A study of the effects of ‘Candidatus Phytoplasma mali’ on the psyllid Cacopsylla melanoneura (Hemiptera: Psyllidae)

Abstract

Cacopsylla melanoneura is a univoltine psyllid vector of ‘Candidatus Phytoplasma mali’, the etiological agent of apple proliferation (AP), a severe disease in European apple orchards. The influence of ‘Ca. P. mali’ on the fitness of C. melanoneura was studied. In the spring of 2007, male–female pairs of field-collected adults were exposed to ‘Ca. P. mali’-infected or healthy ‘Golden Delicious’ apple shoots. Exposure to these diseased shoots did not affect the life span of the adult psyllids. However, significantly fewer eggs were laid on the diseased shoots. Furthermore, fewer of the eggs that were laid on the infected plants hatched. Data suggest a detrimental effect of AP phytoplasma on the fitness of C. melanoneura.

Introduction

Phytoplasmas and their vectors interact in various ways, ranging from the beneficial to the deleterious (Severin, 1946, Beanland et al., 2000). Survival and fecundity are often the main parameters in studies of vector–mollicute interactions (Weintraub and Beanland, 2006). Data concerning changes in the life span and fecundity of vectors are available for a few strains of phytoplasma belonging to the X-disease (‘Candidatus Phytoplasma pruni’) and the aster yellows (‘Ca. P. asteris’) groups, as well as for Spiroplasma spp. (Jensen, 1959, Madden and Nault, 1983, Garcia-Salazar et al., 1991, Ebbert and Nault, 2001).

Phytoplasma infection can make infected plants more suitable hosts for insects, by reducing their chemical defenses or increasing the availability of nutrients. Consequently, infected plants are more attractive to insects (Weintraub and Beanland, 2006). Very little is known about the effects of phytoplasmas on the psyllid genus Cacopsylla (Hemiptera: Psyllidae), which is involved in the transmission of ‘Ca. P. mali’, the etiological agent of apple proliferation (AP) disease. In the apple orchards of the Trentino region, the main vector of the disease is C. picta Förster. However, in recent years, populations of this insect have decreased dramatically (Mattedi et al., 2007). Despite these changes in C. picta populations, the incidence of AP disease has increased.

The full role of C. melanoneura Förster in the spread of ‘Ca. P. mali’ is still unclear (Mattedi et al., 2008), but it has been shown to be an efficient vector of the disease in northwestern Italy (Tedeschi et al., 2002). Despite observed decreases in C. melanoneura populations (Mattedi et al., 2007), large populations can be found in some areas of Trentino, such as Valsugana.

In light of these observations, we chose to focus our attention on C. melanoneura, to better understand the factors that may affect its population dynamics and its role in vectoring the disease. C. melanoneura is a univoltine species. Overwintering adults usually return to apple orchards at the end of January. The new generation of adult insects leaves apple orchards between mid-May and mid-June and overwinters on conifers at high altitudes (Pedrazzoli et al., 2005).

This study investigated the interaction between ‘Ca. P. mali’ and C. melanoneura. In particular, we studied the effects of ‘Ca. P. mali’ on the longevity of overwintered adults, the numbers of eggs laid, the rate of egg hatching and the juvenile survival.