Gerbera as a Cut Flower or Pot Flower
There are few plants on the planet that are as attractive to pests as Gerbera. We actually use Gerbera plants as a control strategy in many other crops, either as a trapping/banking plant or as a trapping/killing plant. In the Gerbera growing industry more chemicals are being removed than introduced and growers have little choice but to move toward an IPM (integrated pest management) system. Hybrid systems have their own challenges. The newer “soft” chemicals are expensive and don’t usually work as well as the old standards and yet they can still harm the beneficial insects.
The cut flower Gerbera is a much larger challenge. This guide, as written, should help the pot Gerbera grower achieve a very high level of success, with bigger and brighter plants at a lower cost than doing the program chemically. Cut Gerbera, on the other hand, require a “heavy hand” to get things turned around. This is because of the very dense foliage. To get a runaway whitefly problem turned around, you must be patient and aggressive. Physical trapping of whitefly, in the beginning of the turnaround will shorten the time frame, but not necessarily save you any money. We know a cut Gerbera grower in California who went for a full year without any chemical or Biological control of whitefly. All he used was yellow sticky tape, 1 foot wide, down every row. His costs were very high however. If you have overhead spray booms, you can convert them to disturbance booms by dragging ribbon or tape over the crop. By fitting either a vacuum system or just lots of sticky yellow tape, you can pull off a great number of adults. Each adult you trap is 150 eggs that won’t be laid. This trapping doesn’t affect the Encarsia very much, or even the Delphastus, as the Delphastus tend to drop when disturbed.
The greatest success that I have seen in this crop is when applications have been extreme, for a short period of time. Consider releases of up to 20 Encarsia per square meter in bad varieties for 2 consecutive weeks. As a general rule, with all Biological Control, we never put enough in early, and we always put too much in at the end.
We have found that the use of “fresh” beneficials makes a huge difference in the effective control of the major pests of Gerbera. Delphastus (for whitefly control) and fallacis (for spider mite control) are newer beneficials that have proven to be well-suited to pest control in Gerbera. Fresh, non-refrigerated Encarsia has, among other traits, a much greater effective temperature range, making it able to control Greenhouse whitefly down to 10 degrees C (50 degrees F.)
Climate control, anti-fungal practices, and plant nutrition have a major effect on pest dynamics and the ability of the beneficials to perform. High humidity has a negative impact on whitefly, spider mites, and to a lesser extent, on thrips. Naturally occurring fungal pathogens will completely wipe out most pest populations if the humidity remains above 80% for more than 48 hours. These are also the conditions that are needed for most of the commercial fungal pathogens to work well. While Gerbera growers will not allow that high humidity to persist, it is important to know what a prolonged period of high humidity can do to the pest population. Consistent temperatures over 30 degrees C will reduce the greenhouse whitefly adult lifespan from weeks to days and suppress egg laying. Hot and dry conditions are ideal for spider mites. While you don’t want to be playing with any of these conditions, it is important to understand how these conditions will affect the pest dynamics.
The use of sulphur pots is probably the single largest impediment to successful biological pest control. Recent research in Holland has shown that even short burns of only a few hours will kill all parasitoids within 2 hours of exposure to the leaf surface. We strongly recommend replacing sulphur pots with an equally effective technique that has been in use for many years. Apply sulphur directly to the floor of the house and allow it to vaporize naturally. This technique is gentle on the beneficials and produces good results. The bacterial fungicide “Rhapsody” appears to be very effective when fogged and does not have any significant negative effect on Beneficials.
Plant nutrition is a critical consideration when dealing with pests. Sucking insects, such as Whitefly and Aphids, are attracted to well-fertilized plants, especially if the nitrogen source is ammonium. By switching your fertilizer mix away from ammonium based products, you make the plants less attractive to the sucking insects. Unfortunately, Spider Mites and possibly Thrips are not influenced by the presence or absence of ammonium, and generally are attracted to stressed, under-fertilized plants. There is very little known on the effect of plant nutrition on beneficials. Because the beneficials feed on the pests, plant nutrition is second-hand for them.
The starting point for most of our strategies is with our soil mites, Stratiolaelaps scimitus and Gaeolaelaps gillespiei. Either are used at planting, or after the planting, to control Fungus Gnats. Both mites have the added effects of preventing Thrips from cycling in the house, by feeding on the pre-pupae and pupae stages for most thrips, including WFT. In addition, they feed on overwintering Spider Mites and any root stages of Aphids. The release rate for both species is 250 per square meter, just once, for the life of the plant.
Whitefly absolutely love Gerbera. The density of the foliage and the succulent nature of the leaves is a perfect refuge. The density of the leaves is also why most spray programs don’t work well, and why beneficial insects have a hard time finding all of the hot spots. All Whitefly predators and parasitoids rely on the smell of the honeydew, produced by the Whitefly, to find the hot spots. A house that is evenly infected by Whitefly will make it difficult for the beneficials to pinpoint the areas of most concern, thus negating their searching ability. Ideally, starting with a clean house, “fresh” parasitoids and Delphastus will find the new areas of infection and deal with them effectively. In houses that have an existing population, you must over-inoculate with beneficials, manually spread them out, and, if possible, wash off the existing honeydew prior to the release of the beneficials. To correct an existing whitefly problem Encarsia formosa should be released at a rate of 10 per square meter for 4 consecutive weeks, before dropping back to 4 per square meter. Additionally, the beetle Delphastus catalinae should be released in the affected areas at a rate of 10 per square meter, just once.
If the Whitefly is Greenhouse Whitefly, Encarsia formosa works well. Our “fresh” Encarsia has a tremendous searching ability and will maintain control (in a house that started clean) at a rate of 2 to 4 wasps per square meter, weekly. Encarsia must be considered like an insurance policy; you must continue with it at the low rate even if the Whitefly population appears to have crashed. The “Achilles Heel” of Encarsia, and all other parasitoids, is that once a hot spot has developed and the leaf surface has become sticky, they lose their ability to suppress the hotspot. The tiny wasps just can’t handle the gooey mess and leave. Luckily, another predator, Delphastus, thrives in the goo. They are attracted to the densest hot spots and eat both the Whitefly as well as the honeydew. Delphastus always prefer the eggs of the Whitefly, progressing up the development ladder until they get to the adult Whitefly, which they will also consume. Rates for Delphastus in Gerbera range from 1 adult per square meter, every 3 weeks, to 1 adult per square meter weekly.
The essential issue for effective Whitefly management is the use of fresh, un-refrigerated Encarsia. This is mostly because of the winter temperature range. Gerbera like to be cool, but stored Encarsia are ineffective below 18 degrees C, allowing a healthy population of Whitefly to build up over the winter and early spring. Fresh, un-refrigerated Encarsia, on the other hand, can easily manage a Whitefly problem down to 10 degrees C. In Gerbera houses managed with our fresh Encarsia, Whitefly is not the main pest issue.|
If the Whitefly is Bemisia or Silverleaf, the strategy has to change. While some parasitoids are naturally inclined to Bemisia, all commercial strains are reared on, and conditioned to the Greenhouse Whitefly. We do not recommend using parasitoids to control or manage these pests. The good news is that Bemisia tends not to spread out as fast as Greenhouse, so the hot spots are more clearly defined and easier to target. Delphastus naturally prefers Bemisia and will go to the Bemisia hot spots before the Greenhouse hot spots. So, while the Bemisia is a more serious threat because of virus transmission, it is also much easier to manage, if you use Delphastus.
As stated before, shifting your fertilizer mix away from ammonium will also assist with Whitefly management. Even subtle changes in the feeding program can have dramatic results.
You seldom find both Whitefly and Spider Mite rampant in the same variety. This is because Spider Mites are attracted to different plants from that of the Whitefly, and because Spider Mites also hate honeydew because of the stickiness and the higher humidity associated with it. Therefore, once you have your Whitefly under control, you will begin the battle of the Spider Mites. Fortunately, we have a few excellent Spider Mite controls that are well-suited for Gerbera. Aside from the Ss or Gg in the soil, Amblyseius fallacis will provide long term Spider Mite suppression. It is usually applied at a rate of 2 per square meter, only once. The fallacis will build up in the crop by feeding on the Spider Mites, but also by eating the pollen and other pests such as young Thrip larvae and other spider mite species. Being a true generalist, fallacis will also feed on Whitefly eggs, helping prevent the spreading of the Whitefly.
If a Spider Mite hot spot becomes too developed, too fast, the “Achilles Heel” of the generalist predator mite becomes a significant factor: they can’t tolerate significant webbing. Phytoseiulus persimilis is then used to treat the hot spots. P. persimilis works very well with A. fallacis as the two do not significantly interfere with each other. P. persimilis easily works at all levels of webbing, but it is only effective against the 2-spotted Spider Mite. We do not recommend A. californicus as the generalist because they significantly affect P. persimilis by eating their eggs and actually repelling them. P. persimilis avoid leaves that had A. californicus on them.
In extreme conditions, during high heat and light, none of the predatory mites are comfortable working in exposed areas, such as the flower heads. This can be a very serious situation for the grower. The beetle, Stethorus punctillum is a spider mite specialist and it will readily move onto the exposed spots to consume spider mites. Release rates for Stethorus are still being developed but we have used 10 per square meter effectively for the hot spots. Being flyers, Stethorus should also be released preventatively once the warmer weather begins at a rate of 0.1 per square meter every two weeks, while the weather is hot.
Thrips: A lot of effort is expended to manage Thrips populations. Thrips also love Gerbera, and can cause economic damage very quickly, as they are attracted to the flowers. Having Ss or Gg in the pots, and on the floors is a starting point, but with Thrips, we need all of the help we can get. Our strategy is to introduce A. cucumeris very early and often. A. cucumeris, like fallacis, is a generalist mite, capable of living on pollen, Whitefly eggs, Spider Mites and Thrips. They are a very effective Thrips predator. Under balanced conditions, they will wait by the Thrip emergence holes and then bite the Thrip’s head off when it emerges. They are also capable of consuming the 1st instar larvae of the Thrip and harassing the 2nd instar thrips, reducing their successful pupation.
Cucumeris should be applied at 50 per square meter, per month, until Thrips become rampant. They should then be applied at 50 per square meter every 2 weeks until the Thrips season is over. We do not recommend swirski for most situations due to the cost. In extreme heat, during the summer, swirski may be the better choice, but only if the average temperature is above 26 degrees C. In addition, swirski interferes with the aphid predatory midge Aphidoletes and can have a negative effect against P. persimilis. An unsprayed Gerbera crop, or at least, a spot-sprayed crop will also allow Orius to naturally enter the house. Orius are true bugs and specialize in feeding on Thrips, tackling even the adults. You can purchase Orius, but Orius is quite expensive and very prone to diapause, therefore preventing early establishment.
There are many physical controls that are effective with Thrips. Yellow or blue sticky cards can actually remove a significant number of flying adults. The trick is to keep the cards low, at or below flower height. Thrips fly just over the “treetops” so hanging cards over 2 inches above the flowers will miss most of the Thrips. Thrips are also attracted to volatile plant esters. The best two to try are Vanilla and Almond extracts. You can purchase the extracts at any grocery store. Put a drop on every card, or even better, stick a cotton ball on the card and apply the Vanilla or Almond to the cotton ball. You can purchase “pheromones” for Thrips, but they are expensive and probably work the same as the extracts. Another technique is to fill a white pan with 1 inch of soapy water, with vanilla or almond or peppermint as an attractant and place the pan under the affected area. The pan idea should also be used by every doorway, as thrips tend to tumble into the greenhouse through the doors. This trap will catch them before they can work their way into your crop.
This pest can overwhelm a Gerbera crop very quickly. Fortunately, the parasitoid Diglyphus works extremely well. As in all parasitoids, early introduction and appropriate numbers are essential. Good control goes along with good Whitefly control, as the honeydew of the Whitefly also impairs the Diglyphus. Another parasitoid, Dacnusa, can be effective but we feel that Diglysphus does a better job for the price.
Aphid can be a big problem, but seldom shows up in Gerbera. The presence of the Foxglove Aphid in North America should be a concern to all growers, as the plant range appears to be increasing for this formidable pest. At any sign of Aphid, the Gerbera grower should not hesitate to apply 0.5 Aphidoletes aphidimyza per square meter weekly. They will quickly eliminate any Aphids, but, more importantly, Aphidoletes is proving to be a very effective Whitefly predator, totally unaffected by the honeydew. We are currently running trials with Aphidoletes on Whitefly hot spots, with very promising results.
|Weekly:||1 to 4 per square meter
0.5 to 2 per squ. Meter
|Encarsia formosa, for Whitefly
Aphidoletes aphidimyza, for Aphid
|Monthly:||1 per square meter
50 per square meter
|Delphastus catalinae, for Whitefly
Amblyseius cucumeris, for Thrip
|Once:||250 per square meter
2 per square meter
|S. scimitus or G. gillespiei, for all
Amblyseius fallacis, for Spider Mite
|As needed:||1 per 100 Spider Mite10 per square meter
1 per square meter
10 per square meter
|Phytoseiulus persimilis, Spider MiteStethorus for Spider Mite
Orius, for Thrip
Delphastus for whitefly
|As directed:||Diglyphus, or Dacnusa||For Leaf Miner|