Psyllids - A major pest of tomatoes and potatoes - © Duncan Smith MSc

In April 2006, Chris Charrington, a crop consultant in Gisborne, was monitoring field tomatoes with a colleague, when they noticed something unusual about the crop. On closer inspection, Chris found insects he'd not seen before, living under the leaves and attacking it. Their find would have significant implications for other major solanaceous crops grown in New Zealand, particularly potatoes.

Tomato plant exhibiting “Tomato Yellows” symptoms, caused by a severe infestation of the Tomato/Potato Psyllid (TPP). © Duncan Smith

There are many psyllid species of insects in New Zealand. They are in the same family as aphids, cicadas, shield bugs and leafhoppers. However, this one was new and identified as Bactericera cockerelli, the Tomato-Potato Psyllid (TPP). It quickly spread across the whole of New Zealand and has become a severe pest in many outdoor and greenhouse crops. It can also be found in some native plants.

This psyllid originated in Central and North America where it bred on tomatoes and potatoes. No one can say for sure how it got into New Zealand. However, their nymphs may have gained entry here on smuggled chilli peppers, from the west coast of the USA.

They have been found here on a range of solanaceous plants, e.g. tomatoes, potatoes, capsicums, tamarillos, Cape gooseberry, chilli and aubergines. Solanaceous weeds can also act as hosts, e.g. black nightshade; African boxthorn; Poroporo and Jerusalem cherry. They can also be found on Kumara, the only psyllid in New Zealand to do so, but it doesn't breed well on them.

Life cycle

This psyllid breeds all year round and its success depends on the climate and whether the crop is grown outside or in greenhouses. The period from egg hatch to the adult stage will depend upon the ambient temperature. It will take longer to go from the egg to the adult stage in winter than in summer. In greenhouses there may be many generations produced a year due to the equitable climate within.
Psyllids develop slowly in spring, with the peak period from late summer to early autumn.

Psyllids have three stages of development - adult, egg and nymph.
Adult psyllids - They are a light yellow colour when they first emerge but darken to a brown or green shade after a few days. They have pale markings and a white band on their abdomens. They have wings, but in New Zealand, they have not been shown to travel very far. Being small, they can be dispersed by wind and by the movement of infested plant materials. They feed by inserting their long, needle-like mouthparts into the plant to draw in the plant sap.

The female TPP can mate more than once and lay up to 200 eggs over a three week period, depending on the host it is infesting.

Eggs are yellow and borne on small stalks. They are commonly seen on the edges of leaves but can be found on most parts of plants and stems. As the eggs mature they darken just before hatching.

Psyllid eggs, on little stalks, on tomato leaf edges. © Duncan Smith

Nymphs emerge from their eggs as flat scale-like insects. They have six legs and can move around, but generally remain in one place to feed, usually underneath young leaves. They feed on plant sap, like adults.
The nymphs go through five stages of development (instars). At each stage, they have to shed their tough outer skins (exoskeletons) in order to get bigger (moult). Their cast skins can be seen on leaves, next to newly emerged nymphs. At later nymphal stages, their wing buds begin to develop, and when they reach the final instar, as adults, they can fly.

Psyllid nymphs on tomato leaf. © Duncan Smith

Around the Auckland region, studies have found that up to 7-8 generations of psyllids can be produced a year.

Feeding

Psyllids are the size of a small aphid, and, like aphids, plunge their needle-like mouthparts into plants to feed. The sap they feed on contains large quantities of water and sugar, but very little protein, which they do need to produce their eggs.

A TPP nymph surrounded by "Psyllid Sugars". © Duncan Smith

Psyllids take in vast amounts of unwanted sugar, of which they require only small amounts, so need to eject the rest. This sweet, sticky secretion, called "honeydew", is cleverly wrapped in small parcels of white wax by the insect before passing it out. These are called "psyllid sugars", and in high infestations, these tiny parcels can cover leaf surfaces.

Disease

When TPP feed, they can pick up a bacteria (Canditatus Liberibacter solanacearum) from an infected plant and transmit it to another plant when they feed. The bacteria produces a potent toxin that has a devastating effect, not only the plants' development but also on the quality of the fruit or other edible parts cropped for human consumption.

Symptoms of the disease in tomatoes are yellowing of the leaves, called "psyllid yellows", curling/cupping of leaf edges, weakened plants and reduced vigour. Black "sooty mould" may also be present on severely infested plants, which reduces photosynthesis.

“Potato yellows” caused by TPP infestation.

Symptoms in potatoes are similar to tomatoes, with yellowing of leaf tips and curled leaves, and also have a purple or pink appearance. Plants may be stunted, with swollen stem nodes. A cross-section of stems will be brown instead of the usual white. Infected potatoes will eventually collapse and look burnt. Tubers from infected plants can transmit this disease.

If potatoes are infected early, they produce many small tubers. When infected later, the bacteria prevents plants from converting sugars into starch, for normal storage, and tubers become rich in high concentrations of sugar instead. When these tubers are fried, they exhibit symptoms called "zebra chip", a striped effect, due to the caramelisation and burning of these sugars.

Maori potatoes seem to be resistant to this disease.

Malformed, bacterially-infected tomato fruit, on the left.

Tomatoes plants may collapse and their fruit will no be properly developed.

Control

Cultural methods

There are ways to reduce the incidence of TPP without necessarily resorting to sprays.

1. When buying potatoes always buy certified seed, free of the "zebra chip" disease. Don't be tempted to plant ones you buy from a supermarket or even your own home-saved potatoes; these could be infected and carry forward this disease. Supermarket potatoes could have also been sprayed with a chemical that prevents them from sprouting in storage, so will not sprout properly.

2. Carefully check for psyllids on all plants you buy from nurseries before buying them. You don't want to inadvertently introduce them into your garden. Make certain to look under the leaves as this is where you are more likely to find them lurking.

3. TPP numbers tend to build up slowly from spring onward, so it is wise to plant tomatoes, potatoes and other susceptible plants as early as your location allows. You will be more confident, to a degree, in avoiding the peak period when psyllids are at their most active. This depends on where you live in New Zealand of course, but in general the earlier you can get them into the ground the better. Choose early varieties of these crops to help you.

4. If your plants become severely infested with psyllids, in greenhouses or outside, carefully remove them and put them into sealed bags and either bin or burn them.

5. Before winter starts, remove and destroy any volunteer potatoes, tomatoes or other solanaceous crops in case they don't get killed off by frosts. Psyllids could overwinter on these in more sheltered spots in your garden. Don't leave psyllid-infested plants in your greenhouse to overwinter either, for the same reason.

6. Most solanaceous weeds are annuals and will die over winter, however, some may survive until spring. These will act as a reservoir of infestation, ready to transfer to your crops next year. It is best to remove any of these around your garden anyway. Poroporo, a New Zealand upright shrub, and tamarillos are perennials; they can harbour overwintering psyllids, so carefully remove, bin or burn any infested plants/leaves.

7. You can encourage predators and parasitoids into your garden to help reduce pest populations. These won't give 100% control but will help lessen their impact. Predators eat other insects, e.g. ladybird adults/nymphs; lacewing and hoverfly larvae too. Parasitoids are tiny wasps, the smallest of all insects, and are often inconspicuous to the naked eye. They lay their eggs onto or into insects which hatch out and feed on the pest, slowly devouring it. Sometimes many eggs are laid inside one pest, and the larvae will feed together and eventually break out to pupate. These will hatch out into new adults, and the life-cycle continues.

8. Plant flowers that they will attract them. Parasitoids have very small mouthparts so need tiny flowers to be able to feed on them. Include some or all of these - fennel, dill, parsley and coriander. Predators, like ladybirds, like alyssum, cosmos and marigolds, while hoverflies love phacelia. Chrysanthemums and chamomile also attract predators and parasitoids.

Parisitoid wasp laying egg on psyllid nymph - Copyright Hort NZ.

Parasitic insect control method

The New Zealand Environmental Protection Authority (EPA) studied a new parasitoid that is specific to the TPP, it's only host. It’s a tiny wasp (Tamaraxia triozae) found mainly in North America and Mexico, where this psyllid originated. An application, from Horticultural New Zealand Inc., on behalf of grower industry groups, was sought, in order to have it released under the Hazardous Substances and New Organisms (HSNO) Act 1996. All risks and benefits were carefully studied to ensure that it was a safe option to help control this important pest. On 17th June 2016, the EPA approved its release. It will be self-sustaining and reproducing without any further human intervention.

Horticultural mesh

In recent years the use of horticultural mesh has become a popular, preventive way of protecting plants. It follows studies by The Biological Husbandry Unit Organics Trust (BHU), a joint venture between Lincoln University and the New Zealand Organic Movement. They initially looked into the effectiveness of it in preventing TPP from attacking potatoes.

Horticultural mesh has tiny holes, (0.6mm), preventing small insects from getting through and attacking the crops. It can also fend off larger pests like cabbage white butterflies, root flies, flea beetles, shield bugs, birds, wasps, possums, rabbits, cats, dogs and even deer!

The mesh is permeable to rain, wind and sunlight, so there is no concern about shading or watering crops adequately. The micro-climate underneath is also improved, so less watering is necessary. However, it is not designed for frost protection. Any sprays of foliar fertilizers, fungicides or other liquids applied to the plants can penetrate this mesh.

Many types of crops can be protected, including brassicas, carrots, peas, lettuces etc. Taller crops, like sweetcorn, are less easy to cover. As maincrop potatoes are planted later in the season, it is ideal for them, being more susceptible to this pest at that time.

In recent "World first" trials, carried out by the BHU, this mesh has even controlled potato blight. Experiments carried out in Israel later, also confirmed this.

The mesh must be applied to crops as soon as they are sown or planted because it is a way of preventing insects from landing on them in the first place. Once the mesh is in place, the edges are covered with soil or stones to prevent insects from getting underneath.

The mesh is very light and can be laid directly onto plants. However, in the case of potato aphids, it's been shown that the smell of potatoes attracts adults of this pest to the plants. They stay on the outside of the mesh and lay their eggs on it. The tiny aphids hatch out and are small and soft enough to be able to penetrate the mesh to feed on the crop. As aphids transmit potato viruses, it is essential to keep the mesh off the leaves by making a cloche out of it or supporting it in some way.

Where some crops need pollinators, the mesh can be taken off for a few days to allow bees or other insects to do their work. Hand pollination is also possible without having to risk this. Field trials of tomatoes under this mesh have shown that they are successfully wind-pollinated, without having to remove it.

Research has also shown that if a few of these psyllids do manage to get onto the crop under this mesh, the level of UV light penetrating it is at a level that the insects don't like, so won't cause much damage.

Good garden centres should supply this mesh, and it is also offered online

Pesticides

The word "pesticide" is an "umbrella" term grouping insecticides, fungicides, herbicides, molluscicides, bactericides, algaecides, etc., together.

Here we consider insecticides which not only kill your pests but will also kill the "good guys" in your garden, i.e. the predators and parasites that are there to help you. Pesticides are not specific to your pest problem as most are generally broad-spectrum. The way they act on any insect, good or bad, is to target particular biological pathways within their bodies and kills them.

According to the literature, the use of organic sprays to control psyllids has not been proved to be very useful. However, Koanga Nurseries, near Wairoa, have produced a spray (Koanga Psyllid Solution) that they claim will control psyllids. It is based on a non-toxic, water-soluble version of diatomaceous earth and applied as a foliar spray. It contains 85% silicon dioxide in the form of dispersible powder. It is also said to be highly effective against a broad range of other insect pests. (Ed. Predators and parasites could also be killed). I have had no personal experience of using this product.

Light mineral oils have shown some promise, but good coverage is essential, as with all contact sprays. You need to ensure that all surfaces of the plants, especially the undersides of leaves, are treated, as this is where most TPP hide. Spray early during the season, and on young plants, to prevent this yellows disease occurring. (Ed. Predators and parasites may also be killed).

Happy gardening.

Further reading - If you have a problem with SOOTY MOULD on your plants and trees, maybe you would like to look at my blog on the subject. https://duncangoeswalking.squarespace.com/blog-pages/sooty-mould-what-is-it-and-how-to-control-it-organically-duncan-smith-msc