INFLUENCE OF TEMPERATURE ON SPRING EMERGENCE OF EUROPEAN CORN BORER MOTHS

(LEPIDOPTERA: PYRALIDAE)I,'

J. A. DuRant Department or Entomology, Clemson University

Pee Dee Research and Education Center Florence, SC 29503

ABSTRACT

European corn borel', Ostrillia 'lUbilatis (Hubner), spring moth emergence from field­coged corn find sorghum stalks in Darlington County, S.C. was monitored during the 5-yl' period 1985-89. Regression analysis of 10·90% moth emergence (Y) on degree-day accumulations (x) (base lOoC; 1 March starting dllte) revealed a highly significant logarit.hmic relationship (r~ = 0.759; n = 30; P < 0.001) expressed as Y = -374.51 + 79.56(1og x). Obsolved dales for 10, 50, und 90% emergence always occurred within 4. II, and 12 d, respectively, of the predicted dates. Emergence for 1988 required substant.ially greater degrec-day accumulations than for thc other ycaTS, indicating that factors other than temperaturc may have been involved. Omission of the 1988 data Crom the analysis produced a highly significant linear relationship (r2 = 0.901; n = 25; P < O.(Xll) expressed as Y = -54.56 + 0.51:<. With one exception, this equat.ion predicted 10, 50, and 90% emergence within 3, 2 and 3 d of the observed emergence, respecti\'ely, with actual dates of 10, 50, and 90% emergence varying from the mean calendar dates by 15, 13, and 9 d, respectively.

Key Words: Ostrinia rwbilalis (Hubner), European com borer, degree·days, spring emergence.

J. Agric. 8ntomol. 7(3): 259-264 (July 1990)

The European corn borer (ECB), Ostrinia nu.lJilalis (Hubner), is a seriolls pest of corn, cotton, sorghum, wheat, and vegetable crops in the southeastern United Stat.es. This insect overwinters as a fifth-instal' larva in above-ground plant debris and moth emergence occurs in South Carolina during March, April, and May (DuRant 1969, DuRant et a!. ]986). Since ECB larvae tunnel into the host plants within a few days after eclosion, effective chemical control involves timing of insecticide applications to coincide with the presence of newly hatched larvae. Legg and Chiang (1984) proposed a b ....o-step model for predicting ECB oviposition based on moth captures in pheromone traps. A reliable method of predicting ECB moth emergence would enhance management of the first generation of this pest by enabling growers and pest management specialists to more precisely time activities such as pheromone trap monitoring and in· field scouting.

Temperal.ure is the primary factor innuencing the rate of ECB development (Beck 1968). Both Caffl'ey and \OVorthley (1927) and Matteson and Decker (1965) reported that the lower threshold temperature fol' ECB development is ca. WOC. Many degree·day (DD) methods for predicting the rate of insect development have been proposed which est.imate, with differing degrees of precision, actual DD

Hcceived for Jlublication 24 November 1989; accepted 28 t,'ehnIUI)' 1990. Technical contribution 110. 3015 of the South Coralino Agriculturol Experiment Station, Clemson University.

259

260 J. Agric. Entomol. Vol. i, No.3 (l9oo)

accumulations (Pruess 1983). Several studies have been conducted on the effects of DD accumulations on ECB development. (Apple 1952, Jalvis and Brindley 1965, Anderson et al. 1982a,b. Eckenrode et al. 1983, Despins and Roberts 1984, Boivin et al. 1986). Despins and Roberts (1984) and Boivin et al. (1986) obsCI"Ved thaI DO requirements for ECR night peaks varied among different geographic locations. Eckenrode et al. (1983) found that ECa flight patterns varied due to different biotypes. Umcozor et al. (l985) investigated the effects of different tillage practices on the timing and duration of overwintering geB moth emergence and rcported that emergence occurred significantly later and was longer in duration in unhan'ested com plots than in mowed and disked plots.

This study was conducted to determine the relationship between DD accumu· lations and spring emergence of ECB moths in the South Carolina Coastal Plain region.

MATERIALS AND METHODS

Corn (1985, 1986) and sorghum (1986-1989) stalks infcsted with diapausing fifth-instal' ECB lalvae were collected from fields in Darlington County during October and November. These stalks were cut at or slightly above the soil surface and immediately transferred to 2.4 by 2.4 by 1.8 m Lumite natural Saran 7.1 by 5.5 mesh/cm field cages which at been erected in a recently disked field within 1 km of the collection site. Each cage conmined ca. 1,000 stalks. The stalks were stacked loosely on the soil in 1985-86, and suspended ca. W cm above the soil surface in wire mesh cylinders in 1987-89. The mesh was sufficiently coarse so that ECB moth movement was not impeded. Two cages containing corn stalks in 1985, one containing corn and one containing sorghum stalks in 1986, one containing sorghum stalks in 1987 and 1988, and two containing sorghum stalks in 1989 were monitored. A hygrothcnnograph was hOllsed in a standard weathel' shelter, ut a height of ca. 0.9 JIl, inside one of the cages. Emerging moths were counted and removed from the cages daily, Monday through Fliday. Degree-days were calculated beginning 1 M81'ch using WoC as the base tempel'8ture. The daily DO accumuJation was computed as the average of the daily maximum and minimum temperatures minus WoC; negative accumulations were defined as zero (LoeweI' et al. 1974). Alt.hough selection of 1 March as the starting date was arbitrary, this date provided slightly more accurate predictions than did 1 January. Regression analysis was conducted to quantify the relationship between DO accumulations (x) and percent emergence (Y) (Lotus Deve!. Corp. 1987).

RESULTS AND DISCUSSION

Emergence began al 39-82 DO, reached 10% al 113-166 DO, 50% at 181-285 DD, 90% at 256-416 DO, and 100% at 368·515 DD. An exception WllS ]989, when emergence began on 16 February, prior to the 1 March starting date for DD accumulation. However, only 11 of 3728 (0.30%) moths had emerged by this date. Emergence plotted over accumulated DO produced a sigmoid curve, but when the extremes (0-10% and 9O·100%) were omitted a significant logarithmic relationship (1'2 = 0.759; n = 30; P < 0.001) existed (Fig. 1). A linear regression, while only slightly less significant (r"l = 0.681; n = 30; P < 0.001), tended to underestimate the DO requirements fOl' LO% emergence.

261 DURANT: European Corn Borer Spring Emergence

100,------------------------,

w U 80 Z W CJ 0:: 60 W 2 w I- 40 Z W U 0:: 20 W 0...

Y = -374.51 + 79.56(log x) cooo o •r 2 = 0.871

00000 • o CD •

00.

00 o. Ol-_-'-_---'-_~__..L__ _'___ _L__"'______'

50 100 150 200 250 300 350 400 450

DEGREE DAYS - BASE 10° C Fig. 1. Relationship between emergence of overwintering European corn borers

and cumulative degree~days from I March in Darlington County, S.C., L985~89. Solid circles represent J988.

Examination of Figure 1 reveals that observed emergence fol' 1988, especially for 50% and greater, was I'elativcly stow compmed with predicted emergence. For each percent (Y axis), the solid circle on the far right represents the 1988 emergence. Although the cause of this relatively slow emergence is unknown, only 161 moths emerged. Beck (1967) reported that partially dehydrated diapausing ECB larvae were incapable of complet.ing prepupal development until after they had access to water. Following access Lo contact moisture, pupation occurred rapidly. Dehydration appeared unlikely to be a faclor in the delayed 1988 emergence, since precipitation totalled 6.1 and 9.4 cm for ~'larch and April, respectively. Emergence was not delayed for 1.986, when precipitation totalled 10.5 and 0.9 em for March and April, respectively. Additionally, cumulative precipitation for January and F'ebrulll'y was 10.7 cm for 1988 compared with only 4.0 ern for 1986. This (1988) was the only year in which females outnumbered males. Since male moth emergence always peaked before female emergence, this may partially explain the delayed emergence. The sex ratio (males: femHles) was 0.9:1 for 1988; it vuried from 1.1:1 to L.4: L for the other years.

\Vhen the L988 data were omitted from the regression analysis, a highly significant linear relationship existed (1'2 = O.90L; n = 25; P < 0.001); the equation Y = ·54.56 + O.51x provided a more accurate prediction of emergence (Table 1, Fig. 2). The observed dates for 10% emergence, which ranged from 28 Murch (L986) to 17 April (1987), nevel' differed from the predicted dates by more than 3 d (19 DD). The observed daLes for 50% emergence ranged from 4 April (1986) to 24 April (L987), and ah..'ays occurred within 2 d (24 DD) of the predicted dates. The

262 J. Agric. Entomol. Vol. 7, No.3 (1990)

Table I. Obsolved versus predicted European corn borer spring emergence from field· caged corn and sorghum stalks in Darlington County, S. C.

Host No. 10% emergence" 50% emergence- 9O'jf Emergence"

(year) moths Observed Predicted Observed Predicted Observed Predicted

Corn (1985) 1355 2 Apr. 31 Mar. 15 Apr. 13 Apr. 22 Apr. 21 Apr. (146) (127) (227) (205) (308) (283)

Com (1986) 196 29 Mar. 30 Mar. 7 Apr. 6 Apr. 18 Apr. 18 Apr. (118) (127) (2"21) (205) (28·1) (283)

Sorghum (1986) 257 28 Mar. :10 1\'101'. 4 Apr. 6 Apr. 15 Apr. 18 Apr. (113) (127) (182) (205) (273) (283)

Sorghum (1987) 1088 17 Apr. 14 Apr. 24 Apr. 24 Apr. 1 May 3 MilY (146) (127) (206) (205) (256) (283)

Sorghum (1989) 3728 29 Mar. 29 !\'laf. 7 Apr. 17 Apr. 24 Apr. 26 Apr. (127) (127) (181) (205) (263) (283)

" Numbl!rs in Jlllrcnlheses are degrce·dllYll.

100

W U Z 80

Y = -54.56 + 0.51 X

r 2 = 0.901 000 0

w 0 coo 0

0:: W 60 :::2: W 0 00

I-Z

40 W U

o <lD

0:: W 20 (L

CO 0

0 50 100 150 200 250 300 350

DEGREE DAYS - BASE 100 C

Fig. 2. Relationship between emergence of ovcl"\vintering European corn borers and cumulative degree-days from 1 March in Darlington County. S.C., 1985·89 (excluding 1988).

263 DURANT: European Corn Borer Spring Emergence

observed dates for 90% emergence ranged (rom 15 Aplil (1986) to 1 May (1987), and always occurred within 3 d (27 DD) of the predicted dates. The only exception was for 50% emergence in 1989, which occurred 10 d prior to t.he predicted date of 17 April. Temperatures were unseasonably low during this time, with only 17 DD accumulating between 7 and 17 April. Emergence from sorghum ran slightly ahead of emergence from corn during 1986.

This study indicates that ECB spring emergence in the South Carolina Coastal Plain region can be predicted with relative accuracy using DD accumulations. Pruess (1983) recommended that any claim that a DD model is superior to simple calendar date predictions should include deviations in actual days from mean calendar dates fOl' comparison. The mean calendar dates for 10, 50, and 90% emergence were 2, 11, and 22 April, respectively. Actual dates for 10, 50 and 90% emergence were 28 March - 17 April, 4 - 24 April, and 15 April- 1 May. respectively. Although the method used here underestimates DO accumulations when the threshold (lO°C) lies between the daily maximum and minimum temperatures, ECB development during these days probably would be relatively minimal. Caution must be observed when using trap capture data with this predictive equation, since the time between moth emergence and trap capture will vary depending on such factors as trap type (Oloumi-Sadeghi et al. 1975, Legg and Chiang 1984), location, and weat.her conditions. Blacklight and pheromone traps were present in the vicinity of the field cages during each year, but they captured extremely low numbers of ECB moths. Trap captures lagged several days behind caged moth emergence. Future validation studies must include attempts to determine the chronological relationships among emergence, trap capture, and oviposition. This study utilized unharvested corn and sorghum stalks; emergence probably would occur several days sooner in fields subjected to mowing and disking (Umeozor et al. 1985). Although determination of the reason for the slight difference in rate of emergence f!'Om corn and sorghum observed during 1986 was beyond the scope of this study, microclimate could have been a factor. Average temperatures possibly were lower within the larger corn stalks, resulting in fewer DD accumulations than within the sorghum stalks. Other possible factors include differences in moisture le\'els and nutrition between these two plants.

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