Population dynamics and seasonal variation in the embryonic dormancy of Pilophorus gallicus (Hemiptera: Miridae): ‘don't put all your eggs in one basket'

Abstract

Pilophorus gallicus Remane is a generalist predator in southern European pear orchards. Nymphs and adults are present in orchards from March to November; their winter absence suggests either migration to other hosts or embryonic dormancy on pear trees to overcome the adverse period. In addition, it has been hypothesized that aestivation takes place to cope with extreme summer conditions. The present study aimed to investigate the reproductive strategy of P. gallicus to overcome unfavourable periods. Accordingly, (i) its population dynamics were followed during several years and (ii) females were sampled in three different seasons to study the condition (diapausing or nondiapausing) of the laying. The results obtained show that nymphs were always the first mobile instar to show up in pear orchards and the presence of adults was delayed, indicating egg overwintering. Nondiapausing eggs prevailed in spring and summer, whereas the autumn eggs were mainly diapausing. Aestivation was rejected. Worthy of note are: (i) the presence of diapausing eggs under favourable conditions and (ii) the existence of females laying both diapausing and nondiapausing eggs; thus, ‘not laying all the eggs in one basket’. The reproductive strategy of P. gallicus is considered as bet‐hedging for the short‐ and long‐term survival of the species.

When natural enemies go to sleep: diapause induction and termination in the pear psyllid predator Pilophorus gallicus (Hemiptera: Miridae)

“…spring, summer) may result in the loss of immediate reproductive opportunity or entail costs with regard to post‐diapause fitness . These observations seem to be related to a ‘bet‐hedging’ reproductive strategy: the distribution of diapausing eggs along the growing season may be a form of risk spreading for the population whenever it faces difficulties in the timing of diapause induction . This strategy would increase the probability of persistence of the population in atypical adverse conditions, but it also decreases the instantaneous biotic potential of the mirid as a biological control agent of pests (e.g., aphids, psylla) because some of the nymphs will not emerge until the following year.…”

“…The requirements vary among the species, mostly depending on the area of distribution, but always with the aim of achieving synchronization between diapause termination and favorable conditions . Pilophorus gallicus females lay diapausing eggs mainly in late September, and no mobile forms are observed until mid‐March . The dependence on a specific cold period to break egg diapause prevents the eclosion of eggs before the end of winter and the exposure of vulnerable stages (e.g.…”

“…Mobile stages of P. gallicus are absent in the field from mid‐October to mid‐March (approx. 150 days) . Thus, we assumed that the exposure time for the termination of the diapause should be shorter than 150 days.…”

“…The choice of the temperatures assayed was based on the approximate average values observed in the field at the appearance of the first adults in spring (18 °C) and at the beginning of the fall (23 °C) (http://siam.imida.es/). Temperatures lower than 18 °C (i.e. 15 °C) are known to substantially reduce the development and oviposition rates .…”

“…29 This mirid bug has a multivoltine lifecycle, normally with three generations per year in southern Europe. 29,33 Unfavorable winter conditions are overcome by entry into a facultative embryonic diapause: P. gallicus females lay mainly non-diapausing eggs in spring and summer, whereas in the fall they lay predominantly diapausing eggs. The development of a diapausing state may also represent an advantage to minimize the negative impacts of winter sprayings on the populations of P. gallicus.…”