Weevil Survival and Emergence
boll weevil, Anthonomus grandis, overwinters as an adult in a state of facultative
diapause in most of the temperate regions of the United States. In the Rolling Plains of
Texas, boll weevils initiate diapause in early August with spring/summer emergence from
winter habitats continuing in some cases to mid-August the following year. The proportion
of diapausing weevils that successfully overwinter (i.e., winter survival and spring
emergence) is believed to be dependent on timing of weevil entry into overwintering
habitat, winter severity as reflected by low temperatures and rainfall, and by the
insulating capacity of the overwintering habitat.
objective of our study was to quantify the effect of climatic data-based variables on boll
weevil overwintering survival and spring/summer emergence. Our approach of constructing a
boll weevil survival and emergence model presented herein offers a significant improvement
in our understanding of boll weevil overwintering biology because it uses weather data
from overwintering habitats (temperature experienced by the weevils during diapause as
opposed to ambient weather) and accounts for the effect of time of entry of diapausing
weevils into overwintering habitats.
were conducted on 16 years of climatological and boll weevil survival and emergence data
from the Rolling Plains of Texas. Daily ambient minimum and maximum temperatures and
rainfall data were collected from a tall growth shinnery oak habitat in Stonewall County
from 1978 to 1985. These ambient temperatures were converted to leaf litter temperatures.
Temperature and rainfall during the years 1986-1995 were recorded on an hourly basis in
the same habitat, but the temperatures were recorded directly from the leaf litter. Leaf
litter temperatures and rainfall were also recorded from 2 other habitats, low growth
shinnery oak and mesquite-grass pasture, in the same locality during 1992-1995.
Boll weevil winter survival
and spring emergence data were obtained from the same experimental sites and for the same
overwintering seasons for which the weather data were collected. Boll weevils used in this
study were obtained from infested squares collected from cotton fields during late summer
and early fall from 9 counties (Concho, Dickens, Haskell, Kent, Knox, Motley, Runnels,
Stonewall, and Tom Green) in the Rolling Plains. Weevils were reared in conditions known
to induce a high level of diapause, after which cohorts were placed in overwintering
dates ranged from 3 September to 22 November, depending on the availability of adult
weevils. In total, 103 cohorts were released during the 16-yr. study, with an average of
~900 weevils per cohort, and ~6 cohorts per year.Each cohort was inspected every 2-7 days
starting from mid-January until mid-August to record the number of weevils emerging from
overwintering cages. Also, Cumulative positive degree-days (DD > 6.1°C),
negative degree-days (NDD < 0.0°C), and rainfall were calculated for each cohort from
the day of the year when a cohort was placed in an overwintering habitat (DOYin)
to when the first weevil emerged from overwintering (DOYstart), and
from DOYin to the completion of emergence. An iterative nonlinear
multiple regression procedure was used to quantify the relationship between the biological
events (e.g., degree-days required to initiate emergence from overwintering habitat (DDstart),
degree-days required to complete the emergence from overwintering habitat (DDstart)
[DDtot - DDstart = DDemerg], overwintering
survival, and the emergence patterns) and the climatic parameters.
The analysis indicated that the start
of overwintering emergence (DDstart) was best estimated as a
function of DOYin and NDDstart, whereas the
completion of emergence was described as a function of DOYin, NDDstart,
Rainstart, Rainemerg, and DDstart.
Thus, our simulation model first estimates the value of DDstart and then
it estimates DDtot using DDstart as one of the
parameters. Once the two points of the emergence curve, DDstart and DDtot,
then the model generates
the emergence profile using a sigmoid curve as a function of NDDstart,
Rainstart, Rainemerg, DDstart,
and DDtot. Overwintering survival was best estimated as a function of DOYin,
NDDstart, Rainstart, Rainemerg,
DDstart, and DDtot.
The physical condition of
weevils that initiate diapause at different times of the season appears to be a major
determinant of the timings of emergence the following spring/summer, with the assumption
that diapause is a metabolite-mediated and hormonally controlled system, as contrasted
with a strictly heat unit controlled system. The notion of early diapausing cohorts
initiating overwintering emergence earlier and late diapausing cohorts later, appears to
be a plausible phenomenon from a biological standpoint, because weevils from cohorts that
enter overwintering habitat earlier in the season may deplete their fat reserves earlier,
which would force them to emerge earlier or die.
Temperature during diapause
affected both overwintering survival and spring/summer emergence. The higher the
temperature above the lower threshold of boll weevil activity (>6.1°C), the higher the
overwintering survival, and earlier the initiation of overwintering emergence. Conversely,
the greater the degree-days below 0.0 °C, the lower the overwintering survival and later
the initiation of emergence. Moreover, variation in overwintering survival and timing of
weevil emergence from overwintering can be directly linked to the quality of overwintering
habitats to which the weevils were exposed.
also indicated that increased rainfall during overwintering contributed to higher
overwintering survival. However, rain occurring before the initiation of overwintering
emergence favored winter survival to a greater degree than did rain which occurred during
emergence. A simultaneous occurrence of both high temperature and high rainfall was more
conducive for winter survival than the occurrence of either situation alone.
overwintering emergence patterns can be predicted with considerable accuracy using our
weevil model, but greater knowledge of factors affecting boll weevil overwintering
survival and the time to completion of overwintering emergence is required for these to be
predicted accurately. Although the data base used for these analyses is possibly the
largest of its kind, it is somewhat limited when considering the wide range of
temperature, rainfall, and times of entry into overwintering habitat represented.
Nonetheless, the derived relationships should serve as the best available tools to predict
boll weevil overwintering survival and spring/summer emergence throughout the Texas Plains
where shinnery oaks, mesquite, and similar types of vegetation are the primary
overwintering habitats for the weevils.
|Send mail to
||Megha N. Parajulee
|Courtesy of Donald
|November 18, 2002
© 1998 AgroEcoSystems Research Group, TEXAS A&M UNIVERSITY