Terry F. Branson

THE SELECTION OF A NON-DIAPAUSE STRAIN OF DIABROTICA VIRGIFERA (COLEOPTERA: CHRYSOMELIDAE)

A non‐diapause strain of Diabrotica virgifera LeConte was selected after nine generations by the technique of mass selection. The passive consequence of chilling the eggs of this species is a synchronization of hatch; the chilling does not break diapause. A hypothetical model (and its mathematical expression) explaining diapause in this species was derived from the data. Initial hatch in the non‐diapause strain occurred in 24 days at 20°. In an unchilled‐unselected population, the mean minimum period of development to initial hatch (both pre‐diapause and post‐diapause development) was ca. 24 days, and the duration of the mean dormant period was ca. 78 days. The duration of the dormant period is probably quantitatively inherited since this character appears to have enormous genetic plasticity.

The contribution of prehatch and posthatch development to protandry in the chrysomelid, Diabrotica virgifera virgifera

Laboratory tests with eggs of Diabrotica virgifera virgifera LeConte showed that during a 10‐day hatching period, hatch of male eggs predominated on the first and second days, eggs of mixed sex, with ca. 1:1 ratio, hatched on the third and fourth days, and eggs hatching from the fifth to the tenth days were nearly all female. Overall, female eggs hatched a mean of 2.9 days later than male eggs. Not only did female eggs hatch later, but the time for posthatch development to the adult stage was 1.8 days longer for females. The later egg hatch and longer posthatch development for females resulted in female adults emerging a mean of 4.7 days later than male adults. Total adult emergence lasted 14 days; of this, males predominated during the first 5 days, and females predominated during the last 9 days. Males of D. v. virgifera appear to have evolved protandry (the tendency for males to emerge before females) by developing both a postdiapause embryonic stage and a combined larval and pupal stage of shorter duration.

DIAPAUSE IN DIABROTICA VIRGIFERA (COLEOPTERA: CHRYSOMELIDAE): A COMPARISON OF EGGS FROM TEMPERATE AND SUBTROPICAL CLIMATES

Eggs of Diabrotica virgifera from subtropical Mexico, when compared with those from South Dakota, are more resistant to desiccation and have a more intense diapause. Diapause occurs at the same embryological stage in the two populations and both can be dormant at humidities below 100%. The data suggest that the diapause mechanism evolved in a tropical climate to overcome a dry season and that same mechanism also adapts the species to a temperate climate.

VIABILITY AND HATCHING PATTERN OF EGGS OF THE WESTERN CORN ROOTWORM EXPOSED TO CHILL PERIODS OF DIFFERENT DURATIONS

After a chill (5°) period of >16 weeks, the mean post‐chill time to initial hatch of eggs of the western corn rootworm, Diabrotica virgifera, was 19 days; peak hatch occurred a mean of 5 days after the initiation of hatch, and hatch lasted a mean of 5 weeks. The post‐chill time to initial hatch was influenced negatively by chill periods lasting up to ca. 3 weeks; however, chilling for >3 weeks and <60 weeks did not significantly affect the time to initial hatch. The duration of hatch was not significantly altered by chill periods up to 4 weeks, but it was significantly reduced after 8 and 12 weeks of chill. Chilling for 16 or more weeks, however, no longer had an effect. The viability of rootworm eggs was not influenced by chill periods up to 12 weeks, but, as chill periods were extended beyond 16 weeks, the eggs slowly begin to lose viability; over a period of about a year, the viability had been reduced by 80%.

Influence of corn planting date on the life stage development and phenology of Diabrotica virgifera virgifera.

Under field conditions in eastern South Dakota, USA three different planting dates of corn and three times of egg infestation were used to imposed synchronous and asynchronous timing of corn growth and Diabrotica virgifera virgifera LeConte egg hatch and subsequent development. Median occurrence of each of the three larval stages and of the adult stage for each of the infestation‐planting treatments was determined by relating occurrence to thermal units and to days after infestation. No significant differences were found among the treatments for median occurrence of each life stage when the thermal unit approach was used. However, significant differences were found among treatments for median occurrence when the day after infestation approach was used. Consistent parameters are needed for prediction of the occurrence of life stages of this insect. Despite the various imposed conditions, this study indicates that the least variable method of predicting life stage occurrence and adult emergence of D. v. virgifera was the use of thermal unit accumulations (base 11 °C).