If you take a poll and ask commercial greenhouse growers what aphids, thrips, and whiteflies have in common, it will likely raise a lot of different responses depending on the plants they grow. Most will have a broad-encompassing response such as, “these are all major insect pests of ornamental plants,” which is true. Some might say, “they are all relatively small insects.” While their sizes can vary within each group, they are generally small enough to be difficult to spot with the naked eye and especially when compared to larger insects, such as beetles and caterpillars.

Some of the more entomologically savvy may point out that all three insects have mouthparts adapted for piercing and sucking fluids from plants. They can all also reproduce rapidly leading to the development of large populations under favorable conditions. Aphids, thrips and whiteflies often have short reproductive cycles and produce multiple generations within one crop cycle.

If there are any scouts participating in the poll, I’m sure life stages will come up as all three have winged and wingless forms that will show up in the greenhouse. The winged forms allow them to disperse and colonize new plants or locations, while the wingless forms mostly remain on the host plant.

Getting to the Root of the Aphids, Thrips, and Whiteflies

Now if you ask this same question to a plant pathologist, the most likely response will be vector abilities. All three insect groups can act as vectors for the transmission of plant viruses. They can acquire viruses from infected plants and transmit them to healthy plants during feeding. The specific vector-virus interactions can vary depending on the region and host plants, but aphids, thrips and whiteflies are among the most common insect vectors of plant viruses. This factor makes them all double trouble.

A Double Trouble Pest

The most common mechanism of insect transmission of plant viruses is through their feeding behavior and can be broken down into 4 steps:

1.       Acquisition: The vector insect feeds on virus-infected plants. As it feeds, the insect probes into plant tissues with its mouthparts, and the virus particles adhere to the mouthparts or enter the insect's gut.

2.       Circulation within the insect: The virus particles move through the insect's circulatory system, either within the hemolymph (insect blood) or within specific tissues.

3.       Replication and accumulation: The virus multiplies within the insect's cells, often in specific tissues, such as the gut or salivary glands. This replication process allows the virus to reach high concentrations within the vector.

4.       Inoculation: The infected vector then feeds on a healthy plant. During feeding, the virus is injected into the plant's vascular system through the insect's saliva. The virus moves from the insect's salivary glands into the plant tissues, establishing a new infection.

What Aphid Vectors Can Transmit?

Some common examples of viruses transmitted by aphids include:

·         Bean Yellow Mosaic (BYMV)

·         Bidens Mottle (BMoV)

·         Broad Bean Wilt (BBWV)

·         Cucumber mosaic (CMV)

·         Dahlia Mosaic (DCMV)

·         Turnip Mosaic (TuMV)

There are more than 20 aphid vectors of BBWV and just one, the green peach aphid of DCMV. CMV is one of the most widely prevalent viruses because of its wide host range, including more than 1200 plant species and many aphid vectors.

Thrips Unique Transmission

Thrips have a unique probing behavior where they insert their stylets and puncture the outer (epidermal) layer of plant host tissue. Then saliva is injected into the plant and cellular contents are sucked up. The adult thrips transmit viruses as saliva injected into the plant tissue. Thrips feeding causes mechanical damage to the plant, but the greatest loss occurs from the species that vector viruses transmit, such as Tomato spotted wilt (TSWV), Impatiens necrotic spot (INSV), and Tobacco Streak (TSV).

TSV has a broad host range, including species in more than 30 families of monocots and dicots. TSWV and INSV are very similar diseases caused by two distinct tospoviruses, both with broad host ranges. But TSWV is important for ornamental and for vegetable and field crops, and INSV is a concern primarily for ornamental plants.

Greenhouse Whiteflies

The viruses transmitted by whiteflies such as Cucurbit leaf crumple (CuLCrV) and Tomato yellow leaf curl (TYLCV) are most well known for the damage they cause on cucurbits and solanaceous crops. However, with greenhouse production of ornamentals and vegetables overlapping, we’ll continue to learn more about the ornamental hosts that are susceptible.

The greenhouse whitefly remains an important vector for Criniviruses and especially Beet pseudo yellows. BPYV’s have an extensive host range causing symptoms that mostly resemble nutritional deficiencies, often resulting in misdiagnosis of virus infection as an abiotic disorder.

Growers Solutions to Aphids, Thrips, and Whiteflies Today

Fortunately, growers have many solutions to choose from for the management and control of aphids, thrips, and whiteflies. But a challenge remains as plant parasitic viruses can spread by other means, including infected plant material during vegetative propagation, transmission by viruses persisting in contaminated soil, infected seeds, and pollen. Some of the most stable viruses are mechanically transmitted through various means, including contact with infected plant sap or by tools and equipment used during plant cultivation. This can occur through activities such as pruning, grafting, or using contaminated tools without proper sanitation. It's important to note that the mode of transmission can vary depending on the specific virus and host plant. Understanding transmission mechanisms is crucial for implementing appropriate control measures and preventing the spread of plant viral diseases.

If you have further questions regarding plant disease, diagnosis, treatment, or product recommendation, please reach out to your local Harrell’s Representative.