Harnessing the Power of Parasitic Wasps: A Guide to Efficient Whitefly Management in Greenhouse and Ornamental Crops

Whiteflies are common pests in greenhouse, ornamental, and agricultural settings, causing damage to a wide range of crops. Among the various whitefly species, the Greenhouse Whitefly (Trialeurodes vaporariorum), and the Tobacco Whitefly (Bemisia tabaci – also commonly referred to as Bemisia Whitefly) are particularly notorious. These two pests have distinct characteristics, necessitating different control strategies. In this article, we will focus on the strategies for managing these whiteflies in greenhouse vegetable/ornamental crops and how parasitism-based and host-feeding based control methods both play a crucial role in their management. 

Understanding Greenhouse Whitefly 

Greenhouse Whiteflies (Trialeurodes vaporariorum) are the most prevalent species in Canada and an ongoing threat to greenhouse vegetable and ornamental crops. These tiny insects deposit their eggs on the newest plant growth and develop sequentially down the plant as the crop matures, severely damaging the crop in numerous ways, including:  

• Feeding on plant sap by piercing the plant’s vascular system with their needle-like mouthparts. This feeding robs the energy reserves, weakens the plant, and reduces its ability to transport essential nutrients and water. As a result, the plant can become stunted and exhibit symptoms of nutrient deficiency, which reduces plant defenses causing a decrease in crop quality (economic losses).  

• Secreting a sugary substance known as honeydew (a bi-product of their sap-feeding). Honeydew can accumulate on plant surfaces, promoting the growth of sooty mold (a black powdery fungus). Sooty mold will cover leaves, reducing photosynthesis and causing fruit & flowers to be discarded for aesthetics. 

• Acting as a vector for various plant diseases, transmitting pathogens from infected plants to healthy ones.  

Understanding Tobacco Whitefly 

Tobacco or Silverleaf Whiteflies (Bemisia tabaci) are a substantial threat to ornamental and greenhouse vegetable crops. These tiny insects will scatter their eggs all over the plant, causing extensive and multifaceted damage to the crop. Some of the damage will be just like other whitefly damage (detailed above), however the Bemisia Whitefly is also known to transfer viruses through enzyme injection. This discolors fruit and causes irregular ripening, making them inedible.  

Additionally, unlike other whitefly species, Bemisia whiteflies have developed an elevated level of resistance to most chemical pesticides commonly used on agricultural crops and are not easily controlled through parasitic wasps. The parasitic wasp's marginal success rates typically range from 20% to 30% at most. These issues cause a big hurdle for growers relying solely on chemical control or parasitism control methods, so other control methods are needed for this pest.  

Parasitism-Based Control 

Parasitism is a biological control method that involves using natural predators, in this case parasitic wasps, to control whitefly populations. Parasitic wasps lay their eggs inside whitefly nymphs or pupae, where they develop inside or underneath their host consuming it from within until eventually pupating inside and hatching out into a new adult parasitic wasp. 

Here is how to best employ parasitism-based control:  

Choose the Right Parasite: 

• En-Strip: Encarsia formosa wasps are typically the more effective parasite in cooler conditions (below 77°F (25°C)) and in lower dispersed whitefly situations as they live longer so they can oviposit eggs over a longer duration. 

• Ercal: Eretmocerus eremicus prefers warmer temperatures, typically above 77°F (25°C) and responds more quickly to high-pressure whitefly situations because they oviposit all their eggs over a shorter period than Encarsia. 

• Enermix: Often considered the safest choice, Enermix wasps (both E. formosa and E. eremicus) provide a buffer for a variety of situations, making them suitable for a range of temperature and pressure conditions. 

Timing of Introduction: 

• Determine whether to introduce parasitic wasps preventatively or once the first whiteflies appear on monitoring cards (the choice depends on your crop, risk factors, and your previous years whitefly pressure). 

• When applying under winter conditions, the diminished light and colder temperatures can have an adverse impact on the behavior of the parasites. Their flying capabilities are reduced, and they tend to predominantly crawl in search of whiteflies. To compensate for these challenges, a higher introduction rate and an increased number of introduction points will be required for effective pest control. 

Adjust Introduction Rates: 

• For greenhouse whitefly control, monitor the levels of parasitism by inspecting the oldest leaves; yellow or black pupae indicate they have been parasitized by ERCAL (yellow) or ENSTRIP (black). The target is to achieve and maintain 80% parasitism levels. Adjust the weekly introduction rates of parasitic wasps accordingly. 

• EX: if approximately 60% of pupae are parasitized, increase the weekly introduction rates by 30% until the 80% parasitism target has been achieved.  

De-leafing Strategy: 

• De-leafing strategies can significantly affect both whitefly levels and parasitism levels in a crop depending on how they are used.  

• When parasitism levels are high: removed leaves can be left on the floor to allow parasites time to hatch out and cycle back into the crop. 

• When parasitism levels are low and whitefly pressure is high: remove leaves from the greenhouse to prevent whitefly adults from hatching back out into the crop. 

• When you have Bemisia whitefly: de-leaf hot spots and remove the leaves from the greenhouse. Due to low parasitism rates on Bemisia whiteflies, we are relying on host feeding as the primary mode of action from the parasites.  

Host Feeding Strategy:  

Host feeding is important for the control of both the greenhouse whitefly and the tobacco whitefly, however more so with Bemisia because of the chemical and parasitism control issues. Host feeding refers to the process where adult female parasitoids use their ovipositors (egg-laying devices) by piercing the body of a host, creating wounds or punctures, subsequently ingesting the body fluids that exude from these wounds. 

Note: When using a host feeding strategy, parasite introduction rates will start higher than when doing a parasitism-control strategy.  

Additional Measures: 

• If you find adults are regularly present in the head of your plants, it might be necessary to add supplementary measures, such as sticky tape, insecticidal soap or entomopathogenic fungi, to further control the population.  

• Sticky tape or traps placed 18-24" above the crop can be a remarkably effective control to remove adults from the population.  

• For Bemisia, you will want to add a second line of sticky tape or traps placed in between the crop. This is because unlike the greenhouse whitefly, all stages of Bemisia are found all over the plant, not just in the heads.  

• For extra Bemisia control, you can blow the adults into the tape/traps by using a blower or vacuum type piece of equipment. Do this prior to the release of parasites.  

• Sprays of soap or entomopathogenic fungi directed only into the top of the plant can also be highly effective at targeting adult whitefly without disrupting biocontrol which are mostly active lower in the crop.  

• Note: Pests are highly unlikely to develop a resistance to soaps or entomopathogenic fungi sprays.  

Effectively managing whiteflies demands an integrated approach. This approach involves gaining a deep understanding of the pest's vulnerabilities, deploying parasitism-based control, and implementing host feeding strategies tailored to the specific whitefly species. Additionally, it necessitates the flexibility to adjust introduction rates based on the timing of introduction and in response to varying parasitism levels. 

Crucially, parasitic wasps like En-Strip, Ercal, and Enermix assume a key role in the battle against whiteflies across a spectrum of crops and conditions. By integrating these multifaceted strategies into pest management practices and vigilantly monitoring whitefly populations, growers can safeguard their crops and minimize the detrimental impact of these persistent and troublesome pests.