David Owens, Extension Entomologist, owensd@udel.edu
Greenhouses
The first watermelon transplants will soon be hardening off. Scout greenhouses for aphids and spider mites. Mite feeding will appear as small yellowish stipples on the upper surface of the new leaves. Check flats around the edges of tables, near vents and any weeds that might be growing in the greenhouse. To scout for aphids, kneel down and try to look at the leaves from underneath so that any aphids are backlit. Heavily infested plants will also have leaves that cup downward, and you will see small honeydew specks on tray surfaces. Also beware of aphid colonization when plants are on transplant wagons. The easiest and most straightforward way to treat either pest is to wait until plants are on transplant wagons and apply an aphid material or a miticide. If using a neonicotinoid, read the label carefully, as the rate for a tray dip or treatment is different than that used in the field for cucumber beetle. Check to make sure that a tray treatment does not preclude you from using that active ingredient later on for cucumber beetle. Anything applied to seedlings will only protect them for a short period of time, and may not provide protection once cucumber beetles become active somewhere between the second and fourth week of May. For mites, there is a second option to use predatory mites in greenhouses. They will not immediately control the population, it takes them a couple of weeks to establish themselves. However, they will colonize the plants and then disperse from the field or greenhouse looking for spider mites in weeds along field edges. The most readily available species is Phytoseiulus persimilis, which is a spider mite specialist. Without spider mites, this species will starve. It reproduces as quickly as spider mites. Another species, Neoseiulus fallacis, is less readily available and more expensive but is a native species that we have observed previously in melon fields.
Allium leafminer oviposition scars on an onion leaf
Allium Leafminer Alert!
Allium leafminer has been detected infesting onion and garlic at a farm in Sussex County. It also damages leeks, green onions, and wild onions. This damaging fly oviposits in leaves as well as feeds on oviposition scars. Look for rows of circular puncture scars on leaves. Only the adult stage can be controlled. Once eggs hatch, larvae enter leaves and feed towards the base of the plant for several weeks. Infested leaves will twist and distort. Bulbs may be more susceptible to bacterial and fungal infection and bulbs will be smaller.
Plants can be covered with row covers to prevent infestation during the flight period. Pennsylvania research suggests that the first flight is active around when pear begins blooming. I suspect in our area the flight would run from March to the end of April. Extension recommendations include destroying infested material at the end of harvest. Be sure to also control wild onion.
Larvae pupate in the soil or at the base of bulbs and go dormant until late September or early October. During the flight period, adults can be controlled with Scorpion, Assail, Exirel, Radiant and Entrust (organic). M-Pede may also help improve control. Be sure to get good coverage on the waxy upright leaves. This can be done with twin flat fan nozzles. Research with Radiant and Entrust suggests that two applications can provide acceptable level of reduced infestation, with sprays applied 2-3 weeks after initial detection and applied 1 to 2 weeks apart. This will not eliminate an infestation, but reduce the number of larvae present.
Cole Crops
Continue scouting for worms. Cabbage whites have been active several weeks now and are ovipositing into various cole crops. Early thresholds are pretty high, between 20-30%. Imported cabbageworm, or cabbage whites, are fairly easy to control with numerous materials. Be sure to get good coverage on plants, spreader sticker type adjuvants are helpful to get good deposition on the waxy leaves. If you use Radiant or a diamide like Coragen, Harvanta, or Exirel, do not use a sticker. These products try to get into the leaf tissue while a sticker tries to keep it on the leaf surface.