Insect Management

Aphid, Asparagus

DOES SPRAYING FOR APHIDS CONTROL VIRUS?  WHEN

SHOULD I SPRAY FOR APHIDS?

    Several years ago I was talking with a vegetable grower who mentioned that he thought it was about time to start spraying for aphids.  I asked if they were starting to build up in the field.  He didn’t know, but said he wanted to use preventive treatments to control aphids so they would not bring a virus disease into his crop.  We checked plants randomly throughout the field and found no aphids.  The grower decided not to spray at that time.  This saved him a few hours of time, the cost of material and equipment operating costs.  It only took us about ten minutes to check the field.

    This is not an uncommon situation. Throughout July and August, aphids show up in many crops, and growers are concerned about whether and when to spray.  Many growers feel they must have a rigorous spraying program for aphids to protect their crops from viruses.  All too often this practice is not effective in preventing the occurrence of virus diseases, and can cost the farmer money as would have been the case in the above situation.

    I think it would be helpful to briefly review some of the basics of how viruses are spread.  Virus diseases require a living host, and when the host plant dies, any virus within the host plant cannot survive.  (An exception is tomato/tobacco mosaic, which can survive in dead host tissue.)  For the most part, viruses survive the winter in certain perennial weeds.  During the growing season, viruses can be transmitted from perennials to a susceptible vegetable crop.

    Most vegetable virus diseases that are important in New England are spread by insects.  Cucumber beetles, thrips, leafhoppers, and nematodes can spread certain viruses, but aphids are the most important vectors (carriers).  Viruses can be classified as persistent and nonpersistent.  This is related to the manner in which the virus is spread by insects and is important to know in choosing the appropriate management strategy.

    An insect must feed for a minimum of ten minutes to an hour to pick up a persistent virus from an infected host.  The virus must then undergo a dormant period of at least 12 hours within the insect before it can be transmitted to another plant.  Aphids will remain infective (able to vector a virus) for at least a week and maybe throughout their life.  A good insect management program can be very helpful in dealing with persistent virus diseases.

    Aphids pick up nonpersistent viruses by merely probing (exploring) an infected leaf.  This happens rapidly--within seconds or minutes.  A dormant period is not required and the aphid can immediately transmit the virus by probing another plant.  Aphids remain infective with nonpersistent viruses for a short time (minutes).

    Systemic materials are generally the most effective insecticides available for aphid control.  Systemic insecticides are taken into the plant and become present in the plant juices.  Aphids feed by sucking juices from the plant, and when they do so they also ingest some of the insecticide.  However, when just probing a leaf an aphid is not feeding and does not ingest plant juices or insecticide.  In fact, the presence of an insecticide may actually stimulate probing and cause aphids to move from plant to plant in an effort to fine a suitable feeding site.  This can increase the spread of nonpersistent viruses. For this reason nonpersistent viruses are very difficult to manage.  There are no pesticides that kill viruses and, as we have seen, they may actually make matters worse. 

   Eradication of perennial weeds around fields can reduce the source of the virus.  The green peach aphid is not the only aphid that transmits viruses, but it is important because it is a universal vector.  Prunus species (peaches, cherries etc) are attractive to green peach aphids.  Removal of  wild cherry trees from around fields can make the area less attractive to green peach aphids.

    Reflective mulch such as aluminum foil on paper has been used successfully to repel aphids and thus can be effective in reducing virus problems.  However this material is expensive and tears easily when laying.  Row covers such as Remay can keep aphids off a crop, but they are generally used during the cool days of spring whereas aphids are most active during warm weather. (In fact, use of row cover over a crop that is already infested with a small number of aphids can result in an outbreak of aphids,  because the natural predators are excluded while aphids reproduce rapidly in the high temperature.)

    Direct damage from aphids: Besides spreading virus diseases, aphids in high numbers can cause economic damage by their feeding activities.  Leaf curling and yellowing or deposits of honeydew on leaves or fruit can affect crop quality or yield.  For this reason it is important to manage aphids even if viruses are not a concern.  However, beneficial insects such as ladybeetles, lacewings, and parasitic wasps often keep numbers low enough to prevent direct damage.  Early sprays targeting aphids may actually result in further aphid outbreaks, because the natural enemies that keep them in check have been killed.

    Scouting across the field gives you an estimate of current numbers.  If aphids are present, check back in a few days to see if the numbers are increasing or decreasing. Take note of which natural enemies are present.   Check undersides of leaves, including lower and mid level leaves.  The following thresholds can be used to determine if insecticides are needed (sampling routine in parenthesis):

--Pumpkin and winter squash: 20% of leaves have more than 10 aphids (based on 50 leaves).

--Pepper: 10 per leaf (based on 4 leaves per plant, 25 plants).

--Tomato: 6 per leaf (based on 2 leaves per plant, 25 plants).

--Potato: 4 to 10 per leaf (based on 25-50 compound leaves; higher threshold near harvest).

--Sweet corn: 50% of plants with >50 aphids at emerging tassel (based on 100 plants). 

    When spraying for aphids, whenever possible select a systemic insecticide, or a selective insecticide that wilh not harm natural enemies.

--John Howell, Rob Wick, Ruth Hazzard