Spider Mites, Aphids, Leafhoppers, More: Don’t Let Your Guard Down Just Yet

By Jane Fyksen
Thursday, August 9, 2007 11:35 AM CDT

Spider mite damage

Crops Editor

Soybean growers should be out in their fields, looking for spider mites. Aphids are also abundant, but farmers cautioned about spraying R6 beans; no yield benefit is gained.

Lack of rain, coupled with high temperatures, is increasing the possibility of economic spider mite populations. “Recall that in 2005 in Eastcentral Wisconsin and in 2006 in Northwestern Wisconsin, spider mite populations started earlier in July and were at very high levels in some fields by early August,” says UW-Madison entomologist Eileen Cullen. “Currently, we have not yet seen widespread outbreaks, but have had a few reports from drier areas.”

Lee Milligan, St. Croix County agricultural agent, says that “in addition to soybean aphid being a problem in several St. Croix County soybean fields, the two spotted spider mite has been reported in a few soybean fields for several weeks. The numbers have not warranted treatment, however, if the drought condition continues, they could become a serious problem very quickly. The potential exists for another outbreak similar to 2006.”

At this point in the season, its essential that growers confirm a mite problem prior to taking action, stresses Cullen. Scout the edges and corners of fields first. Pull plants and shake some of the bottom leaves over a sheet of white paper and look for “moving dirt,” i.e. mites. If they’re on the field edges, then check the rest of the field. By the time economically treatable populations are present, stippling (yellow spotting) and general yellowing of leaves will be visible.

Cullen says mites insert their mouthparts into leaf cells extracting the contents of individual living cells that results in many small white or yellow spots (i.e. stippling). “Another good word for the leaf symptoms would be ‘speckling’,” she says. “From a distance, affected fields are apparent by leaf yellowing.” Although infestations often start at field edges, as noted, they can also be seen within fields on drier knolls and can be “patchy” in distribution within the field.

Don’t assume rain will eliminate either aphids or mites. It’s important to scout for mites if you’re contemplating spraying for soybean aphids. Spraying for aphids with pyrethroid sprays could aggravate the situation with mites. Pyrethroids perform poorly against spider mites and could even “flare” a mite increase.

The only products recommended for spider mites are the organophosphates chlorpyrifos (Lorsban) and Dimethoate.

There’s no specific economic threshold for spider mites in soybeans. Before spot treating, thoroughly scout the entire field. If mite injury is evident in the interior, there’s potential for economic populations within one to two weeks. Eggs hatch in two to four days; nymphs develop in two to four days; and adults can live up to 21 days with better survival when it’s hot and dry. Depending on temperatures, spider mite generations are completed in 4 to 14 days, with the fastest developmental rates above 91 degrees.

Cullen recommends moving at least 100 feet into the field before making your first stop. Walk a “U” pattern, checking at least two plants at each of 20 locations.

Assess mite damage using the following scale:

0 n No spider mites or injury observed

1 n Minor stippling on lower leaves, no premature yellowing observed

2 n Stippling common on lower leaves, small areas or scattered plants with yellowing

3 n Heavy stippling on lower leaves with some stippling progressing into middle canopy; mites present in middle canopy with scattered colonies in upper canopy; lower leaf yellowing common; small areas with lower leaf loss. (Spray threshold)

4 n Lower leaf yellowing readily apparent; leaf drop common; stippling, webbing and mites common in middle canopy; mites and minor stippling present in upper canopy. (Economic loss)

5 n Lower leaf loss common, yellowing or browning moving up plant into middle canopy, stippling and distortion of upper leaves common; mites present in high levels in middle and lower canopy.

Again, Cullen recommends treatment only if damage and mites are detected throughout the field. Use the above scale and treat when injury progresses to a rating of 3. Fields with ratings of 5 or worse may not be salvageable. Check fields every four to five days if drought persists since damaging infestations can develop quickly.

When considering a late season treatment growers must be aware of 21 and 28-day pre-harvest intervals of Dimethoate and Lorsban, respectively.

Edge treatments are not considered to be effective since mites are usually spreading throughout the field before any visual symptoms are noted.

Cullen says that R4 (full pod) and R5 (beginning seed) are critical stages for determining soybean yield. Spider mite feeding reduces photosynthetic area and accentuates drought stress, reducing pod set, seed number and seed size. “If plants are killed, pod fill is stopped in its tracks,” she discusses, noting that “pods on mite-stressed plants are more likely to shatter, which compounds yield loss.” Only a 10 to 15 percent reduction in effective leaf area creates yield losses that’ll justify application. “Unfortunately, it’s not easy to estimate a 15 percent reduction in effective leaf area,” she notes.

“With increased mite injury, leaves become yellow, bronzed, brown and may eventually drop off the plant,” says Cullen, reporting that mites disperse within and between fields by climbing to the top of the plants and spinning silk strands that, when caught on the breeze, let them drift to new hosts.

Two spotted spider mites overwinter as adult females in sheltered field margins. In most years, adequate rainfall and a fungal pathogen that is the primary natural control prevent outbreaks. In their absence, mites reproduce fast, with eggs, nymphs and adults found together on plants.

Cullen says that “following any August rainfall, relatively cooler temperatures and high humidity can foster” the fungal pathogen “enemy” of mites. Mites look discolored, waxy or cloudy and they die within one to three days of infection. It has to be cooler than 85 degrees with at least 90 percent humidity. Mite populations will decline rapidly once fungal disease takes hold.

According to Krista Hamilton with the Wisconsin Department of Agriculture, Trade and Consumer Protection, many fields in the east central and west central counties have been treated for mites to reduce economic populations and minimize leaf drop. Some soybean fields in the northwest are also experiencing heavy mite populations, she notes. Hamilton, too, warns that growers statewide should continue to be alert to the possibility of mite outbreaks in soybeans. As noted, treatment is suggested if damage is before R6 to R7.

Soybean aphids increasing

Soybean aphid densities have continued to increase over the past week and economic infestations are prevalent in the southern districts. A report from Columbia County indicates erratic densities within fields there. Some plants are saturated with aphids on new growth and along stems and other plants have relatively few individuals. Inconsistent infestations such as these can complicate management decisions, says Hamilton.

To decide if a foliar spray is warranted, examine a minimum of 20 to 30 plants distributed throughout the field, and treat only if 16 to 24 plants (80 percent) or more have 250 soybean aphids per plant. Treatment decisions should be made immediately before fields surpass the R4 (full pod) and R5 (beginning seed) growth stages. No yield benefit is gained by treating fields at R6 (full seed) and beyond, stresses Hamilton.

Annual aphid survey conducted

Results of an annual survey of 227 soybean fields carried out from July 12 to 31 by the state ag department indicate high soybean aphid populations for the west central district and portions of the southwest, south central, and central districts, moderate populations for much of the central and north central districts, and low populations for the southeast (with the exception of Walworth County), east central, northwest, and northeast districts.

An overwhelming majority of the survey sites (82 percent) contained non-economic levels of soybean aphids below 250 per plant. A total of 73 percent of the fields sampled averaged fewer than 100 aphids per plant; 10 percent averaged 101 to 250 aphids per plant, 9 percent averaged 251 to 500 aphids per plant; and 8 percent averaged greater than 501 aphids per plant. The highest average number of aphids per plant recorded was 3,250 in a Columbia County field. Approximately 35 percent of the soybean fields examined were at full bloom (R2), 51 percent were at beginning pod (R3), and 14 percent had reached full pod (R4).

Despite generalized regional trends, scattered fields with economic populations of soybean aphid were detected in most of the nine agricultural districts. For example, the average number of soybean aphids per plant was 56 for the entire southeast district, but two of the five fields examined in Walworth County had economic densities of 293 and 346 aphids per plant. Similarly, the average number of soybean aphids per plant was 109 for the north central district, but Price County fields contained an average of 407 aphids per plant.

The survey revealed a large geographic area north and west of the Wisconsin River, and south of Highway 29, in which economic densities of soybean aphids developed in most fields. This area included Crawford (899 aphids per plant), Richland (1,071 aphids per plant), Vernon (253 aphids per plant), La Crosse (744 aphids per plant), Monroe (808 aphids per plant), Juneau (295 aphids per plant), Jackson (618 aphids per plant), and Wood (332 aphids per plant) counties. Columbia and Marquette counties also had high county average soybean aphid densities of 810 and 292 aphids per plant, respectively.

The statewide average number of soybean aphids per plant was 164, which compares to 69 per plant in 2006, 108 per plant in 2005, 11 per plant in 2004 and 618 per plant in 2003. The department’s annual soybean aphid survey is conducted during the R2 to R4 stages of soybean growth to detect peak seasonal soybean aphid densities and to assess fields while treatment may still be beneficial. Many fields statewide continue to be sprayed for this pest. A report from Columbia County indicated several R4 stage fields were treated from July 31 to Aug. 2, and sprayer tracks were noted in some Dane and Sauk County fields surveyed last week.

Soybean rust in new state

USDA's soybean rust website (www.sbrusa.net/) reported the first detection of Asian soybean rust in Oklahoma on July 26. The Oklahoma finds represent the most northern occurrence of soybean rust in 2007. As of Aug. 1, Asian soybean rust has been reported in Alabama, Arkansas, Florida, Georgia (on kudzu), Louisiana, Mississippi (on kudzu), Oklahoma, and in Texas. Weather conditions have been favorable for rust development in many parts of the south and to the states just north of the Gulf States.

Corn needs watching, too

The summer flight of European corn borer moths has produced higher numbers at the Arlington, Lancaster, Mazomanie, Sparta and Chippewa Falls black light trap sites. Egg laying is underway in sweet corn and snap beans, and only a few days remain for cornfields in the southwest, west central, and south central districts to be assessed for damage by the second generation of European corn borer larvae. The treatment window is expected to close in these areas following the accumulation of 2,100 GDD (base 50 degrees) this week.

Western and northern corn rootworm beetles were very active last week, according to department pest-watchers, and present in field corn in moderate to high numbers. Surveys in Columbia County revealed counts of 1.1 to 24 beetles per plant, with the western specie predominating. Individuals of the northern specie were most numerous in the tips of corn ears. An exceptional Columbia County field had as many as 11 northern corn rootworm beetles in a single ear tip. Surveys in the east central district yielded lower counts of 0.7 to 2.7 beetles per plant. An average of one beetle per plant indicates the potential for larval injury to continuous corn next season. Corn rootworm beetle populations in fields with brown silks are expected to decrease as the adults move to preferred feeding sites with fresh silks.

Colonies of corn leaf aphid have become more evident on corn in the east central and southern areas of the state. High populations of 50 to 110 aphids per plant on 30 percent of the plants were noted in Sheboygan and Washington counties, and populations of 10 to 85 aphids per plant on 15 to 20 percent of the plants in scattered Columbia, Dodge and Sauk County corn fields. Aphids were concentrated on tassels and ear leaves. In most instances, fields with higher populations are already pollinated and should not be greatly affected.

As for corn earworm, the Insect Migration Risk Forecast predicted a “moderate” risk of migration into Wisconsin through Aug. 7. The potential exists for a sizeable flight of corn earworm moths to arrive during this period. To view the complete forecast, visit http://agweather.niu.edu/IMRFForecast.html.

Potato leafhoppers: vigilance

Cullen urges alfalfa growers to “remain vigilant” for potato leafhopper threshold densities and treatment potential through August and into early fall. “Routine scouting is advised as long as hot and dry conditions continue to promote the rapid development of this insect,” she maintains.

According to Hamilton, ag department surveys in the south central, southeast and east central districts indicate that potato leafhopper counts in alfalfa are not uniformly above treatment thresholds. Counts in the south central district varied from 0.1 to 2.4 leafhoppers per sweep with an average of 0.8 per sweep. Two of twelve fields surveyed in Dane, Green and Rock counties had economic populations of leafhoppers. Averages in 18 to 2-inch Dane County alfalfa were very low and ranged from 0 to 0.7 per sweep, suggesting these fields had been treated recently.

Counts in the southeast and east central districts ranged from 0.1 to 3.0 per sweep with an average of 1.2 per sweep. Five of 11 fields surveyed in Dodge, Fond du Lac and Washington counties had economic populations of potato leafhoppers. The highest numbers were noted in Washington County where surveys yielded 2.3 to 3 leafhoppers per sweep in 10 to 12-inch fields and 0.7 per sweep in one 12-14 inch field. Populations of two or more leafhoppers per sweep are at or above the treatment threshold, she notes.

“Despite generally lower sweep net counts, adults have been very numerous at lights and make up a high percentage of the insects caught in some black light traps. Reproduction, as indicated by a strong representation of nymphs in virtually all of the fields sampled, has not slowed noticeably in the past week. Hopperburn is particularly severe in those fields under drought stress,” says Hamilton.

Cullen adds that some county agents have been questioning the efficacy of fertilizer pellets/granules coated or impregnated with Lorsban insecticide for potato leafhopper control. She says this is not recommended or economically justified. While some organophosphate insecticides have varying degrees of systemic activity (i.e. mild), Lorsban isn’t considered systemic, “nor expected to have significant uptake in a plant with fertilizer granular activity.” Stick with foliar treatment.

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