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Greenhouse Grower notes: Leafminer control options

This past summer, we have seen greater problems with leafminer than we have for a number of years.

This past summer, we have seen greater problems with leafminer than we have for a number of years. The crop that has been most affected is chrysanthemum, including those grown for cut flowers, as well as potted mums and garden mums. Damage caused by leafminer can be devastating, and within a very short period, crop quality is reduced, often to the point where it is not marketable.

Leafminer damage on potted chrysanthemum. PHOTO COURTESY GRAEME MURPHY, OMAFRA. Advertisement

Gerbera crops have also been affected. Growers trying to deal with this pest have very few options. There are only three pesticides registered for use against leafminer in Canada: one adulticide (permethrin – Pounce or Ambush) and two larval control products (Citation and Avid). None are providing control, and some growers are suffering heavy losses.

It has been interesting to note that growers of garden mums are experiencing different levels of problems depending on whether their crops are in the greenhouse or grown outside. In general, the outdoor crops are not damaged nearly as badly as those grown indoors. Even for individual growers who have crops both inside and outside, there is often a difference. The most likely explanation is that outdoor crops are not sprayed as much, especially early in the cropping cycle, and naturally occurring beneficials are providing significant levels of control. Inside, the environment is more favourable for the leafminer: it is more difficult for beneficials to get inside the greenhouse and pesticide residues are likely to be more of an issue by not allowing the natural enemies to establish.

BIOCONTROL OFTEN THE BEST APPROACH IN DEALING WITH LEAFMINER
■ This suggests that biocontrol may be the best approach in dealing with leafminer, and in fact there are many growers who have controlled it successfully in this way. However, some of the growers I have spoken to in recent months have been spraying two to three times per week, without getting control. The heavy pesticide load is damaging the crop, as well as leaving residues on the plants that can affect biocontrol agents for up to several months. This creates a downward spiral in trying to control the pest. The more pesticide that is applied, the worse the problem gets and the more difficult it is to implement the most effective control option – biological control.

So, how should we approach the problem? Firstly, it needs to be said that there is no easy way out. Leaves damaged by “mines” are going to remain on the plant and detract from its quality for the life of the crop. However, I have worked with growers who have managed to fight their way through it and get back to the point where they are able to introduce biocontrols and their leafminer population has been reduced enough that it doesn’t overwhelm the biocontrol agent. There are a number of strategies that can be used and a diverse IPM program has to be put in place.

The first thing to do is stop spraying. As difficult as that may be to do, logic tells us that if multiple sprays every week have done nothing, then there is no point in continuing, despite the therapeutic value of holding a spray gun.

STEINERNEMA FELTIAE IS ACTIVE AGAINST LEAFMINER LARVAE
■ There is one biocontrol agent that can be introduced even if there are pesticide residues on the crop. The nematode Steinernema feltiae has been shown by a number of researchers to be active against leafminer larvae. Presumably, they enter the mine either through the wound made by the female when the egg was laid, or through the wall of the mine. They then attack the leafminer larva in the mine. The nematode should be applied as a foliar spray to the whole crop. Because S. feltiae is very susceptible to dehydration and UV radiation, the best time to spray is in the evening. The longer the foliage can stay wet, the greater chance there is for the nematode to infect the leafminer. (Although the thought of spraying the foliage and leaving it wet into the evening may horrify plant pathologists, we have to choose between the lesser of two evils. If any crop can handle such a treatment, it is chrysanthemum (especially pre-flowering), and there are fungicides available that won’t affect the nematodes or other biocontrol agents if diseases such as Botrytis become an issue).

NEMATODES HAVE BEEN EFFECTIVE IN ONTARIO
■ In a number of Ontario greenhouses where nematodes have been applied on a weekly basis, I have found levels of control in the 40-50 per cent range. Where growers have sprayed more frequently (up to twice per week), control is even higher (I have seen up to 70-80 per cent of larvae in the leaves dead).

While these levels of control are better than what can be achieved with pesticides, on their own, nematodes are unlikely to bring the pest population back to an acceptable level quickly enough, so other control strategies also have to be used. If a grower has his own stock plants, they should be kept isolated from the rest of the crop, so that when leafminer (or other pests such as thrips) is under control, they are not being transferred in the cuttings with every new crop. Although this may not always be feasible, it should be seriously considered as a longer-term part of the program.

Many Ontario growers are dipping their cuttings (whether from their own stock or brought in from a specialist propagator) in a concentrated nematode solution. This can be useful in a number of ways:

The high concentration of nematodes means that it is more likely some will find their way into the leaf mine.
By dipping the cuttings, growers achieve 100 per cent leaf coverage, again improving the opportunities for the nematodes to contact the leafminer.

The cuttings are then planted on a rooting bench under conditions of high humidity, allowing better nematode survival.

Once on the rooting bench, cuttings should continue to be sprayed with nematodes every four to five days to maintain their presence on the leaves. This practice should continue throughout the life of the crop. Use a wetting agent to improve coverage. And make sure that nematodes are mixed and applied exactly according to instructions to maximize their survival and effectiveness.

If you know a researcher or someone with a microscope, you can assess whether this approach is working by dissecting out leafminer larvae from the leaves. It is quite easy to do as well as being easy to tell whether they are dead or alive – it is also very satisfying as you start to find more and more dead leafminers.

INCLUDE SOME MEASURES TO CONTROL ADULT STAGES
■ Until now, I have only been talking about controlling the larvae, but it is important to try to reduce the adult population as well.

Use yellow sticky tape in as large a quantity as you possibly can, without interfering with regular work practices. Leafminer adults are strongly attracted to the colour yellow and it is very difficult for them to lay eggs while glued to sticky tape. I have seen growers use two to three vertical layers of tape: I have seen it strung above benches, from post to post under the gutters (put Saran Wrap around the posts first so they don’t become too messy), and even laid horizontally between rows of pots on troughs to capture pupae as they fall from the plant. There is no question that sticky tape can be horrible stuff to work with, but the alternative is worse. The tape should last the life of the crop or perhaps even longer as populations decrease. Use the widest tape possible to increase the catching area. It should be noted that if a biocontrol program using parasitic wasps is put in place, sticky tape should not be used in this way, since it will also catch many of the biocontrol agents.

If growing cut chrysanthemums in the soil, make sure that the soil is disinfested between crops. The leafminer pupates in the soil and in a heavy population there can be many in the soil waiting to emerge after the crop has been harvested. Place a tarp on the soil as soon as plants are harvested to minimize the number of new flies emerging. If possible, steam the soil between crops to kill pupae.

Some growers have considered compartmentalizing their greenhouse by installing plastic drop sheets under the gutters to minimize the movement of adult leafminer from older crops into newer ones. While this will not completely protect new crops, it will slow down the speed with which new plantings are infested.

BEST TIMING TO APPLY PARASITIC WASPS
■ After pesticides have been discontinued for two to three months, consideration should be given to introducing parasitic wasps such as Diglyphus isaea, which is a very effective natural enemy of leafminer. A number of Ontario growers have successfully used both nematodes and Diglyphus at the same time.

Bringing a chrysanthemum crop back from the ravages of a heavy leaf miner infestation is not easy and requires a lot of hard work. However, it can be done, and when the alternative is a lost crop, the incentive to make it work is very strong.

Graeme Murphy is the greenhouse floriculture IPM specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs at Vineland. • 905-562-4141, ext. 106, or graeme.murphy@ontario.ca.