|dc.description.abstract||Argentine stem weevil (ASW) Listronotus bonariensis (Kuschel) is an introduced species that attacks improved graminaceous pasture and crop throughout much of New Zealand. It is recognised as the most important arthropod pest of ryegrass in this country. While most research over the last 10-15 years has focused on plant resistance and biological control, little consistent information is available on aspects of the biology of the weevil. The main aim of this research was to investigate, in detail, the biology of ASW considering particularly preimaginal and imaginal development.
A method for ASW rearing under laboratory conditions using natural food sources was developed. The effect of adult population density on egg laying and other reproductive variables was assessed in the laboratory. A weak trend with adult density was shown for the variables recorded, with females laying fewer eggs, having shorter mean ovariole length, fewer oocytes in their ovarioles and fewer eggs in their calyces at higher densities. Eggs were laid in ryegrass sheaths and leaves; the percentage of eggs laid in the leaves was not significantly different between adult population densities. The fecundity, fertility of females and the viability of eggs laid in a "mass" rearing situation was also assessed. A mean of 384 eggs were laid per female with a mean of 4 eggs per day. Mean fertility was 97% and 95% of the eggs were viable. The time to peak oviposition was 19 days and 50% of the eggs had been laid 35 days after emergence. A method for egg incubation, larval rearing and prepupal and pupal extraction under laboratory conditions was established. A mean of 75% of prepupae plus pupae were recovered from the rearing pots and on average, 92% developed into adults.
The fecundity, fertility, longevity and other development parameters of oligopausing and non-oligopausing adults were assessed at 20°C. The average duration of the preoligopause, oligopause and postoligopause periods was 76.2, 538.0 and 96.3 days, respectively. Longevity averaged 590.7 days for females and 617.2 days for males, with no significant difference between the means. An average of 150.8 eggs were laid per female. The average number of eggs laid per female, per day was 1.3 during preoligopause and 1.1 during postoligopause. On average, 56.4% of the eggs were laid during the preoligopause and 43.6% during the postoligopause period. Fertility and viability of eggs averaged 96.2% and 94.2% during preoligopause, and 99.4% and 96.9% during postoligopause.
Couples were reared at various temperatures to determine any effect on fecundity and other reproductive and development parameters. The mean total number of eggs laid per female ranged from 150-263 at 35 and 25°C, respectively. The time to peak oviposition after emergence was 24-25 days at 20°C, 20-21 days at 25/10°C (alternating temperatures), 14-15 days at 25°C and 10-11 days at 30 and 35°C. Fifty percent of the eggs had been laid 56-67 days, 44-45 days, 32-33 days, 20-21 days and 14-15 days after emergence at 20, 25/10, 25, 30 and 35°C, respectively. No oviposition was observed at 40°C and only 18% of the females laid eggs at 15°C. The mean percentage of fertile and viable eggs laid at 35°C (66.9 and 61.6%, respectively) was significantly lower (P < 0.05) than at the other temperatures (> 95%). Mean longevity for non-oligopausing females ranged from 41.4-144 days at 35 and 20°C, respectively. The development time tended to be longer at a constant temperature of 20°C than at alternating temperatures of 25/10°C with the same means, except for the postoviposition period. There was a significant effect (P < 0.05) of temperature on the occurrence of oligopause in the laboratory; the higher the temperature, the lesser incidence of oligopause was observed. Oligopausing females and males were dissected to check their reproductive condition. A regression was fitted to the development rate (1/day) of the preoviposition period versus temperature and was well described by a linear regression model (r²= 0.99). The lower threshold temperature for the preoviposition period was 12.1°C and 141 day-degrees were required before oviposition.
The effect of photoperiod and its interaction with temperature on oviposition was investigated using laboratory-reared adults and overwintering field-collected adults. The mean duration of the preoviposition period of laboratory-reared females was not significantly different (P> 0.05) between photoperiods within the same temperature, but was significantly different (P < 0.05) between temperatures. There was a significant effect (P < 0.05) of photoperiod on the mean number of eggs laid per day by laboratory-reared females, but there was no significant effect (P> 0.05) for overwintering field-collected females.
The effect of temperature on the development and survival of ASW preimaginal stages was assessed under laboratory conditions. No hatching occurred at 10 and at 40°C and no survival was observed after the second instar for larvae maintained at those temperatures from the beginning of the first instar. Between 15 and 35°C the only significant mortality observed was in the prepupal stage at 35°C. The development time for the preimaginal stage was significantly shorter (P < 0.001) as temperature was increased and ranged from an average of 19.3 days at 35°C to 115.8 days at 15°C. The development time of preimaginal stages was significantly longer (P < 0.05), except for the prepupal stage, at a constant temperature of 20°C than at an alternating temperature of 25/10°C, both with the same mean temperature. ASW female pupae were significantly heavier (P < 0.05) than male pupae at all temperatures, whereas there was no significant influence (P > 0.05) of temperature on the average weight of female or on the average weight of male pupae, except that at 35°C female pupae were significantly heavier (P < 0.05) than at 30°C. The sex ratio at each temperature was not significantly different (P > 0.05) from 1:1.
Based on the development time at constant temperatures, the lower threshold temperatures (T₀) and the day-degrees (DD) requirements were calculated for all preimaginal stages and validated at varying temperatures in the glasshouse and in the laboratory. Regressions were fitted to the development rates (1/days) versus temperature for each preimaginal stage and period, and were well described by a linear regression model as indicated by high r² values (> 0.97). There was no significant difference (P> 0.05) between T₀ of the preimaginal stages, larval period and preimaginal period, which ranged from 10.7°C for the 3rd instar to 13.3°C for the 1st instar. The average DD requirements for most of the stages at varying temperatures in the glasshouse and in the laboratory were lower than the DD requirements estimated at constant temperatures, however, most of them fell within the confidence intervals given for the DD requirements at constant temperature. The T₀ and DD requirements to complete a generation were calculated and the number of generations possible at Lincoln (Canterbury) was estimated to be 1.7. The implications of these findings are discussed.||en