Insect Update - June 20, 1999

Leafhoppers
Populations of both Aster and Potato Leafhopper are high in Minnesota this spring. These insects become re-established in the state each year as individuals are blown north from southern states where they overwinter. Leafhoppers are currently being reported in potato, dry beans, alfalfa and even small grains. Although only the first three crops are at risk from leafhoppers, populations in small grains may move into crops that can be damaged by these insects.
Potato leafhopper is a small (1/8"), pale lime-green, wedge-shaped insect. The adults are winged and move very rapidly. The nymphs are similar to the adults, only smaller and lack wings. Aster leafhoppers are roughly the same size and shape as Potato Leafhoppers but are brownish gray with a line of dark spots between their eyes. Both insects have sucking/piercing mouthparts, resembling straws, with which they suck plant sap. Leafhoppers inject saliva into the plant as part of the feeding process. The repeated piercing and injection of saliva reults in a physiological response in the plant. Initial feeding often appears as small pitting or discolored spots. Heavier feeding results in crinkling of the leaf and 'hopperburn', a characteristic change in coloration as the leaves turn from green to yellow to reddish brown. Hopperburn first appears at the terminal edge of leaves and impairs plant growth. Fields with extensive hopperburn take on a reddish color. Potentially damaging populations of leafhoppers should be controlled before the onset of these symptoms.
Leafhopper nymphs tend to feed on the underside of leaves and walk to the other side of the leaf when disturbed. Nymphs cause more damage to crops than do adults due to their greater numbers and higher feeding rates. In addition, adults are more mobile and tend to feed on a number of different plants while nymphs tend to remain on the same plant until they become adults.
In dry beans, the treatment threshold is one leafhopper per trifoliate leaf. Leafhopper damage from adult feeding is unusual but has been reported in dry-bean fields this year. Reduced rate applications have been effective against adults. When treating nymphs, however, it is recommended to use the lower level of the label rates. Populations of nymphs are generally more numerous than adults and by the time nymphs are present, the plant canopy is fuller, resulting in more plant area to be treated. Insecticides labeled for control of 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, treatment thresholds for populations of nymphs are 1 nymph per 10 leaves. Scouting for nymphs must be done by selecting mid-leaves and counting the number of nymphs present. Adult leafhoppers can be scouted with a sweep net. Sweeping in a pendulum motion (one swing across the body and back) through the canopy, complete a series of 20 pendulum sweeps in at least 5 locations, ensuring good coverage of the field. Treatment thresholds are 10 - 20 adults for every 20 pendulum sweeps. Insecticides labeled for control of leafhoppers in potato include: permethrin, Asana XL, carbaryl, Dimethoate EC (Cygon, De-Fend), endosulfan (Thiodan, Phaser), Furadan 4F, Guthion, Imidan, Vydate, and Penncap-M. Reduced rates are also recommended for treating adult leafhoppers in potato.

Grasshoppers
First and second instar grasshopper nymphs are starting to show up throughout the state. We probably are not looking at an outbreak year, especially in those areas that have recently had cool wet weather (these conditions help build up the fungal diseases that control grasshopper populations). However, we will likely experience hotspots as we do every year. Grasshopper populations don't just suddenly appear, they build up over years. So, if you had an area last year which had high grasshopper populations, that's where you'll probably find high populations this year. Grasshoppers prefer to lay their eggs outside of crop fields (with the exception of soybeans and alfalfa) so you'll find them first at the field edges. Now's the time to be scouting field edges for grasshopper nymphs. Walk along the edge, picture a 1 ft. sample square ahead of you and count the number of hoppers that are in the square. Take the average of at least 20 squares per edge, multiply by 9 and you have the estimate of grasshoppers per sq. yd. The treatment threshold for nymphs is 25-35 nymphs / sq. yd. At the field edge.
The Grasshopper management webpage for Minnesota provide detailed scouting, threshold and treatment information and can be found at either of the following sites:
http://www.nwes.umn.edu/ent/gh98/hopper1.html
http://rrc2.rrcnet.org/~swes/SWMNPEST/hopper1.html
A hardcopy brochure summarizing the information is also available at the above sites or can be obtained by contacting the authors.

Update on Cereal Insects
Aphid populations in small grains have started to rise in some areas of the state. The recent cool, wet weather is ideal for aphid reproduction. Cereal aphids do not overwinter in Minnesota, rather they overwinter in southern states and populations are re-established by individuals carried north on weather fronts each spring. Aphids have piercing/sucking mouthparts and feed on plant sap. This causes long-term physiological stress on the plant, and heavy aphid populations can cause stunting and grain loss. However, yield loss generally results from either very high populations on plants or moderate populations established on plants for extended periods of time. Consequently, it is important to treat early; ideally treating for aphids should be done when wheat is at the flag leaf stage. This means scouting wheat at or before flag leaf. Select 100 tillers in a way that ensures even coverage of the field (see figure 1), treat if >80% of the tillers have aphids on them. In the past, aphid insecticides have been tank-mixed with fungicides targeted against leaf diseases and applied when wheat was at the flag leaf stage. Since Fusarium Head Blight has become the major disease concern in small grains, most fungicide application is now occurring when wheat is heading. There is a temptation to delay aphid treatment until heading and tank mix the insecticides with fungicides to save money on application costs. Research suggests this is too late to avoid loss of yield from aphid feeding; by heading, most of the yield loss from aphid feeding has already occurred. Treating earlier will prevent aphid populations from developing to the point where yield is lost. One way to save on insecticide inputs is to scout fields to ensure insecticides are only applied when necessary.
Early and continued scouting of aphids this year is also recommended due to the amount of late planted wheat. These plants are at greater risk of being exposed to aphids vectoring Barley Yellow Dwarf Virus (BYDV). This plant disease will cause discoloration and stunting of cereals. Isolated fields have been reported in North Dakota with low infestation rates but as the season progresses and earlier planted wheat matures, aphid populations will move into younger, later planted fields. As aphid populations grow through the year, a higher percentage of aphids will be vectoring BYDV and so these later planted cereals will be at a greater risk of contracting this disease.
Barley thrips have been noted in a number of cereal fields in the Red River Valley. Populations of this insect usually expand when the weather is hot and dry. However, high populations in cereals now may indicate problems later. The threshold for Barley Thrips is 8 per head. To scout for Barley Thrips, you must select and open tillers, counting the thrips that are present.
Orange Wheat Blossom Midge (OWBM) emergence will soon start throughout northwest Minnesota. Accumulated growing degree days (GDD) for OWBM indicate that the counties in the southern valley have reached the 1200 accumulated GDD (most recent GDD map for OWBM can be found at http://www.ext.nodak.edu/weather/idd/idd40.gif). This marks the start of the emergence of OWBM. Peak emergence is at 1400 GDD and emergence subsides at 1600 accumulated GDD. This early emergence is good news. Many late planted fields will escape the OWBM due to the fact that emergence will be completed before heading of the late planted crop. Early planted fields, especially those planted on wheat on wheat, are at risk though and should be scouted for presence of OWBM. Scouting is only effective just before dusk when wind speeds are below 6 mph and temperatures are above 59 degrees Fahrenheit as those are the conditions necessary for egg laying activity. Treatment thresholds are 1 adult midge per 5 heads. The only insecticide labeled for control of OWBM in small grains is Lorsban 4E-SG.