Occurrence of different development time patterns induced by photoperiod in Anagrus atomus (Hymenoptera: Mymaridae), an egg parasitoid of Empoasca vitis (Homoptera: Cicadellidae)

Abstract

The wasp Anagrus atomus L. parasitizes eggs of the cicadellid Empoasca vitis (Göthe) on grapevines during the vegetative season. In vineyards, abundant parasitoid emergence occurs in the autumn as a result of a developmental delay in part of the A. atomus population. In this context, physiological times required by the parasitoid to develop in E. vitis eggs are recorded during the summer. In early summer, all individuals develop fully, with physiological times in agreement with those reported in the literature. From early August onwards, an increasing proportion of parasitoids show a delay in development involving pre-imaginal stages. Under transmission microscopy, larvae of A. atomus inside the eggs of E. vitis can be clearly differentiated as 'light' and 'dark' types, which are associated with normal and delayed development, respectively. The decreasing photoperiod as autumn approaches appears to promote the retardation of growth. This phenomenon can be considered a typical case of risk spreading because, when the availability of E. vitis eggs in vineyards is high (i.e. in early summer), A. atomus shows (i) only a normal developmental pattern that allows a faster population increase, whereas (ii) when E. vitis egg availability begins to be scarce (i.e. from mid summer), an increasing proportion of individuals having delayed development provides better fitness because it allows the parasitoid to synchronize its life history with leafhopper species that lay overwintering eggs on plants other than grapevines.

Morphological and Molecular Identification ofAnagrus‘atomus’ Group (Hymenoptera: Mymaridae) Individuals from Different Geographic Areas and Plant Hosts in Europe

“…In this way we can know not only the plants but also the leafhopper species as potential sources of A. atomus for E. vitis biocontrol in vineyards. It is also possible to know what leafhopper species is parasitized by an Anagrus species by marking and exposing to parasitization leafhopper eggs laid on a plant by identified females ( Zanolli and Pavan 2013 ). This knowledge is crucial to set up conservation biological control strategies based on habitat management.…”

“…These individuals were identified as A. atomus on the basis of cuticular hydrocarbons ( Floreani et al. 2006 ) or because they emerged from E. vitis eggs, known to be parasitized only by this species ( Zanolli and Pavan 2013 ). All individuals were frozen as soon as they emerged and stored at −80°C until used.…”

“…2006 ; Viggiani et al. 2006 ), but in Northern Italy on grapevines only A. atomus was associated with the eggs of Empoasca vitis (Göthe) ( Chiappini 1987 ; Zanolli and Pavan 2011 , 2013 ), whereas A. parvus emerged only from the eggs of Zygina rhamni Ferrari ( Zanolli and Pavan 2011 ). However, the number of potential hosts for Anagrus spp.…”

Behavioural Evidence and Chemical Identification of a Female Sex Pheromone in Anagrus atomus (Hymenoptera: Mymaridae)

“…As with many other parasitoids, the mating system of A. atomus is characterized by protandry, with males emerging first followed by females (Zanolli and Pavan 2013). The sex ratio is generally 1:1 (Zanolli and Pavan 2011).…”

Behavioural Evidence and Chemical Identification of a Female Sex Pheromone in Anagrus Atomus (Hymenoptera: Mymaridae)

“…As many other parasitoids, the mating system of A. atomus is characterized by protandry with males emerging rst and followed by females (Zanolli and Pavan 2013); the sex ratio is generally 1:1 (Zanolli and Pavan 2011). A. atomus females begin to lay eggs as soon as they emerge and have a life expectancy of 9 days at 24 °C (Agboka et al 2004).…”