ISSUE 4 May 25, 2000
LEAFHOPPERS PRESENT IN THE REGION
It is time to begin monitoring for leafhoppers in the region. During the 1999 season Potato and Aster Leafhoppers caused problems in many local crops. The Potato leafhopper affected dry beans, potatoes, and alfalfa. The aster leafhopper moved the mycoplasma-like organism Aster Yellows (AYP) to flax, canola, sunflower and others.
Adult Aster Leafhopper can be found in grassy areas and moving into some small grain fields. They are the greyish looking leafhoppers active in field borders. If you look closely, six spots on the top of the head are visible.
The Aster leafhopper migrates into the region AND overwinters as an
egg in grass leaves. Typically, the migrating population of adults from southern states
are the ones with the greatest risk of carrying AYP and they are the primary contributor
to AYP transmission to hosts. The local population typically has a lower level of
infectivity. Factors that contribute to infectivty are: feeding on AYP infected plant;
and, an incubation period of 2 to 3 weeks before the AYP can be transmitted. Adults
migrating from southern states are more likely to be AYP carriers because of infected host
plants where they originated and they have the
necessary time needed for AYP to replicate itself in the insect.
Aster yellow infections are very difficult to predict. Risk of problems relates to the size of the insect population and what level of infection is present. The economics of control to try and prevent AYP are not very good for most field crops. High value vegetable crops, such as carrots, should be monitored and treated when aster leafhopper is present. In this case, the insect is controlled to limit transmission. Once AYP is aquired by a plant, there is nothing that can be done. Symptoms require a week or more to appear in infected plants. Typically, a longer time is required for symptoms to appear in older plants. The later the infection, the less impact on the overall health of the plant. Therefore, later planted crops are at greater risk to AYP.
Over 300 species of plants have been identified as hosts of AYP. The vegetable crops (carrots, lettuce, celery) get most of the attention because of their production value. Other plants infected include grains (oats, barley, wheat, rye) and weeds (quackgrass, lambsquarters, sowthistle, dandelion). The aster leafhopper also moves the Oat blue dwarf virus to small grains.
Because of so much uncertainty about infection and poor economic incentive to control the vector, treatment has not been encouraged for field crops and small grains.
The Potato leafhopper invades North Dakota fields each year following migration of the leafhoppers from southern states where they overwinter. An early arrival can mean problems for a number of crops. Places to look for leafhoppers now include alfalfa, and young dry beans.
The small (1/8 in.), pale green, wedge-shaped winged adults move rapidly by jumping. Sweep nets are useful for confirming their presence in a field. The nymphs will be paler green, lack wings, and characteristically walk sideways when disturbed on the leaf surface. The nymphs will be found on the undersides of the leaves. The nymphs are generally more damaging than the adults since they feed for several weeks on the leaves where they hatch. Adults move around much more.
Feeding injury by potato leafhoppers results in a symptom called "hopperburn". The visual symptoms include leaves changing from green to yellow to brown as they deteriorate. When leaf injury is present, plant growth is impaired. Control of potentially damaging populations should not be delayed to the point where damage symptoms are visible.
In alfalfa, potato leafhoppers that arrive early can cause problems with regrowth after the first cutting is complete. Monitoring regrowth with the use of 15 inch sweep nets to detect adults is highly recommended.
Leafhoppers/net sweep (average)
8 - 11
1.0 adult or nymph
12 - 14
2.0 adults or nymphs
Insecticides approved for use to control leafhoppers in alfalfa include: permethrin*, Baythroid*, carbaryl, dimethoate EC (Cygon, De-Fend), Furadan*, Lannate*, Lorsban, Malathion 57 EC, Penncap-M*, and Warrior*.
In dry beans, the threshold for basing spray decisions is when an average of one leafhopper per trifoliate leaf is found. Last years early arrival of leafhoppers resulted in adult infestations in seedling dry beans. Many people treated these fields, however, reduced rates of insecticide were generally used. Reduced rate treatments were effective at reducing the adult populations on these small plants and allowed for growth before additional adults moved back to the field. On larger plants with nymphal populations at threshold levels, labeled rates would be recommended. Insecticides approved for use to control leafhoppers in dry beans include: Asana XL*, Dimethoate EC (Cygon, De-Fend), endosulfan (Thiodan, Phaser), Lannate*, Malathion 57 EC, Orthene 75S, and Penncap-M*.
In potatoes, it is suggested that an adult threshold
based on sweep net sampling be used to make decisions. If an average of ½ to 1 adult
leafhopper per pendulum sweep is found, treatment would be recommended. Later, treatments
are recommended when leafhoppers can be found at a level of 1 nymph per 10 leaves. At that
time, sample 35 leaves from 5 locations in a field. The nymphs are sampled by selecting
leaves, and counting the number present. Sample for the presence of adult leafhoppers by
using a sweep net. Do not let infestations and damage progress to the point that yellowing
of foliage is easily detected. Insecticides approved for use to control leafhoppers in
potato include: permethrin*, Asana XL*, carbaryl, Dimethoate EC, endosulfan, Furadan 4F*,
Guthion, Imidan, Vydate*, and Penncap-M*.
Sugarbeet root maggot flies have started emerging from old beet ground. The degree day model for emergence is getting close to peak emergence time. Next week, we would expect significant increase in fly emergence in the region. Hopefully beet growers have sticky stakes out to monitor adult densities around fields in areas where the SBRM has been a problem. The fly catches should be used in combination with degree-day (DD) accumulations for post treatment decisions.
Using ambient (air) DD accumulations
Calculating ambient DDs uses 47.5EF as the base temperature. With this procedure, the first 80E day following accumulation of 600 DD usually coincides with peak activity in new beet fields. Currently we have reached the upper-400 DDs.
Post liquid treatment for SBRM control need to be applied 3 days before peak fly activity, which would be around 520 to 550 DD and when the daily temperature gets up to 80E. High winds and low temperatures can keep SBRM from moving from the old beet fields to the new beet fields. It is very important that 520 to 550 DD have accumulated and the weather has warmed up to 80E for post liquid applications.
Post treatments of granular insecticides may be a very good option for controlling the SBRM in high population areas. Granular insecticides can be applied up to ten days ahead of peak fly activity.