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Boll Weevil Survival and Emergence

Adult weevil feeding on cotton boll

The 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.

The 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.

Analyses 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 cages.

Weevil traps in tall shinnery oak

Cohort release 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 > 6.1C), negative degree-days (NDD < 0.0C), 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.

S. Carroll is checking weevil traps in low shinnery brush

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, are estimated,

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 (>6.1C), 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.

Weevil trap in mesquite-pasture

Our analyses 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.

Boll weevil 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.

 

Document Author:

Megha N. Parajulee
Send mail to Megha N. Parajulee

Photos:

Courtesy of Donald R.  RUMMEL      

Revised:

November 18, 2002
Copyright 1998LadyBug.gif (1020 bytes) AgroEcoSystems Research Group, TEXAS A&M UNIVERSITY