Timing of Short-Day Exposure Influences Diapause Response of Western Tarnished Plant Bug

The western tarnished plant bug, Lygus hesperus, enters adult diapause in response to short photoperiods.  However, the instars or stages receiving the photoperiodic cue are poorly defined.  Lygus were reared under a short (10-h) day length until they were dissected to determine diapause status as 10-d-old adults.  For comparison, bugs were similarly reared under the short day length but were transferred to a long (14-h) day length at either 1st or 3rd instar, or as newly eclosed adults.  Reciprocal treatments were also observed where bugs were initially reared under the long day length, where they were either maintained or switched to the short day length.  No bugs reared under short days and switched as nymphs to long days exhibited diapause characters.  Only a small portion (<10%) of bugs switched from short to long days as young adults retained the diapause characters by 10 days of adult age.  Incidence of diapause was highest (55%) for bugs receiving only the short-day photoperiodic cue, or for those switched from long to short days as 3rd or earlier instars (40-53%).  No diapause response was observed for bugs switched from long to short days as young adults.  These results indicate the photoperiodic cue is received at least by 3rd and earlier instars, and the incidence of diapause appears increased slightly with increased exposure to the short day length.  Exposure to long days was efficient at reversing diapause induction irrespective of earlier exposure to short days.