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Non-Destructive Detection of Diapause in the Western Tarnished Plant Bug, Lygus Hesperus Knight (Hemiptera: Miridae) Dale W

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Non-Destructive Detection of Diapause in the Western Tarnished Plant Bug, Lygus Hesperus Knight (Hemiptera: Miridae) Dale W The Journal of Cotton Science 22:162–170 (2018) 162 http://journal.cotton.org, © The Cotton Foundation 2018 ARTHROPOD MANAGEMENT AND APPLIED ECOLOGY Non-Destructive Detection of Diapause in the Western Tarnished Plant Bug, Lygus hesperus Knight (Hemiptera: Miridae) Dale W. Spurgeon* and J. Joe Hull ABSTRACT he western tarnished plant bug, Lygus hesperus TKnight, is a key pest of western crops including The western tarnished plant bug, Lygus hespe- cotton (Gossypium spp.). Despite the economic rus Knight, overwinters in an adult diapause. How- importance of this pest, several aspects of its basic ever, the ecological implications of diapause to L. biology remain poorly investigated, including hesperus population survival are poorly understood. the ecology of overwintering. Lygus hesperus is Enhanced understanding of L. hesperus overwinter- considered to overwinter as diapausing adults in ing ecology might reveal opportunities to develop response to short days (Beards and Strong, 1966; ecologically-based management tactics. One fac- Leigh, 1966). Diapause in southern populations tor limiting efforts to understand the dynamics of is thought to terminate in late-fall to early-winter overwintering is lack of a non-destructive method (Beards and Strong, 1966; Strong et al., 1970), but to distinguish diapause. We evaluated a suite of the mechanism facilitating population survival from external characters for utility in distinguishing dia- early-winter until the availability of spring-time pausing from non-diapausing L. hesperus. Abdomen hosts has been a matter of conjecture. coloration of female L. hesperus was a highly reliable In Arizona, only a portion of the L. hesperus indicator of diapause status. During these studies, population exhibits the short-day diapause response a single female (0.6%) was misclassified using the (Spurgeon and Brent, 2015), and the ecological fate of abdomen color criterion. For male L. hesperus, the reproductive (non-diapausing) insects is unknown. corrected abdomen length (abdomen length / head Cooper and Spurgeon (2012, 2013, 2015) found the capsule width) was used to predict diapause status durations of all L. hesperus life-stages were substan- with ≈84% accuracy. Both criteria provided im- tially extended by low temperatures. These findings proved accuracy compared with earlier reports, in suggest the potential for a portion of southern L. hes- part because their application was limited to adults perus populations to overwinter as slow-developing of a specific age (10 d) and reared at a specific tem- eggs, nymphs, and reproductive adults. There is also perature (26.7°C). Application of these criteria to potential for a second generation of diapausing adults. individuals that were subsequently starved allowed Brent (2012) illustrated a color phenotype associated us to unambiguously distinguish the survival func- with diapause in adult L. hesperus, and this phenotype tions of diapausing and non-diapausing males and has been observed in alfalfa (Medicago sativa L.) in females. Although neither criterion (abdomen color late-winter (Spurgeon, 2018). Studies of diapause for females, corrected abdomen length for males) induction in the laboratory, using F1 progeny of field was error-free, both offer sufficient accuracy to adults collected throughout the year, have not exhib- justify their use in studies of the ecology, physiology, ited seasonal patterns (Spurgeon and Brent, 2015; or molecular biology of L. hesperus diapause. These Spurgeon, 2017), which implies absence of a mater- criteria provide the ability to non-destructively dis- nal effect on diapause induction. These observations tinguish the diapause status of adult L. hesperus with prompted Spurgeon (2017) to suggest that some L. reasonable accuracy, and should enhance efforts hesperus developing during winter and early spring to better understand diapause and overwintering may exhibit the same diapause response as insects ecology in this important pest species. developing in late-summer and fall. Collectively, these reports suggest a more dynamic overwintering ecology for L. hesperus than is generally recognized. Better understanding of L. hesperus overwintering D.W. Spurgeon*, and J.J. Hull. USDA, Agricultural Research Service, Pest Management and Biocontrol Research Unit, ecology will require improved knowledge of the 21881 N. Cardon Lane, Maricopa, AZ 85138 timing and duration of adult diapause, as well as its *Corresponding author: [email protected] ecological impact on population survival. SPURGEON AND HULL: NON-DESTRUCTIVE DIAPASUE DETECTION IN THE TARNISHED PLANT BUG 163 Ecologically relevant estimates of host-free lon- ventral aspect, and they are obscured by the wings gevity of diapausing L. hesperus are not available. from the dorsal aspect. Displacement of the wings to Brent et al. (2013) reported the host-free longevity of allow observation from the dorsal aspect often results diapausing L. hesperus adults was about three times in injury to the insect. We hypothesized that the ac- that of non-diapausing adults. However, the insects curacy of color-based diapause assessments might be were obtained from an established colony known to improved if temperature conditions and adult ages exhibit a reduced diapause response (Brent and Spur- were narrowly restricted, and that compared with color geon, 2011), and the starved adults were not provided classes, measurements of the abdomen taken from the a source of moisture. The importance of moisture to ventral aspect might provide less subjective criteria for overwintering success of L. hesperus was previously distinguishing diapause status of L. hesperus adults. reported by Fye (1982). Consequently, the median survival of diapausing adults observed by Brent et al. (2013) was <200 h at 28°C. In contrast, Cooper and Spurgeon (2015) provided newly-eclosed, un- fed, non-diapausing adults a source of moisture and observed median host-free longevities of 6.1 (males) to 8.8 d (females) at 26.7°C. Therefore, improved estimates of host-free longevity of diapausing L. hesperus might be obtained in experiments where the insects are provided a moisture source. A major impediment to accurate assessment of host-free longevity is the inability to distinguish diapausing from non-diapausing individuals at Fig. 1. Illustration of abdomen distension caused by fat body L. hesperus the beginning of the survival period. The diapause hypertrophy in a diapausing male (a) compared with a reproductive male lacking fat body hypertrophy (b). condition in L. hesperus is usually determined by dissection, which precludes subsequent observa- tions of the same individuals. Although estimates MATERIALS AND METHODS of the incidence of diapause based on a dissected subsample of each survival cohort are useful, this We evaluated external morphological characters approach increases necessary sample sizes and in- as indicators of diapause in three separate experi- troduces the potential for sampling error in the dia- ments: 1) an initial experiment to determine the most pause estimate. A reliable non-destructive indicator promising characters for further evaluation, 2) a sur- of diapause would decrease the cost of the research vival experiment to determine whether application of while improving the accuracy of survival estimates. the initial diagnostic rules were related to host-free Brent (2012) reported that diapausing L. hesperus survival, and, whether those rules could be modified could be distinguished from reproductive bugs by to improve accuracy of classifications, and 3) a vali- abdominal coloration, with average accuracies of 84% dation experiment utilizing an independent observer for females and 67% for males. Spurgeon and Cooper and small sample sizes typical of laboratory studies. (2012) used abdomen color to predict reproductive Each repetition of each experiment used the F1 prog- status of adult L. hesperus, but error rates of those eny of about 500 L. hesperus adults collected from predictions were generally high and varied markedly fields of alfalfa near Maricopa, AZ. Field-collected with rearing temperature. In both of these studies, dia- adults were maintained at 27±1°C with a 14:10 (L:D) pause or reproductive status was assessed at a variety h photoperiod within 0.03-m3 screened cages. Cages of adult ages. However, Spurgeon and Brent (2010) were provisioned with shredded paper, pods of green recommended diapause assessments at adult ages bean (Phaseolus vulgaris L.), raw seeds of sunflower ≥10 d at 26.6°C. In addition, fat body hypertrophy (Helianthus annuus L.), and a water-saturated cotton associated with diapause typically causes the dorsal pad. The bean pods, which also served as oviposition plate of the male’s abdomen to be less flattened, and substrate, were replaced three times weekly. Bean the intersegmental membranes to be more prominent, pods containing eggs were held on shredded paper compared with reproductive males (Fig. 1). However, within 3.8-L plastic buckets with screened lids under these characters are difficult to observe from the the same conditions as the parent adults. JOURNAL OF COTTON SCIENCE, Volume 22, Issue 3, 2018 164 Experimental insects were obtained as newly- hatched first instars which were held individually within 18-ml snap-cap vials (Thornton Plastics, Salt Lake city, UT). Vial caps were ventilated with nylon organdy. Each insect was provided a section of green bean pod (≈5 cm long) that was replaced three times weekly. Cut ends of the pod sections were sealed
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