The intensity of parasite attack in relation to the abundance of Cardiaspina albitextura (Psyllidae)

In areas where outbreaks of the psyllid, Cardiaspina albitextura, were taking place the percentage destruction caused by the three primary parasites (Psyllaephagus gemitus, P. xenus, and P. discretus) was usually low, and varied inversely with psyllid abundance. The numbers of primary parasites were limited greatly by hyperparasitism and sometimes also by breakdowns in host-parasite synchronism. In areas in which outbreaks did not occur and in areas where psyllid numbers remained low after outbreaks, effective parasitism varied independently of psyllid abundance according to a uniform seasonal pattern. The mean mortality in nymphs that escaped destruction by other agencies was 30-40% in the spring and summer and about 75% in the autumn. It appears that there was a limit (75-85%) to the fraction of psyllid nymphs that could be parasitized and that this limit was approached frequently in the autumn. Although the parasites did not operate as "density-governing agents" it is probable that in some areas they contributed materially to the prevention of psyllid outbreaks.

The influence of predation by Syrphus Sp. on the numbers of Cardiaspina albitextura (Psyllidae)

Field observations have shown that the larvae of an unnamed species of Syrphus (closely related to S. viridiceps) are predacious on late instar nymphs of C. albitextura, the fifth (final) instar nymph being the stage attacked most heavily. It was found that Syrphus larvae killed up to about 35% of third to fifth instar nymphs. However, because of the overriding influence of other factors, the mortality caused by Syrphus played only a minor part in the determination of psyllid abundance.

Factors affecting the attractiveness of foliage for oviposition by Cardiaspina albitextura

Field observations and experiments show that, when Eucalyptus blakelyi is heavily infested by Cardiaspina albitextura, the attractiveness of leaves for oviposition is affected greatly by the psyllid itself. It appears that the attractiveness of leaves depends not only on such factors as leaf age and position on the growing shoot but also on the quantity of favourable food available in them. Heavy feeding either by nymphal or adult psyllids greatly reduces the attractiveness of leaves as oviposition sites. On the other hand, the presence of eggs increases the attractiveness of leaves in proportion to the number laid on them. At times of very high psyllid abundance, the majority of eggs are laid on part only of the small quantity of foliage which escapes severe damage by the nymphal stage. The contagious distribution of eggs laid on such foliage is probably due largely to the increased attractiveness of some leaves as oviposition sites, owing to the presence of eggs, and to the reduced attractiveness of others resulting from heavy feeding by adult psyllids.

The influence of population density on the number of eggs laid by females of Cardiaspina albitextura (Psyllidae)

Field observations and experiments suggest that crowding during the period of nymphal development does not greatly affect the mean number of eggs laid by Cardiaspina albitextura females. Under experimental conditions, crowding in the adult stage on foliage undamaged by nymphs resulted in reduction of the mean number of eggs laid, the supply of favourable food being probably the limiting factor. In association with increase in the number of adults per shoot, mean egg production declined at a progressively decreasing rate. At the highest population densities tested, the mean number of eggs laid per female was approximately one-third to one-fifth of the number laid by uncrowded females. Both at low and high population densities, the mean number of eggs laid per female on previously unoccupied foliage was about twice as high as on foliage which had been occupied previously by numerous other adults of the same psyllid generation. At times of peak abundance in areas of outbreak, the foliage damage caused during the period of nymphal development greatly limits both the quantity of favourable food and the number of oviposition sites available when C. albitextura reaches the adult stage. The experimental results suggest that the crowding of adults which occurs on the small amount of foliage that escapes very severe damage by nymphs contributes towards the observed limitation of egg production. However, the scattered distribution of such foliage and the poor dispersal powers of C. albitextura females probably play a more important part in determining the very low mean number of eggs laid per female reaching the adult stage. It appears that many females fail to find favourable foliage and consequently lay scarcely any eggs.

The general bioloy of Cardiaspina albitextura (Psyllidae) and its abundance in relation to weather and Parasitism

Cardiaspina albitextura Taylor is a test-forming psyllid which inhabits the foliage of redgums, particularly Eucalyptus blakelyi Maiden, in inland southeastern Australia. In warmer areas, as on the South-West Slopes of New South Wales, this psyllid usually completes three generations per year, whereas in the cooler Southern Tablelands region it normally takes about 18 months to do so. It is a small insect with limited powers of dispersal. The majority of females oviposit within 100 yd of where they originated. The average number of eggs laid per female varies greatly even when individuals are not crowded, the observed range being 14-289. The reproductive rate appears to be influenced by weather conditions, especially by temperature. A few individuals of E. blakelyi are unfavourable to the psyllid and therefore resistant to attack. The susceptible majority exhibit considerable differences and variability in favourableness. In general, however, although there may be marked differences between individuals within stands, average favourableness does not differ greatly from place to place. When the numbers of C. albitextura become very high, i.e. when outbreaks occur, susceptible E. blakelyi suffer severe foliage damage. Where high psyllid numbers occur repeatedly at intervals of less than 3 years, they cause increasing "die back" and host plants produce progressively less new foliage. If outbreaks continue for years, trees are destroyed at an increasing rate. For the 15-20 years prior to 1950 it appears that localized outbreaks of C. albitextura were not uncommon, particularly on the Tablelands, but did not persist for long. Since then extensive outbreaks have occurred in both the Tablelands and Slopes regions. Some have persisted for 10 years and show no signs of ending. Others have lasted only for several psyllid generations. They have been more numerous and persistent in cooler areas than in the Slopes region.

An analysis of the outbreaks of the Australian Plague Locust, Chortoicetes terminifera (Walker) during the seasons 1940-41 to 1944-45

Outbreaks of the Australian plague locust, Chortoicetes terminifera (Walker), occurred each locus: season from 1940-41 to 1944-45. The largest outbreak, that of 1942-43, was less severe than those recorded in 1937-38, 1938-39, and 1939-40. Consideration of the course of these recent outbreaks in relation to temperature and moisture has again demonstrated the importance of these factors as determinants of the regional and seasonal occurrence of swarms. The curve of limiting conditions for the persistence of swarm populations, obtained by Key (1942), was a useful guide for the interpretation of events in the present analysis. However, a number of discrepancies occurred. These were due probably to a variety of causes, e.g, the kind of infestation concerned, the crudenessof the monthly Meyer ratio as an index of soil moisture and pasture conditions, and regional differences in the upper moisture limits at high temperatures. The results of the present investigation suggest that:(a) The limiting Meyer ratios for the persistence or development of a spring infestation of small, weakly gregarious swarms at temperatures between 60 and 70�F. exceed 4-5; either when the September Meyer ratio is 5 or less, or when the spring is preceded by a severe winter drought.( b ) At temperatures around 75'F., the maximum Meyer ratio at which swarms can exist is approximately 20. (c) At temperatures near the estimated hatching threshold of 57.5'F., swarm hatchings can occur at Meyer ratios as low as 4-5. The collapse of the 1942-43 outbreak in part of the main area infested was due apparently to parasitism by an unidentified fly. Similarly, a biotic factor, possibly the same parasite, was largely responsible for the decline of the 1943-44 outbreak during the late spring and early summer.

The ecology of Chrysomela gemellata Rossi and c.Hyperici forst, and their effect on St. John's Wort in the Bright District, Victoria

This paper describes the basic entomological study necessary in an investigation of the effectiveness of Chrysomela gemellata Rossi and C. hyperici Forst. as controls of St. John's wort, Hypericum perforatum L. var. angustifolium DC, in the Bright district, Victoria. The investigation has shown that the conditions prevailing in the Bright district during the study period must have been generally near the limit of tolerance of both C. gemellata and C. hyperici, parts being a little above and parts definitely below this limit. Furthermore, largely because of certain behaviour responses, neither insect has been able to colonize the timbered areas infested by Hypericum. The total area favouring the multiplication of the insects forms only a small part of the area colonized successfully by Hypericum. Within the zone favourable to the beetles, they have failed to control Hypericum effectively except in small, treeless areas adjoining densely timbered country infested by the weed (in such places the timbered area has acted as a damping factor to population fluctuation, enabling the insects to persist in moderate numbers). Although adverse conditions have played an important part in limiting the effectiveness of the insects, their failure has been due primarily to their own characteristics. Both species have poor powers of dispersal and a relatively poor ability to multiply. In the most favourable sites, both species are able to multiply within a few years to a density high enough to cause complete defoliation of the host plant, which usually dies out. By destroying their food and reducing their protective cover, both species suffer very heavy losses and greatly limit their own numbers. Although many individuals succumb to starvation, a large proportion of the mortality is caused by density-independent factors, e.g. frost and predation, which are allowed to operate in an intensified manner by removal of the protective cover provided by the foliage of the host plant. After completing the defoliation of the weed, the surviving beetles migrate elsewhere in search of more food. Their desertion of the site gives the host plant a chance to recover by regeneration from seed. Numerous environmental factors unfavourable to the insects, combined with their poor mobility and slow rate of increase, frequently allow the plant to recover almost completely before beetle numbers become sufficiently high to defoliate it again. Where other wort-controlling agencies are lacking, the insects have caused their host plant to fluctuate violently in density in both space and time without producing a great overall reduction in quantity. By destroying stands of Hypericum in areas in which the soils are capable of growing a dense pasture, the insects have paved the way for successful wort control by other factors, e.g. shade-producing herbage and grazing animals. Such areas virtually cease to provide habitats for the insects after the original stand of Hypericum is destroyed.

On the abundance of the Australian plague locust Chortoicetes terminifera (Walker) in relation to the presence of trees

By the use of belt transects through selected tree stands, in the Bogan–Macquarie Outbreak Area of Chortoicetes terminifera (Walker), it was possible to show quantitatively that the abundance of non-swarming locusts is greatly limited by the presence of trees. Thus, if the former community dominants were re-established in those outbreak centres which were originally densely timbered, the production of swarms in them would probably be prevented, and locust damage thereby reduced. It was found that the abundance of adult locusts with in tree stands is not related to tree density, as such, but to a "barrier effect" of trees in depth, acting in such a way as to diminish the chances that an individual will penetrate to a given depth in the stand as that depth is increased. Hoppers (predominantly 4th instar), the non-swarming residue of bands hatching outside tree stands, appear to be similarly affected. On the other hand, the population density of hoppers hatching within stands, both at the time of hatching and when the majority reach the 4th instar, is correlated with tree density as well as with distance within the stand, suggesting that adult female locusts are influenced hy tree density in their selection of oviposition sites. The effect of tree density, as such, on the density distribution of locusts apparently disappears gradually as locusts grow and become increasingly mobile, because after they reach the adult stage the relationship no longer exists. The evidence obtained suggests that the limiting effect of trees on locust abundance is largely due to the reduction of light intensity that they bring about, the insects being affected per medium of their positive phototactic response.