The annual wholesale value of poinsettias in the U.S. is valued at over $181 million. Greenhouse ornamentals, including poinsettias, represent an important option for diversification to supplement traditional crop production. Ornamentals are grown for their esthetics and growers’ tolerance for pests is low, and thus chemical pesticides are commonly used repeatedly. Growers recognize their reliance on chemical insecticides is neither sustainable nor desirable from an economic, human-health or environmental perspective. In recent years, pesticide resistance among whiteflies on poinsettias has been reported. Growers are eager to use biological control instead of chemical pesticides but they must be sure it works and is cost-effective. IPM approaches are being developed using plants that are highly attractive to the pest as either trap plants to draw the pest away from the crop or indicator plants for early detection. Natural enemies can be released onto these plants where they attack the pest, reproduce and then disperse throughout the crop, providing a continuous supply of biological control even if pest populations are low. This system is commonly called a “habitat plant”. We assessed use of eggplants as a habitat plant in a greenhouse in the pre-finishing phase of poinsettia production. This provided an ongoing source of biological control to address whitefly problems early in production and reduce pest pressure in the later stages of plant finishing. This system would reduce the number of insecticide applications required to keep whitefly numbers below damaging levels.
Because ornamentals are grown for their esthetic value, growers’ tolerance for pests and damage is necessarily low, far less than for greenhouse grown vegetables. Therefore chemical pesticides are commonly used repeatedly, though growers in general recognize their heavy reliance on chemical insecticides is neither sustainable nor desirable from an economic, human-health or environmental perspective. In recent years several examples of pesticide resistance among key pest species have been reported. Specifically in poinsettias, a whitefly biotype (Q) has been found that is resistant to Marathon, the most commonly used insecticide for whitefly control. The Q biotype surfaced in Vermont a few years ago, resulting in a persistent pest problem. Growers are eager to use biological control rather than chemical pesticides as part of IPM but they must be assured that these alternatives work and are cost-effective. The key to successful biological control is to minimize pest problems at the start of the season, i.e., ‘Start clean to stay clean.’ Several aspects of poinsettia propagation make this goal difficult to achieve.
A few of the larger US growers propagate poinsettias from cuttings under mist tables and sell them to growers to finish off and sell retail in November and December. Stock plants are started in April and grown to full size over the summer. Thousands of cuttings are taken from these stock plants from July-August and propagated under misters before going into the production and then finishing phase. Because the propagators are continually exposed to these cuttings for 8 hours a day for ~2 months, growers sometimes do not use chemical pesticides on their stock plants. As a result, cuttings and young plants are commonly lightly infested with whitefly. These low-level infestations serve as a pest source that can increase and spread throughout the crop over the summer and early fall because environmental conditions (day length and ambient temperature) are ideal for whitefly population growth.
To maximize on the potential of biological control in poinsettias during the finishing phase, strategies are needed to minimize pest infestations during propagation and production. However, if there are no pests, the beneficials have no host material on which to reproduce. Repeated releases of natural enemies are necessary in these situations which are expensive and time consuming. IPM approaches are being developed using plants that are highly attractive to the pest as either trap plants to draw the pest away from the crop or indicator plants for early detection. Natural enemies can be released onto these plants where they attack the pest, reproduce and then disperse throughout the crop, providing a continuous supply of beneficials even when pest populations are very low. This system is called a “habitat plant” because it serves multiple IPM roles, i.e., that of indicator and/or trap plant and natural enemy production system.
We assessed use of eggplants as a habitat plant in a greenhouse where poinsettia cuttings are grown. This addressed whitefly problems early in production, thereby reducing pest pressure in the later stage of plant finishing. In our other recent studies Eretmocerus eremicus was released on eggplants and clear evidence of host feeding and parasitism was observed suggesting that the eggplant habitat plant system played a role in sustaining the wasp population over time. Parasitism rates of >45% were observed on the habitat plants, showing that they have great potential as a sustainable biological control system. If whitefly infestations could be minimized or eliminated from the stock plants, the newly propagated plants would have fewer pest problems, improving successful, less expensive biological control throughout the rest of the growing season.
Determine the effectiveness of eggplant habitat plants as early detection tools and trap plants for whitefly in greenhouse-grown poinsettia cuttings.
Determine the reproductive ability of Eretmocerus eremicus on eggplant habitat plants naturally infested with whitefly and assess the sustainability of parasitoid releases throughout the production and finishing phases of poinsettia production.
Evaluate the efficacy of eggplant habitat plants as an IPM tool in poinsettia cuttings throughout production and finishing phases for managing whitefly and other arthropod pests.
Research was conducted at a northern VT commercial greenhouse and was integrated into the standard poinsettia production system. The plants produced were for wholesale and retail sale. From July–August (the production phase) young poinsettia cuttings were grown in several 4,500-sq. ft greenhouses. From August onward (the finishing phase) these young plants were either sold to other growers or transferred to other greenhouses for finishing.
Because the habitat plant system relies on a total IPM approach, in July before the poinsettia cuttings were planted, greenhouses were sanitized to eliminate fungal diseases and algae. Weeds were removed and all vents were covered with fine mesh screening to reduce infestations of thrips, whiteflies and aphids from outside.
Eggplants (var. Fairy Tale hybrid) were grown from seed in a pest-free environment at the UVM Entomology Research Laboratory. This variety was selected because we found it to be highly attractive to whiteflies in previous trials. In addition it small and compact at maturity and has a high stress tolerance that is ideal as a habitat plant. The eggplant habitat plants were grown to a height of at least 15 cm before they were introduced into the poinsettia greenhouse.
To determine base-line pest population levels, habitat plants and yellow sticky traps were placed in the greenhouses 1 week before the cuttings were planted. Cuttings were inspected carefully prior to planting them into the production house for any signs of whitefly infestation. Eggplants and sticky traps were introduced at a rate of 1 per 1,000 sq. ft (total of 4 in these greenhouses (4,500 sq. ft./house). The following treatments were applied: 1) E. eremicus released weekly on the eggplants in the Biocontrol treatment house; 2) no parasitoid releases in the conventionally managed treatment house (control). Eggplants and sticky cards and twelve cuttings per 1,000 sq ft (selected at random) per greenhouse were scouted weekly for live adult and immature whiteflies and parasitoids. The incidence of parasitism and host feeding by the parasitoids and distance dispersed from the eggplant were also recorded. Persistence of parasitoids in the biological control greenhouse was monitored from September – early December during the finishing phase of poinsettia production. Parasitoid releases were discontinued after plants were spaced for finishing (19 Sept.). Sticky cards, eggplants, and poinsettias were scouted, and data on pest and parasitoid populations were taken as described above. This provided information on the persistence of the pest and natural enemies from the production to the finishing phases of production and demonstrated the sustainability of the habitat plant system.
Throughout the study, low to very low levels of whiteflies (average: 0-7 adults/sticky trap, 0-0.5 adults per eggplant habitat plant) were present in the conventional control greenhouse and the biological control house. Whitefly adults were detected both on sticky cards and habitat plants, but not on the poinsettia plants during routine crop scouting. Whitefly populations regionwide were unusually low in 2009, most likely because of heavy pesticide treatments in the early phases of propagation prior to shipping to the distributors. Consequently, at our study site, whitefly populations were also very low. Whitefly adults were detected on the same day on habitat plants and yellow sticky cards. We found it extremely difficult to differentiate between whitefly species on sticky cards whereas on eggplants accurate identification to species is easy. Growers need to know the species of whitefly infesting their crop as it will determine what parasitoid to release. With the exception of a slight population increase between week 37-39, the mean number of adult whiteflies on sticky cards was less than 1.
Throughout the pre-finishing phase, 4,000 E. eremicus parasitoids were released weekly on each of four habitat plants. Parasitoids were found in low to moderate numbers on the habitat plants (average: 0-3.75 per plant) and yellow sticky cards (average: 0-46.25 per card).
The mean number of parasitoids found on eggplants remained very low throughout the experiment, with the exception of weeks 38-39. This coincides with the peak in whitefly populations suggesting that the parasitoids responded to the increased pest population. Because whitefly populations on the habitat plants were low, parasitoids dispersed throughout the crop in search of the host, as evidenced by the higher numbers on the sticky cards throughout much of the study. Adult parasitoids were never observed on the poinsettias during scouting. In addition, no whitefly nymphs were found on the poinsettias or on the habitat plants. This demonstrates that spot inspection of poinsettias is not a reliable means of scouting. Though whiteflies were found throughout the season in both greenhouses, based on sticky card and habitat plant inspections, they were not readily detected on the pre-finished crop.
When the poinsettias reached the finishing phase (based primarily on plant size), some were moved to a new greenhouse and others were left in the original greenhouse to maintain proper spacing. At this time, all parasitoid releases ceased, though the habitat plants remained in place. Parasitoids were found on the habitat plants and on sticky cards 2 weeks after the final release date. This suggests conditions were suitable for the parasitoids in the habitat plants, and there was at least a 2-wk carry over effect. We also found parasitoids on sticky cards 9 weeks after the last release. Whitefly adults were also detected. This shows that though releases had ceased parasitoids reproduced within nymphs on poinsettias despite extremely low levels of their host.
This project addressed several practical questions that growers face in the pre-finishing and finishing phases of poinsettia production.
Are eggplants more attractive to whiteflies than poinsettias; are they an effective early detection tool? Whitefly adults were detected both on the sticky cards and habitat plants, but not on the poinsettia plants during routine crop scouting. Sticky cards and eggplant habitat plants were effective scouting tools for early detection of whiteflies. Growers can save time on scouting by inspecting cards or habitat plants first, and are able to assess pest populations quickly to determine if intervention is required. However, the sticky cards had one major disadvantage over habitat plants. When whiteflies are stuck to the card, it is extremely difficult to distinguish the two common species on poinsettias (greenhouse and silverleaf whitefly). In contrast, it is relatively easy to identify a live adult whitefly resting on a plant leaf. Species identification is critical for deciding on the appropriate biological control agent to release or pesticide to use. Eggplants have the potential to be a useful tool in pest management by providing accurate species identification in order to choose the most effective management option.
Do whiteflies reproduce on eggplants more than on poinsettias; are whitefly populations reduced by the parasitoid; do parasitoids reproduce within the crop? In the past, we found that whiteflies reproduce in higher numbers on eggplants than on poinsettias and thus provide a site for parasitoid reproduction. Because whitefly populations were so low on the habitat plants and poinsettias, we could not evaluate the reproductive capacity of the whiteflies and parasitoids within this system. Interestingly, in previous years the grower has had severe whitefly problems during all phases of production. This demonstrates the variability in pest populations from year to year. A grower must always be vigilant, anticipating potential pest problems and have a management plan in place for pest prevention and management when infestations occur.
Do the released parasitoids disperse throughout the crop? Parasitoids were not found in very high numbers on the eggplant habitat plant even though it was their focal point of release. Because whitefly populations on the habitat plants were so low, parasitoids dispersed throughout the crop in search of the host. This was observed by the high numbers on the sticky cards. Adult parasitoids were never observed on the poinsettias during scouting. Due to their small size it is often difficult to detect them but their presence was indicated by the consistent sticky card catches. Although it is difficult to detect the parasitoids and their presence in the absence of their host, dispersal did occur as they searched for prey.
Are habitat plants an effective scouting tool for other arthropod pests and/or a source of additional pest problems; are they a source of a continual supply of other natural enemies? Thrips were a significant problem on the eggplants this year. The predatory mite Neoseiulus cucumeris was released onto eggplants to manage thrips there. Thrips were also a problem on the poinsettias. Three insecticide sprays were used targeting thrips. Given the attractiveness of the eggplant habitat plant system to thrips, it has the potential for the early thrips detection and as a habitat plants for predatory mites.
Are whitefly populations reduced in the finishing phase of production as a result of using habitat plants during propagation; are released parasitoids sustained over time? In the conventional control house, chemical insecticides were applied four times. In contrast, no insecticides were applied targeting whiteflies in the biological control greenhouse. This demonstrates that similar control was achieved using biological controls instead of chemical insecticide. After the final release date, parasitoids were found up to 9 weeks later despite the low whitefly host population. Considering this observation, when whitefly populations are low, growers may be able to reduce frequency of releases, though additional research is needed to confirm this. It would save a significant amount of money for growers.
Education & Outreach Activities and Participation Summary
Results from this trial were presented at the 2009 annual Tri-State Greenhouse IPM workshops in Maine, New Hampshire and Vermont for greenhouse growers, extension specialists and professional pest managers. Over 150 participants attended. Over 41% of the growers participants at the workshop indicated they use some form of a plant-mediated IPM system last year, compared with 39% the year before. This IPM tool seems to be particularly interesting to growers, and something they are readily willing to implement. Handouts (see Appendix II) were provided to growers detailing the results of this study along with the results of other IPM based trials underway at the UVM Entomology Research Laboratory.
This project was a highly effective means of transferring IPM technology to the greenhouse owner/manager and his staff. They were actively involved in the project, assisting with data collection, decision making and other technical activities. Staff learned first hand how to correctly identify various greenhouse pests, scout for whiteflies and their natural enemies and the use of habitat plants. Involvement in the project also provided the growers with unique networking and professional development opportunities, such as attending the New England Greenhouse Conference held in Worcester, MA in October.
The value of these workshops and having a specialist provide hands-on research at the commercial site provides a unique experience for growers. It is the direct involvement that provides the most effective education by providing techniques modified for their own personal use. For example, some of these staff had been to workshops on pest id in the past. It wasn’t until they worked directly with a specialist on these trials and were faced with these pests and biological controls in the greenhouse that they really learned how to correctly id them. Furthermore, the effects of the use of fewer chemical insecticides impact the general community as a whole by providing more ecologically friendly plants improving overall human and
Timing is the key to successful biological control to prevent problems before they arise. This study showed it is possible to achieve the same level of pest management using biological control as conventional chemical management when used early on in the growing season. If whitefly infestations are minimized or eliminated from the new plant materials, the plants in the finishing phase will have fewer pest problems improving the chance for successful, less expensive biological control throughout the rest of the growing season. This ultimately increases human and environmental health by allowing the grower to choose “softer” pesticides if necessary that are compatible with natural enemies, and have less negative impacts on humans or the environment.
This was a small project working with one Vermont grower. Taking part in this project strengthened his interest in non-chemical approaches to pest management, but it is too early to state with certainty that the grower will adopt this IPM technique independently.
Areas needing additional study
Further research is needed to refine the habitat plant system. The biggest challenge is finding the most suitable biological release schedule to use, taking into consideration different pest population levels. This information is critical for the grower to save the most money and still provide a reliable source of pest management while producing the highest quality crops possible. Although the eggplant habitat system can be an effective scouting tool, its value in terms of attracting whitefly and serving as a means of enhancing parasitoid reproduction was not fully confirmed because whitefly populations were so low. Ideally, this study should be repeated in future years in multiple greenhouses when whitefly populations are higher.