Effects of constant and changing temperature conditions on diapause induction in Helicoverpa armigera (Lepidoptera: Noctuidae).

The effects of photoperiod and temperature on the induction and termination of facultative pupal diapause in Helicoverpa armigera (Lepidoptera: Noctuidae) were investigated under laboratory conditions. Exposing H. armigera larvae to both constant and fluctuating temperature regimes with a mean of 25°C and 20°C resulted in a type-III photoperiodic response curve of a short-long day insect. The long-day critical daylengths for diapause induction were ten hours and 12 hours at the constant temperatures of 25°C and 20°C, respectively. Higher incidences of diapause and higher values both for the longer and the shorter critical photoperiods for diapause induction were observed at fluctuating regimes compared with the corresponding constant ones. At alternating temperatures, the incidence of diapause ranged from 4.2% to 33.3% and was determined by the temperature amplitude of the thermoperiod and by the interaction of cryophase or thermophase with the photoperiod. Helicoverpa armigera larvae seem to respond to photoperiodic stimuli at temperatures >15°C and <30°C; all insects entered diapause at a constant temperature of 15°C, whereas none did so at a constant temperature of 30°C under all the photoperiodic regimes examined. Although chilling was not a prerequisite for diapause termination, exposure of diapausing pupae to chilling conditions significantly accelerated diapause development and the time of adult emergence. Therefore, temperature may be the primary factor controlling the termination of diapause in H. armigera.

Overwintering survival and spring emergence of Helicoverpa armigera (Lepidoptera: Noctuidae) in northern Greece.

Overwintering survival of Helicoverpa armigera (Hübner) was studied under field conditions during the winter of 2004-2005 and 2005-2006 to clarify whether a local population of this insect is established in northern Greece. Additionally, the postdiapause eclosion times of the overwintering generation was compared with adult male dynamics using pheromone traps. Our study supplies strong evidence that part, if not all, of the late-season generation of H. armigera overwinters as diapausing pupa in northern Greece, suggesting that a local population exists in this area. Pupae enter diapause during September and October in response to declining temperatures and photoperiod. A limited number of degree-days in autumn prevented part of the population from developing to the diapausing pupal stage. Larvae of H. armigera that were placed in field conditions late in October were not able to pupate and died. The termination of diapause and the eclosion of adults in the following spring were determined by the local soil temperatures. Diapause terminated between mid-April and early May, and adult emergence occurred within a 4- to 6-wk period from late April to early June. Females emerged slightly earlier than the males. The emergence of 10, 25, 50, 75, and 90% of adults required 153, 199, 252, 303, and 347 DD, respectively. Differences in timing and degree of overlap between adult emergence from the experimental population and pheromone trap catches suggests that the spring population of this species also includes immigrants.

Development, survivorship, and reproduction of Helicoverpa armigera (Lepidoptera: Noctuidae) under constant and alternating temperatures.

Laboratory studies were conducted to assess the effect of temperature on the survival, development, fecundity, and longevity of Helicoverpa armigera (Hübner) at 11 constant temperatures ranging from 12.5 to 40 degrees C, as well as at five alternating temperature regimes (25-10, 30-15, 32.5-17.5, 35-20, and 35-27.5 degrees C) and under a photoperiod of 16:8 (L:D) h. H. armigera reared at constant temperatures did not develop from egg to adult (emergence) outside the temperature range of 17.5-32.5 degrees C. The alternating conditions expanded this range from 10 to 35 degrees C. The lowest developmental thresholds of the immature stages were estimated by a linear model and ranged from 10.17 (pupal stage) to 11.95 degrees C (egg stage) at constant temperature regimes and from 1.1 to 5.5 degrees C, respectively at alternating temperatures. The values of developmental thresholds estimated using the nonlinear (Lactin-2) model were lower than those estimated by the linear model for constant and alternating temperature regimes except for larval and pupal stages at constant temperatures. Mean adult longevity fluctuated from 34.4 d at 15 degrees C to 7.6 d at 35 degrees C. Females reared under all alternating temperature regimes laid more eggs than females reared at any, except the 25 degrees C, constant temperature treatment. The intrinsic rate of increase was highest at 27.5 degrees C, at both the constant and the corresponding alternating temperature regimes (0.147 and 0.139, respectively). Extreme temperatures had a negative effect on life table parameters.