Whitefly Management in Arizona: Contribution of Natural Enemies to Whitefly Mortality
Steven E. Naranjo, ARS Western Cotton Research Laboratory, Phoenix
Direct-observation studies were conducted to identify causes and estimate rates of mortality of whiteflies over the course of four generations between late June to early September in replicated experimental plots. In plots receiving no whitefly insecticides, predation and dislodgment were major sources of egg and nymphal mortality and overall survival from egg to adult ranged from 1-8.5%. Similar patterns were observed in plots treated with insect growth regulators, except that Knack caused high levels of egg inviability and Applaud was a major source of mortality in small nymphs during the second generation immediately following single applications of these materials. Mortality due to predation was generally lowest for eggs and nymphs in plots treated with a rotation of conventional insecticides reflecting disruption of the predator fauna. Parasitism was a very minor source of mortality throughout. The selective action of the IGRs enhances the abundance and activity of natural enemies resulting in high levels of whitefly control with minimal use of disruptive insecticides. Natural enemies likely contribute to the "extended" residual effects of IGRs so commonly reported by growers.
Each year the scientific and industry community speculate the causes of the patterns of whitefly population fluctuation. Much of the debate has centered on differences observed between 1995 and 1996. The causes range from timely weather in the form of violent monsoons and direct residual activity of the new insect growth regulators (IGRs) in 1996, or to the poor performance of other insecticides due to resistance in 1995. Others credit the increasing effect of natural enemies in cotton due to a reduction in use of broadly-toxic insecticides. In fact, many biotic and abiotic mortality factors impact the population dynamics of whitefly in agricultural systems. The effects of various insecticides are generally well known and recent advances in sampling methods and action thresholds for whitefly have improved the timely and efficient use of this important control tactic in cotton (see Ellsworth et al. 1998). Studying the effects of such factors as predation and parasitism is much more difficult, and we have a poor understanding of how these mortality forces may contribute to pest population regulation. This task is made even harder because of overlapping generations of whitefly in the field and because pest management activities provide further sources of mortality that may enhance or disrupt natural enemies. Understanding the role of predators and parasitoids in the cotton system will require that we examine their effects within the framework of overall pest management.
In 1996 and 1997 we conducted studies to examine natural enemy conservation in relation to different whitefly management practices. These commercial and quasi-commercial scale trials suggested that the use of the IGRs Applaud (buprofezin) and Knack (pyriproxyfen) for whitefly control helps conserve native general predators and whitefly parasitoids (Naranjo and Hagler 1997, Naranjo et al. 1998). Here we present preliminary results of detailed direct-observation studies to identify causes and estimate rates of whitefly mortality in fields where different pest management practices were being examined. These studies were part of an overall effort to demonstrate and evaluate different strategies for whitefly management in Arizona cotton (Ellsworth et al. 1998, Naranjo et al. 1998).
Materials and Methods
The study was conducted using NuCOTN 33B and contrasted four whitefly control regimes; Applaud used first, Knack used first, a rotation of conventional insecticides (1995-IRM), and an untreated control. The threshold for use of IGR treatment was 1 large nymph/disk plus 3-5 adults/leaf (Ellsworth et al. 1996). All conventional insecticide applications were made at 5 adults/leaf (Ellsworth et al. 1995). One insecticide application was made for Lygus hesperus over the entire experiment with Vydate C-LV (1 lb ai/A) on 25 July. All applications were made by ground and seasonal usage of insecticides for the studies described here is summarized in Table 1. Each insecticide regime was replicated 4 times using a randomized block design in a total area of about 8 acres. Additional detail on the entire experiment is provided in Ellsworth et al. (1998).
Cohorts of eggs and settled 1st instar nymphs were established over the course of one pre-spray and 3 post-spray generations between late June through early September in the replicated experimental plots described above. Because there were not enough whiteflies during the pre-spray generation, we used clip cages to introduce adult whiteflies and establish cohorts of eggs and nymphs. Cohorts in the remaining generations were identified from natural populations. Cohorts consisted of approximately 50 individuals of each stage in each plot. The location of each individual was marked on leaves with a non-toxic felt-tip pen. Each stage was then examined every 2-4 days directly in the field with the aid of a hand lens. Sources of mortality were recorded as due to insecticides, predators, parasitoids, inviable (eggs only), unknown and missing. This last category was often presumed to be associated with weather or chewing predation; however, insecticides also may have dislodged insects. In this preliminary report we present only apparent rates of mortality observed for each source. Future analyses will involve the estimation of marginal rates of mortality which corrects for the effects of contemporaneous mortality by multiple factors (e.g. predation, parasitism and insecticides) and allows for more robust comparisons among treatments.
The first post-spray cohort was established one day after spraying and continued for 14 days. The second post-spray cohort was conducted 14-27 days after initial spraying. The final cohort was conducted in untreated plots only. Only one IGR spray was made in the two IGR regimes during these studies. For the conventional regime, two sprays were made during the first post-spray cohort and one spray during the second post-spray cohort.
In the first generation, prior to any insecticide sprays, the major sources of mortality were predation and missing (Table 2). Missing during this period was believed to be due to chewing predation because of large, coincident Collops beetle populations, and the absence of any rain or wind storms. About 6% of the eggs were inviable and 25% of the eggs hatched.
In second generation (1st post-spray generation) inviability was a major source of mortality in all regimes (Table 3). Inviability was highest in the Knack regime (69%), reflecting one of the main modes of action of this insecticide (interference with embryogenesis). Levels of egg predation were similar among the IGR and untreated regimes (19-30%) and lowest in the 95-IRM (9%). Missing was a significant source of mortality in Applaud, 95-IRM and untreated control plots. Egg hatch was lowest in the Knack regime (3%) and highest in the 95-IRM regime (34%).
Overall patterns of egg mortality changed little in the third generation (2nd post-spray generation), except that inviability was very low (< 6%) in all regimes but Knack (19%) (Table 4). Predation was again lowest in the 95-IRM compared with all other regimes. Overall, egg hatch was higher than in the second generation and was highest in the 95-IRM and untreated control (58-66%).
The last generation was observed late in the production cycle, after irrigation termination and only in the untreated control plots (Table 5). Inviability remained low and predation and missing were significant sources of mortality. About 43% of the eggs hatched.
In the first generation prior to any insecticide sprays, mortality was highest during the fourth instar (not shown) and the major mortality factors overall were predation and missing (Table 2). As noted for eggs, missing during this period was believed to be due to chewing predation because of large, coincident Collops beetle populations and the absence of any storm activity. Parasitism accounted for only 5% of observed mortality. About 16% of the nymphs survived to adulthood.
The second generation (1st post-spray generation) showed similar patterns of mortality for the untreated control with the majority of mortality again during the fourth instar and predation and missing being the main causes of death (Table 3). The Knack regime produced similar results to the untreated control and had similar rates of mortality across instars. In contrast, the Applaud and 95IRM regimes had the majority of mortality expressed during the first instar (about 60%) with successively declining rates for subsequent instars. This is a reflection of the temporal similarity of insecticidal action in these two regimes. Applaud, a molting inhibitor, killed most of the insects in the first and second instars, and conventional insecticides are most effective against smaller instars. Mortality rates due to insecticides were highest in these two regimes and consequently mortality rates due to predation were lowest. There was essentially no adult emergence in the insecticide treated regimes (< 1%) and only 4% emergence in the untreated check. Parasitism rates were very low (< 1%) in all plots.
In the third generation (2nd post-spray generation), the patterns of mortality shifted, in part because of the interval since spraying with the IGRs (14-27 days earlier). The untreated displayed the typical pattern of highest mortality during the fourth instar and most killed by predation (Table 4). The Knack regime again mimicked the untreated with similar rates of predation. There was, however, significantly greater survival in the untreated and remaining regimes (about 8-10%) compared to Knack (< 1%). There was less mortality overall due to insecticides in any of the regimes including 95IRM which was the only regime with a spray during this generation. In Applaud and 95IRM regimes, there was more mortality due to missing than predation, while in the untreated the reverse was true. Rates of mortality due to predation were lowest for the 95IRM regime, a reflection of the disrupting influence of conventional insecticides on the predator fauna. Parasitism was slightly higher throughout (2-4%).
In the final generation, observed only in the untreated control plots, the pattern of mortality within the nymphal stage was decidedly different from the previous generations (Table 5). The majority of mortality occurred in the first instar, in part due to a rain event resulting in missing individuals, but also due to higher predation in this instar. Mortality in subsequent instars was lower, and overall survival was much higher in this generation (19% adult emergence). Parasitism was still very low (3%), while predation remained the major source of mortality overall.
We estimated total survivorship from egg to adult by assuming complete survival of first instar crawlers, the one immature stage we did not observe (Table 6). Survivorship was extremely low overall, ranging from 1% during the 2nd generation to about 8.5% in the final generation in untreated control plots. The second highest survivorship was observed in the 2nd post-spray generation in the 95-IRM regime (7.5%). For the immature stage overall, predation and missing accounted for most of the mortality, with the exception that high levels of egg inviability were seen in Knack plots during the first post-spray generation. Due to high levels of egg mortality and because neither Applaud nor the conventional materials used in this study are active against eggs, the overall contribution of these insecticides to total immature mortality was relatively low (< 16%).
Mortality factors were identified and quantitatively measured in natural populations of immature whitefly subject to different commercial management practices at MAC. In plots where no whitefly insecticides were used, predation accounted for a large portion of both egg and nymphal mortality. A large fraction of these stages were also dislodged partially due to weather, but also from the activity of chewing predators. Despite the high levels of control by these natural forces, whitefly populations continued to increase through the season in untreated control plots (Ellsworth et al. 1998). Predation was the largest or second largest source of mortality in the three insecticide regimes, but was generally lowest in the 95-IRM compared with the two IGR regimes. Insecticides exerted significant mortality, but mainly early in the life cycle (eggs or small nymphs) with diminishing direct impact two weeks later. One surprise was the very minor contribution of parasitoids to whitefly mortality within a generation in any treatment regime. These findings are at odds with those determined using standard relative methods to assess parasitoid activity (e.g., Naranjo and Hagler 1997, Naranjo et al. 1998) and indicate that researchers need to be very cautious when interpreting parasitoid impact. The selective action of the IGRs led to conservation of predators populations (Naranjo et al. 1998) which translated directly into enhanced levels of predation compared with the more disturbed conventional regime. IGRs combined with Bt cotton and the judicious use of broad-spectrum inputs could lead to better target and secondary pest control through enhanced natural enemy conservation.
We thank Virginia Barkeley, Celso Jara, Jeanette Martin, Elena Odenheim and Greg Owens for expert technical assistance.
This is a part of publication
AZ1006: "Cotton: A College of Agriculture
Report," 1998, College of Agriculture, The
University of Arizona, Tucson, Arizona, 85721. Any
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