

The distribution of Cacopsyllapyri (L.) (Homoptera-Psyllidae) infestation within the tree canopy and in the various organs of the tree was investigated in the region of Magnesia (Greece) during 1988 and 1989. The upper parts of the tree canopy and the sections orientated to south and west were infested heavier during winter and spring. Later in the season the infestation was diffused within the tree canopy and at the end of the season a rather uniform pattern of distribution was observed. During the first oviposition period (February-March), the preferred oviposition sites were flower buds while during spring (second generation) they were the leaves of flower buds and young shoots. Sampling strategy for the study of populations of immature stages of pear psylla is discussed on the basis of these findings.

The action of parasites of Saissetia oleae (Oliv.) (Homoptera-Coccidae) is studied in Corfu from 1980 in the framework of an ongoing project on the population dynamics of Saissetia oleae.Results obtained during the period 1980-1983 from 17 experimental sites indicate the following: a) the population of S. oleae increased in 1981 but from 1982 it declined to very low levels; following the overall decline of the host population, parasitization rates of third instar larvae (mainly by Metaphycus helvolus Comp.) were relatively low and ranged from 2.1 % to 6.7% for the autumn period and from 14.6% to 29.4% for the spring period; this does not minimize the possibility that Metaphycus helvolus plays an important role on the regulation of 5. oleae populations; b) a considerable proportion of females of S. oleae is attacked during summer by Metaphycus lounsburyi How. (an internal parasite on preovipositing females), Moranila calif arnica How. and Scutellista cyanea Motsch. (egg predators), but their significance upon the population system of 5. oleae is not expected to be great. The action of Metaphycus lounsburyi in spring, when the peak of the suitable stage occurs (i.e. fourth instar larvae), appears to be very low, at least for these three years. Its action is delayed and most of the specimens of this parasite emerge from ovipositing females in early summer. Among the species attacking S. oleae females, M. californica (recently introduced in Corfu) was the most abundant and it might be worthy to be introduced to other parts of Greece too.