Hugh Smith, Vegetable Entomologist, University of Florida, Institute of Food and Agricultural Sciences, Gulf Coast Research and Education Center, Balm, FL.
The silverleaf whitefly, Bemisia tabaci, is a serious pest of watermelon, and presently transmits at least three viruses that can greatly reduce watermelon yields in Florida. These viruses are Squash vein yellowing virus (SqVYV), which causes watermelon vine decline, Cucurbit leaf crumple virus (CuLCrV), and Cucurbit yellow stunting disorder virus (CYSDV). Common cucurbit weeds including balsam-apple, smellmelon and creeping cucumber can host each of these viruses and should be destroyed. Green bean (Phaseolus vulgaris) is also a host of CuLCrV, and some types of pigweed (Amaranthus spp.) can harbor CYSDV.
Whiteflies have piercing-sucking mouthparts, and they acquire viruses by feeding on infected plants. Presently there are no commercially available watermelon varieties possessing any level of genetic resistance to whitefly-transmitted viruses. Clean culture is the backbone of whitefly and virus management: harvested fields should be promptly destroyed so as not to serve as a source of whiteflies or virus. Watermelon fields that are not being actively managed for pests due to poor markets or other factors can also serve as a source of whiteflies and virus. Metalized mulches can help repel whiteflies early in the season, but their effect diminishes with time.
Guidelines for whitefly management in watermelon and other crops focus on insecticide use. Table 1 lists key insecticides registered for use on watermelon in Florida that can help reduce whitefly numbers. Insecticides are effective when whitefly numbers are low or moderate, but cannot always reduce high whitefly populations. Whiteflies can develop tolerance to insecticides if the same mode of action is used repeatedly. The mode of action is the specific way the insecticide kills, and it is assigned a code number to make it easier to avoid overusing the same mode of action on a given farm or field. Table 1 lists the modes of action for some common whitefly insecticides. As a general rule, insecticides with the same mode of action being applied for whitefly management can be used more than once within a five-week interval on a field or group of fields, but should not be used during the following five-week interval. The objective of grouping whitefly insecticides in five-week intervals is to avoid treating successive generations of the population with the same modes of action to reduce the development of insecticide resistance. Insecticides with the same number in the code should be grouped together for purposes of resistance management. For example, insecticides in groups 4A and 4D should be grouped together, as should insecticides in groups 9B and 9D.
The most effective insecticides for whitefly and virus management are in group 4, the nicotinic acetylcholine agonists, and group 28, the diamides. These two groups of insecticides are systemic. They can be taken up by the foliage or roots, and move within the vascular system of the plant where they are fed upon by whiteflies. These materials should be deployed early in the crop cycle when protection against whitefly-transmitted viruses is crucial. Amongst the group 4 insecticides, dinotefuran (Venom) and flupyradifurone (Sivanto Prime) have consistently provided superior whitefly control when populations from south Florida have been tested. Thiamethoxam (Platinum, Actara) is also an effective whitefly material. Imidacloprid (Admire Pro, many generics) is effective against some populations, but tolerance has been documented. Of these products, only Sivanto Prime has been shown to have limited negative impacts on commercial pollinators when used according to the label.
Cyantraniliprole is a group 28 insecticide that is sold as Verimark for application to the root zone and Exirel for foliar application. Verimark can be applied to transplant trays in the nursery up to 72 hours prior to planting in the field, but not earlier. The manufacturers of Verimark provide very specific instructions for treating transplant trays with Verimark, which can provide effective early season protection against whiteflies and virus. Growers wishing additional information should contact their FMC representative.
Buprofezin (Courier) and pyriproxyfen (Knack) reduce the viability of whitefly eggs and prevent nymphs from completing their development to the adult stage, but have no direct effect on adult mortality. Spiromesifen (Oberon 2SC) is an effective spider mite material that can also help reduce numbers of whitefly eggs and nymphs. The group 9 insecticides (Fulfill, PQZ, Sefina) kill whitefly adults and first stage nymphs (crawlers) but have limited impact on established nymphs. There are many pyrethroid insecticides registered for use on Florida watermelon not listed in Table 1 that may help manage whitefly populations. Given the potential of pyrethroid insecticides to flare thrips and spider mite problems, they should not be the products of first choice for whitefly management. The Vegetable Entomology lab at GCREC screens whiteflies collected from commercial fields for susceptibility to key insecticides. Growers interested in having whitefly populations tested should contact Hugh Smith (tel.: 813-419-6588; email: firstname.lastname@example.org).