Section F: Integrated and Area-wide Pest Management Approaches, and Crop Management Systems - 2000

Section F (1999)

Plenary Session Summary:

Investigator's Name(s):

Steve E. Naranjo¹ & Peter C. Ellsworth² (Part I)

Peter C. Ellsworth² & Steve E. Naranjo¹ (Part II)

Affiliation & Location: ¹USDA-ARS, Western Cotton Research Laboratory, Phoenix, AZ & ²The University of Arizona, Department of Entomology & Maricopa Agricultural Center, Maricopa, AZ

Research & Implementation Area: Section F: Integrated and Areawide Pest Management Approaches, and Crop Management Systems.

Dates Covered by the Report: 1996-1999.

Whitefly Population Dynamics: Why Use Life Tables & What Do They Tell You?

Studies of population dynamics of any species seeks to explain the underlying mechanisms and consequences of changes in population density. Population density is a function of births, deaths, immigration and emigration. Studies into these questions may be descriptive, theoretical, experimental or demographic. For the past four years we have endeavored to use the demographic approach known as life tables to describe, explain, and contrast whitefly population dynamics in managed and unmanaged cotton systems.

Life tables quantify the probability of dying while assigning a specific cause of death. Underlying mechanisms of population change may be inferred by study of these mortality rates by insect stage (age) and/or by mortality factor. Using procedures that adjust for contemporaneous mortality, marginal rates can help identify the relative importance of various mortality factors in population change. From this, we can conduct K-factor analyses, estimate irreplaceable mortality, and make specific experimental treatment contrasts.

The identification of these factors within IPM systems is crucial to understanding how that system functions to control whiteflies. By contrasting to unmanaged cotton systems, we can demonstrate the role that each mortality factor (and insect stage) plays in regulating population density. We can infer how and when specific insecticides work (conventional and insect growth regulators [IGR]) and when biological control agents might be most compatible with other management practices, especially chemical controls.

Under large plot replicated conditions (>0.25 ha/plot) in Maricopa, AZ, four different whitefly management ‘regimes’ were implemented (Knack™ used first, Applaud™ used first, non-IGR conventional chemistry, and an untreated check [UTC]). To monitor insecticidal inputs for Lygus control, split-plots were established each year (1997-1999) such that a Lygus-untreated and treated split were maintained throughout the season. The final key pest of this system, pink bollworm, was managed through the use of Bt transgenic cotton throughout all studies. No other insecticides were required for any other pests.

Over 14 whitefly generations (1997-1999) in unmanaged cotton (UTC), generational survivorship (egg-adult) ranged from 0% to 27%. Sources of mortality included obvious predation (usually from sucking predators), parasitism (apparent in the 4th instar by Eretmocerus or Encarsia spp.), ‘missing’ (which is a factor that partitions among predation, weather-related or insecticide-related dislodgement), unknown (which may be related to physiological mortality), inviability (which was in the egg stage only and refers to lack of hatching), and "sunken" (an unknown factor that resulted in a typically sunken, intact, fourth instar). Marginal rates of mortality over this period showed the following order for factors (highest to lowest): predation > missing > unknown > inviability > parasitism; and for stage-specific rates: egg > N4 > N3 > N2 > N1. Irreplaceable mortality, or that mortality which could not otherwise be replaced by some other contemporaneous factor, showed the following patterns of importance (highest to lowest) by factor: predation > missing > unknown > inviable > parasitism. K-factor analyses which estimates the importance of a single mortality source or stage on the overall generational mortality showed the following order of importance (highest to lowest) by factor: inviable > missing > predation > ‘sunken’ > unknown > parasitism; and by stage: egg > N4 > N2 > N1 > N3.

These analyses point to the importance of egg inviability to the overall dynamics of whitefly populations in unmanaged cotton. They also show the relative minor impact that parasitism has on whitefly population dynamics. Predation, meanwhile, plays a significant role especially when considering that a portion of the ‘missing’ category was likely as a result of removal by predators. The prominence of ‘missing’ as a source of mortality is also of interest. High rates of ‘missing’ were noted following major weather disturbances associated with the summer monsoon season. Nymphs and even eggs were removed by high winds, dust, and/or rain over the course of these studies.

Whitefly population dynamics were simulated using an unpublished whitefly model (Naranjo, DeGrandi-Hoffman, unpubl) and the specific measured rates of mortality over 4 to 6 successive generations within a year. Where the observed deviates from the model prediction, we might infer the effects of adult movement. In each year simulated, the first generation simulation underpredicts the actual measured adult densities. The discrepancy is likely at least partly due to the immigration of adults from sources external to cotton early in the season. Likewise in the late season, the fourth or fifth generation simulation overpredicts the actual adult densities measured. This would suggest the effects of emigration, when cotton develops through to cut-out and ceases to be an attractive host for whitefly adults. The intervening generations were extremely stable, i.e., there were no apparent deviations between the predicted and observed densities and thus little presumed effects of migration.

Synthetic survivorship curves made up of the 6-12 generations of whiteflies treated or untreated with insecticides show just how fragile a state the ‘managed’ condition is relative to unmanaged. The difference between the two curves implies the impact of insecticides on the system. Somewhat counter-intuitively, the gap between curves is rather low with only a 2% difference in survivorship to adulthood. Thus, it would seem that IPM is challenged to achieve a rather "minor" level of irreplaceable mortality in order to accomplish economic control of this insect.

Chemical control (including IGRs) has been dramatically successful in the campaign to manage whiteflies in Arizona. Yet, our results from unmanaged cotton would suggest that they are assisting with only an extremely small amount of irreplaceable mortality. In situ life tables provide us the tools necessary for identifying and quantifying these changes from the unmanaged condition. Marginal, stage-specific mortality from insecticides clearly identify the mode of action of the respective chemistries used. Knack provided 4-38% marginal mortality in the egg stage and was the only compound that had significant insecticidal effects on that stage. Applaud had the highest rates of marginal mortality in the young instars (N1 & N2), while the conventional chemistry was capable of killing some younger instars. Insecticide mortality was not a major factor in the larger instars, though Knack and conventional chemistry had appreciably higher rates of marginal mortality than Applaud. Knack, as a juvenoid, is known to affect egg viability and metamorphosis in N4. Applaud, a molting inhibitor, disrupts the instar transitions up to but excluding N4.

The other major feature of the IGRs, however, is their selectivity towards whiteflies. This feature is thought to enhance or at least conserve the extant predator populations. Our studies of egg cohorts over each year show differing contributions of insecticides vs. predation effects on mortality. They do, however, show a consistent trend of higher rates of marginal mortality due to predation in the IGR regimes when compared to the conventional chemistry, yet similar to the UTC. This general trend was observed for the nymphal stages as well; however, it was much more pronounced and consistent one generation post-application of the IGRs.

Management of whiteflies at the same location with the same conventional chemistry since 1995 has revealed a different spray requirement for whitefly control each year (1-6 sprays). Clearly, different forces were acting on these populations each year. Yet every year populations initially reached the recommended threshold with a population fate or trajectory of unknown consequence. This presents growers and practitioners with a critical challenge. That is, the ability to predict the whitefly population dynamics in the context of all interrelated mortality factors, including insecticides. Grower experience tells us that the IGRs "last" 30 days or more. Our work which directly measures insecticidal mortality clearly indicates that the bulk of insecticide mortality occurs during the first 14 days. Little if any IGR-related mortality occurs beyond this point. What then is the source of this additional mortality? Our results would suggest that the IGRs provide for an environment where predation can occur at rates similar to untreated cotton. Thus, when predators are abundant, growers can expect a large contribution of predation to the "bioresidual" of the IGRs. When predators are low, however, these effects maybe less dramatic, such as when a broad-spectrum insecticides are required for Lygus control. Looking at insecticide-related effects over 8 generations, irreplaceable mortality rates are surprisingly low (0.001-0.08). This re-enforces the fact that insecticides are providing for an exceedingly small, yet strategic source of mortality, and that with the IGRs an additional boost is provided by the natural enemies that are conserved.

Ultimately, an IPM plan has been implemented that includes a significant component of chemical control. In situ life table analyses have helped explain when and why the current approach is successful and may be pointing to additional opportunities for control of whiteflies within the cotton system. There are, however, significant limitations to this approach. First, it’s hard work! Direct observation under field conditions over long summer periods demands the discipline and training necessary to routinely and carefully identify sources of mortality, insect age, and time of death in the field. Importantly, too, our studies represent in this case 14 different snapshots in time, and extension to other periods or other generational starting points might produce different results. For example, all of our first post-spray cohorts were marked and initiated 1-day post-application. Strictly speaking, our life tables remain incomplete, because they lack any direct estimates of crawler mortality, the 12 hr long ambulatory stage of the first instar. The duration is so short that its impact may be minor; however, anecdotal evidence would suggest that crawlers are subject to conventional insecticides and weather-related dislodgement. Finally, no effort was made to assess adult mortality directly or the impacts of immigration and emigration, though our work can assist in assessments of adult movement.

The power of our approach has yielded some important findings with respect to IPM and biological control. Insights derived from the life tables provide explanation for the large annual variation observed in whitefly population dynamics in Arizona cotton. The specificity and modes of action of the IGRs were identified and quantified. Through the use of simulation our results provide for new hypotheses about the effects of immigration and emigration in the system. Weather as a source of mortality in whiteflies has been discovered, described, and quantified. Even as uncontrollable and unpredictable as this source may be, its impact on the system was fairly consistent over years. It certainly re-enforces the recommendation to hold-off chemical controls if a significant weather event is imminent. Most importantly, however, to IPM, we have a systems level evaluation in which extrapolation and modeling are unnecessary. Mortality was measured directly and in situ within production systems of relevance to growers in the West. Ultimately, life tables become a powerful tool for measuring the bioefficacy of novel compounds and other management tactics.

In terms of biological control, there are a number of important conclusions. Most interesting is the relatively weak role that parasitoids have, even in unmanaged cotton systems here. This fact calls into question even the measurement systems that are most widely used to assess "parasitism rates." Predation and ‘missing,’ on the other hand, were major sources of mortality in both managed and unmanaged systems. Thus, tactics which capitalize on this fact, such as selective IGRs, result in better natural enemy conservation and ultimately better control. This, too, re-enforces the recommendations for IGR use to growers which includes 1) use them first instead of broad-spectrum chemistry, 2) do not tank-mix with broad-spectrum chemistry, and 3) provide for enough time to see their effects and maximize their "bioresidual" through natural enemy conservation.

Investigator’s Name(s): D. H. Akey and T. J. Henneberry.

Affiliation & Location: USDA, ARS, Western Cotton Research Laboratory, Phoenix, AZ 85040-8803.

Research & Implementation Area: Section F: Integrated and Area-wide Pest Management Approaches, and Crop Management Systems.

Dates Covered by the Report: June 1997-September 1999

Progress in Development of IPM for Upland Cotton in Arizona Using Biorational and Biopesticide Agents for Control of Silverleaf Whitefly (SLWF) Bemisia argentifolii and Other Cotton Pests

An Integrated Pest Management (IPM) program of biorational/biopesticide agents was tested for impact on beneficial arthropods. Biorational agents replaced conventional chemistries and considered Insecticide Resistance Management (IRM). Deltapine cotton was planted and furrow irrigated: 1997- DP 5415; 1998-9- NuCOTN 33^B. Plots were 0.10ac; separated by fallow rows and alleys. Spray applications were by ground boom: 1 center nozzle/row, and inter-row drops with 2 or 4 swivel nozzles angled upward, applied at 80 or 250 psi and 30 gal/ac. Sweeps were taken weekly (25/ each plot) for Lygus, predators, parasites, and thrips. Entomopathogenic fungi used against silverleaf whitefly included: Beauveria bassiana, as Naturalis ® L (Troy Biosciences) 10 oz product/ac, 2.3x 10⁷ conidia/ml, as Mycotrol ® (Mycotech)[WP in 1997 0.5 lbs/ac, 2 x 10 ¹³ spores/lb, ES in 1998 and 9 0.5 pt/ac, 2 x 10 ¹³ spores/qt], and Paecilomyces fumosoroseus as PFR- 97 ™ (Thermo Trilogy), 0.025 lbs / gal, 1x 10 ⁹ CFU/gm equivalent 20% product. Insect growth regulators included: azadirachtin as Bollwhip ™ (Thermo Trilogy), 4.5% form., at 3, 6, and 9 oz in 1997 and 6 oz/ac in 1998-9, product/ac, buprofezin as Applaud ™ 70WP (AgrEvo) 0.35lb. AI/ac; and pyriproxyfen as Knack ™ 0.86 EC (Valent USA) 0.054 lb AI/ac. As SLWF populations met action thresholds (Univ. AZ Ext.) applications were made. Biorationals for other insects included: pink bollworm sex pheromone, alone or baited (^1/10rate chlorpyrifos); BT gene-NuCOTN 33^B; diflubenzuron -Dimilin ® ; BT sprays- e.g. DiPel ® ; and K-salt of fatty acid- M-Pede ™ (all per label). Other insects included: pink bollworm, beet armyworm, cabbage looper, and saltmarsh caterpillar. Treatments were random block in design and included a "Best Agricultural Practice" (BAP) and an embedded control; plus, a single1-ac block control. Numbers of treatments (t) and spray applications (sa), respectively, were: 1997,16 t, 12 sa; 1998, 13t, 9sa;1999, 10t, 8sa. Efficacies are given as % reduction from block control. In 1997, Bollwhip ™ was effective at controlling SLWF at all three rates. PFR-97 efficacies were: 83% for eggs; 78 and 75% for small and large nymphs, respectively. BAP (Applaud ™ and Knack ™ ) efficacies were similar. Naturalis ® L and Mycotrol ® gave excellent control of SLWF(>PFR-97). 1998 had unfavorable weather for cotton production. SLWF reached action thresholds late July. In an 8-day period, 2 sprays were applied, then populations dropped sharply and did not rebound. Bollwhip ™ efficacies were: eggs, 37 %; small nymph, 32 %; and large nymphs, 66 %, respectively. Efficacies were: eggs; 41 and 23 % as Naturalis ® L and Mycotrol ® ES, respectively; and PFR-97 ™ , 30%; small nymphs; 6 and 19 % as Naturalis ® L and Mycotrol ® ES, respectively; and PFR-97 ™ ; 24%; large nymphs, 28 and 8 % as Naturalis ® L and Mycotrol ® ES, respectively; and PFR-97 ™ , 4 %. Applaud ™ , the only BAP treatment applied, had egg, small nymph, and large nymph efficacies of 28, 37, 38 %, respectively. Though low, these efficacies held SLWF populations below action thresholds. In 1999, Bollwhip ™ efficacies were: eggs, 37 %; small nymph, 32 %; and large nymphs, 66 %, respectively. Mean number of large nymphs in Bollwhip ™ plots did not exceed treatment threshold. BAP, Buprofezin then pyriproxyfen 2wks later, provided season control with egg, small nymph, and large nymph efficacies of 42, 75, and 95 %, respectively. Efficacies of Mycotrol ® ES and Naturalis ® L respectively, were as follows: 26 and 41% against eggs; 23 and 46% against small nymphs; and 56 and 74% against large nymphs. Efficacies of PFR -97 ™ were: 50% against eggs; 49% against small nymphs; and 78% against large nymphs. Attempts to control all pests, with biorational agents, replacing conventional chemistries, failed because of strong Lygus pressure: in 1997, 1 application of oxamyl, Vydate ® ; 1998 several required but only 1 applied; and 1999, entire season-above threshold but no conventional applications made. Presently, we have no biorational agents for Lygus.

Investigator’s Name(s): Pamela K. Anderson.

Affiliation & Location: Centro Internacional de Agricultural Tropical (CIAT).

Research & Implementation Area: Section F: Integrated and Areawide Pest Management Approaches, and Crop Management Systems.

Dates Covered by the Report: 1996 - 1999

The CGIAR Systemwide IPM Project on Sustainable Integrated Management of Whiteflies as Pests and Vectors of Plant Viruses in the Tropics

In 1995, the Systemwide Program on Integrated Pest Management (SP-IPM) was established by the Consultative Group on International Agricultural Research (CGIAR) in recognition of the importance of IPM to sustainable development. One of the objectives of the SP-IPM is to promote collaboration between CGIAR centers, national agricultural programs (NARS), and basic research institutions, on topics of mutual concern and endeavor. To date, the steering committee has approved 12 areas of collaboration, including whiteflies as pests and vectors in the Tropics. In 1996, the Center for International Tropical Agriculture (CIAT) in Cali, Colombia was designated as the convening center to formulate a systemwide proposal on the Sustainable Integrated Management of Whiteflies as Pests and Vectors of Plant Viruses in the Tropics.

To that end, a Task Force meeting for the CGIAR Whitefly IPM Project was held at CIAT from February 13-15, 1996. The Task Force meeting included 24 participants representing the CGIAR centers, national and regional agricultural programs, and basic research institutions. The Task Force agreed on three priority problems to be addressed by the Project: 1) whiteflies as pests in Tropical highlands; 2) whiteflies as vectors of plant viruses in mixed cropping systems in the low-to-mid-altitude Tropics; and 3) whiteflies as pests and vectors of plant viruses in cassava.

The Whitefly IPM Project initiated in 1997 with Phase 1, start-up, funding from the Danish International Development Agency (Danida). The objective of Phase 1 was to: 1) form a pan-tropical network for professionals working on whiteflies and whitefly-transmitted viruses in the Tropics; and 2) improve the characterization of the prioritized whitefly problems in the Tropics, in order to lay the foundation for a sound research agenda as well as select critical geographical areas (hot spots) to target intensive research activities and IPM component testing for Phase 2 work.

Since 1997, the Project has continued to expand. The formal partners in the network now include:

International Agricultural Research Centers (CIAT, ICIPE, IITA, AVRDC, CIP)

Advanced Research Organizations (in Australia, Germany, New Zealand, UK, and the US)

NARS institutions in 30 countries across the Tropics (12 in Latin America, 10 in Africa, 8 in Asia).

In addition to Danida, the Donor Partners now include: the Australian Center for International Agricultural Research (ACIAR), the Department for International Development (DFID) of the UK, the FAO-Global IPM Facility (GIPMF), the New Zealand Ministry of Foreign Affairs and Trade (MFAT), the United States Agency for International Development (USAID) and the Agricultural Research Service of the United States Department of Agriculture (USDA--ARS).

In August of 1999, the USDA--ARS, signed a Scientific Cooperative Agreement with CIAT. The research objectives of the SCA, to be carried out in collaboration with the USDA--ARS, laboratory in Fort Pierce, Florida, will focus on the epidemiology of whitefly-transmitted geminiviruses. The service objectives of the SCA are to link the US Whitefly Research and Action Plan with the CGIAR Whitefly IPM Project. The objective of this talk is to present a tentative proposal for initiation of those linkages and to stimulate discussion on that proposal.

Investigator's Name: S. J. Castle.

Affiliation & Location: USDA-ARS, Western Cotton Research Laboratory, Phoenix, AZ.

Research & Implementation Area: Section F: Integrated and Areawide Pest Management Approaches, and Crop Management Systems

Dates Covered by the Report: Summer 1999

Reduced Whitefly Infestations in Cotton Using a Melon Trap Crop

Trap cropping involves the manipulation of crop stands in time and space with the objective of concentrating a pest species within the trap crop rather than the main crop. This can be achieved by using a trap crop that is the same species or cultivar as the main crop, but which is grown asynchronously to the main crop in order to concentrate either early or late pest invaders. Alternatively, a trap crop that is contemporaneously grown with the main crop will probably involve a different plant species that is more attractive to the target pest than the main crop. Assuming that either approach is effective at concentrating the target pest, then the real challenge begins with managing the pest in the trap crop effectively to prevent the trap crop from becoming a source of the pest rather than a sink.

Whiteflies are amenable to trap crop management for a number of reasons. Foremost is that whiteflies are highly polyphagous, utilizing many different crop, ornamental and wild hosts, but also are differentially attracted to their various plant hosts. Basic differences among plant species in food quality and as reproductive hosts probably determine why certain plant species accumulate and generate more whiteflies than other species. Whatever the mechanism(s), years of observation and data collection in the field have amply demonstrated that much higher numbers of whiteflies are found on melon plants than on other crop plants. In addition to putative superior host characteristics for silverleaf whiteflies, the use of melons as a trap crop is advantageous in that cultivation and pest management practices are well determined. Thus, there is greater certainty to manipulating an agronomically-proven trap crop for the intention of minimizing the impact of whiteflies on the cotton main crop than would be true for a typically non-crop trap host.

A second year of field experiments was completed in 1999 at the Maricopa Ag Center in Arizona to explore the potential of using a melon trap crop to reduce whitefly infestations in cotton. The experimental design was altered from 1998 to gain isolation among treatment blocks by using 4 separate fields that helped to avoid the influence of one treatment upon the other. A consistent response of significantly fewer whiteflies in cotton planted within a surrounding melon trap crop, relative to the same area of cotton without the trap crop, was observed throughout the July-September sampling period. Better chemical management of whiteflies in the melons during the second season helped to reduce the large differential in whitefly densities between melons and cotton observed the previous year, but preferentially contributed to a greater differential observed between melon-protected cotton and unprotected cotton. Although the infestation buildup was delayed and the season-long densities of whiteflies in the melon-protected cotton were reduced, the action thresholds for treatment with IGRs were ultimately attained and exceeded. In the present management environment of perhaps only 1 IGR treatment per season, it is unlikely that the melon trap crop approach would provide acceptable control unless a grower was willing to tolerate late-season whitefly densities higher than the current IPM recommendations.

Investigator’s Name(s): S. M. Greenberg¹, T. Sappington¹, Tong-Xian Liu², and G. W. Elzen¹.

Affiliation and Location: ¹Kika de la Garza Subtropical Agricultural Research Center, ARS-USDA

2413 E. Highway 83, Weslaco, TX 78596; ² Texas Agricultural Experiment Station, Texas A&M University, 2415 E. Highway 83, Weslaco, TX 78596-8399.

Research & Implementation Area: Section F: Integrated and Areawide Pest Management Approaches, and Crop Management Systems.

Dates Covered by the Report: 1999

Preliminary Data of the Effects of Cotton Defoliant Chemicals on Bemisia Argentifolii Mortality and its Parasitoid Survival

Lethal and sublethal effects of two commonly used defoliants, Def and Dropp, on whitefly, Bemisia argentifolii, and its parasitoids, Eretmocerus eremicus and Eretmocerus hyati, were evaluated in laboratory and greenhouse tests. Whitefly eggs and adults were more susceptible to defoliant treatments than larvae. The reduction in feeding sites differentially affected whitefly nymph mortality depending on instar. Sublethal effects of Def, Dropp or their mixture on whitefly were manifested through reduction of percentage female progeny and the number of eggs deposited per female per day after spraying young nymphs. The timing of application significantly affected parasitoid survival. After defoliant treatments of whitefly nymphs parasitized with early instar E. eremicus larvae, the number of parasitoid female progeny was significantly reduced and their longevity was significantly shorter than those of controls.

Investigator’s Name(s): Luko Hilje¹ & Philip A. Stansly².

Affiliation & Location: ¹ Plant Protection Unit, CATIE. Turrialba, Costa Rica, and ²Southwest Florida Research & Education Center (SWFREC), University of Florida, Immokalee, Florida.

Research & Implementation Area: Section F: Integrated and Areawide Pest Management Approaches, and Crop Management Systems.

Dates Covered by the Report: August 1997-August 1999

Living ground covers are effective for managing whitefly-vectored geminiviruses in tomatoes

The Tomato Yellow Mottle Virus (ToYMoV), so far reported only for Costa Rica, is one of some 17 geminiviruses affecting tomatoes in the Americas, and is vectored by Bemisia tabaci. The impact of diseases caused by these viruses on crop yield depends on plant age at time of infection, and is greatest during the first eight weeks after germination (critical period). In the search for management approaches to deal with both whiteflies and geminiviruses, a preventative scheme suited for resource-poor growers who normally plant staked tomatoes on small plots (< 0.5 ha), has been proposed. This scheme focuses on minimizing contact between the vector and the tomato plant during the critical period, and includes protection of seedbeds with tunnels of fine netting, as well as the use of living mulches after transplanting, which appear to mask or conceal the crop from immigrating viruliferous whiteflies.

The role of living ground covers in masking tomatoes from whiteflies in Costa Rica was appraised in large plots (2400 m²) over two years, to minimize interference between treatments. A randomized complete block design was used, with four replicates, each occupying an entire location over one season. Treatments consisted of six types of ground covers: Arachis pintoi (perennial peanuts) (Leguminosae), the low-growing weed "cinquillo" (Drymaria cordata, Caryophyllaceae), coriander (Coriandrum sativum, Umbelliferae), silver plastic, bare ground treated with imidacloprid (commercial standard), and bare ground untreated (absolute control). Living covers were established well before tomatoes were transplanted. Silver plastic (silver/black, coextruded, 56" x 1.25 Mls; Olefinas S.A., Guatemala) was put in place over the 30 cm-wide bed two weeks before transplanting. Imidacloprid (Confidor 70 WG; Bayer) was applied to the foliage at the recommended rate (9 g/ 40 m² of seedbed surface) a week before transplanting, and two drench applications (250 g/ha) two and four weeks later. No other insecticides were used in any plot during the rest of the season.

Silver plastic was the best treatment in terms of reduction of incoming whitefly adults, delay of ToYMoV dissemination, reduction of disease severity, yield (36 t/ha) and net profit (US$ 30,347/ha). Normal yields in Costa Rica range from 21-35 t/ha. Silver mulch was followed by living covers (with yields ranging from 17-22 t/ha, and net profits from $ 8-16,000/ha) and bare ground with imidacloprid (15 t/ha, $ 5,700/ha). Yield from control plants was as low as 5 t/ha and losses amounted to $ 2,500/ha. Among living covers, on the average, perennial peanuts provided both the highest yield and net profit (22 t/ha and $ 16,000/ha), followed by coriander (19 t/ha, $ 10,000/ha) and Drymaria (17 t/ha, $ 8,000/ha). However, yields in one of the replicates were as high as 25 t/ha (coriander), 36 t/ha (Drymaria) and 40 t/ha (perennial peanuts). Furthermore, since coriander provides additional economic returns when sold ($ 5,000/ha, on the average), and is much easier to establish and remove than the other living covers, it is being recommended for commercial use. Nevertheless, control by either silver plastic or living covers broke down under extremely high inoculum pressure in one replicate. Thus, individually applied preventative and curative management may have to be supplemented with area-wide preventative approaches, such as planting dates and crop-free periods, to successfully manage the geminivirus-whitefly complex in Costa Rica.

Investigator’s Name(s): Mark S. Hoddle¹, R. G. Van Driesche², S. M. Lyon², & J. P. Sanderson³.

Affiliation & Location: ¹Department of Entomology, University of California, Riverside, CA 92521, ²Department of Entomology, University of Massachusetts, Amherst, MA 01003, ³Department of Entomology, Cornell University, Ithaca, NY 14583.

Research & Implementation Area: Section F: Integrated Pest Management Approaches, and Crop Management Systems.

Dates Covered by the Report: July 1997 - July 1999

Compatibility of Selected Insect Growth Regulating Insecticides with the Whitefly Parasitoid Eretmocerus Eremicus for Control of Bemisia Argentifolii on Poinsettias

Five insect growth regulators (IGR's), Applaud (buprofezin), Knack (pyriproxyfen), Precision (fenoxycarb), Fulfill (pymetrozine), Enstar II (S-Kinoprene) were examined in the laboratory and greenhouse for compatibility with the silverleaf whitefly (Bemisia argentifolii) parasitoid Eretmocerus eremicus. Specifically, we quantified adult E. eremicus mortality foraging on poinsettia leaves without whitefly nymphs when exposed to residues of 6, 24, and 96 hours of age. Parasitoid mortality resulting from host feeding by female wasps on B. argentifolii nymphs treated with IGR's 24 and 96 hours of age were determined also. The effect of IGR's on developing E. eremicus larvae was determined by treating parasitized whitefly nymphs 5 and 14 days after they had been parasitized by E. eremicus. The repellancy of aged IGR residues to foraging female E. eremicus was quantified by visually observing wasps that had the choice of foraging on either a treated or untreated section of poinsettia leaf. All experiments used water as a control treatment. Greenhouse level experiments were conducted to determine the efficacy and economics of low parasitoid release rates (one female E. eremicus released per plant per week of the growing season) with selected IGRs for B. argentifolii control on commercially grown poinsettias. Efficacy of combining IGRs and E. eremicus was determined by comparing whitefly control to greenhouses in which parasitoid releases alone were made and greenhouses in which growers used systemic insecticides for whitefly control.

Investigator’s Name(s): Tong-Xian Liu

Affiliation and Location: Texas Agricultural Experiment Station, Texas A&M University, 2415 E. Highway 83, Weslaco, TX 78596-8399

Research & Implementation Area: Section F: Integrated and Areawide Pest Management Approaches, and Crop Management Systems

Dates Covered by the Report: 1999

Population Dynamics of Silverleaf Whitefly on Spring Collard and Relationship to Yield in the Lower Rio Grande Valley of Texas

Seasonal population dynamics of the silverleaf whitefly, Bemisia argentifolii Bellows & Perring [formerly known as the sweetpotato whitefly, B. tabaci (Gennadius) Biotype "B"], was investigated on collard (Brassica oleracea L. variety acephala) during spring 1998 and 1999 in the Lower Rio Grande Valley of Texas. Yield loss caused by whitefly was determined by using insecticides to suppress whitefly populations to a low level. Although B. argentifolii populations of adults and immatures fluctuated greatly from April to June during the two seasons, the relative values were similar. Adult whiteflies first appeared on the plants in early April, increased rapidly within the month, peaked in May, and then declined at the end of the season in early or mid-June. Whitefly eggs appeared on plants soon after adults were found, but high numbers of eggs were observed on foliage until late May 1998 and mid- and late May 1999. Nymphs and pupae increased slowly before June 1998 and increased early in May 1999. Whitefly population levels appeared to be positively associated with the availability and the growth of host plants until plant maturation, afterwards being negatively related with plant quality in the late season. Temperature, rainfall, and natural enemies were not key factors in regulating population dynamics during the two seasons. Collard plants with heavy infestations of whiteflies were unmarketable because of the damage caused by honeydew and sooty mold on the foliage. Application of a combination of fenpropathrin (Danitol) and acephate (Orthene) not only significantly reduced the whitefly infestation levels but also reduced plant foliar damage, resulting in marketable foliage with 6- to 7- fold greater yield and higher quality compared with the untreated plants.