The recent warm weather has caused the sudden appearance of many insect pests in vegetable fields. Insects that overwinter in Michigan need two major factors to emerge: adequate degree day accumulations and adequate moisture. The cool, wet weather we experienced in mid-May provided sufficient moisture. The very warm weather during the past week has meant a rapid rise in degree days. Consequently, some insect pests such as Colorado potato beetle and European corn borer (see photo), which would normally emerge during several weeks of moderate temperatures, emerged all at once, in high numbers in the hot weather.

Colorado potato beetles are present in potato and tomato fields and are mating and laying eggs. Eggs will generally hatch in three to six days, depending on temperature. Fields treated with neonicitinoids (Admire®, Platinum®, Cruiser®, Gaucho®, etc.) should be protected. In untreated fields, insecticide applications should be made at approximately 10 to 20% egg hatch to effectively control larvae. In many cases, only edges or “hot spots” need to be treated in rotated fields. Reducing the number of insecticide applications made, rotating insecticide classes and using non-neonicitinoid insecticides, and spot treating fields reduce the risk of insecticide resistance development.

In many areas of Michigan, large numbers of European corn borer moths have emerged from overwintering. In Monroe County, the number of European corn borer moths caught in pheromone traps during the week of May 23-30 averaged over 160 per trap (the previous week’s average was 0). In Ingham County, (MSU Campus), the number averaged 34 per trap. In Macomb County, an average of 18 European corn borer moths were caught per pheromone trap. European corn borer moths emerge from overwintering and begin to fly and lay eggs at about 450 GDD50. Many areas of the southern and central Lower Peninsula are already at that threshold. The high temperatures have caused high numbers of moths to emerge simultaneously. Sweet corn and potatoes are vulnerable to European corn borers, which cause damage to sweet corn mostly after tassel (from the second generation of larvae). Very small corn is not attractive for egg laying. Even if eggs are laid, larvae don’t generally survive on corn before the 10-leaf stage. Before tassel, some corn borers may feed on leaves and burrow into the midrib. Larger larvae may bore into the stem. Treatment is only recommended if more than 15% of the plants are infested at the whorl stage.

Potatoes are generally less attractive to European corn borers than sweet corn. However, if the corn is small and the potatoes are larger and actively growing, female moths may prefer to lay eggs on potato. Eggs are laid on the underside of potato leaves and larvae eventually bore into the potato vines. Although potatoes can tolerate a good deal of European corn borer damage before yield is affected, holes created by larval boring create an entry for disease. Scout potato fields for evidence of European corn borer eggs or larvae (holes in stems that are surrounded by green sawdust-like frass). An insecticide may be warranted if more than 15-25% of the plants are infested. Neonicitinoid insecticides used to control Colorado potato beetle are not effective against European corn borer or other caterpillars.

Recent weather patterns have also brought in an influx of migratory pests. Potato leafhoppers are present in potato fields in southeast Michigan. Potato leafhoppers cannot overwinter in Michigan, and they feed on many crops. Vulnerable crops, including potatoes and snap beans, respond to the leafhopper’s toxic saliva with a loss in conductive tissue. The visible symptom created by feeding, hopperburn, occurs when the margins of the leaf turn yellow and start to roll. Yield loss, however, occurs before hopperburn is seen. It is important to scout fields regularly for potato leafhopper and apply an insecticide if needed to prevent damage. The treatment threshold is five potato leafhoppers per ft of row (2 per ft of row for seedlings) for snap beans. For potatoes it is five adult leafhoppers per 10 sweeps or 1 nymph per 10 leaves. Neonicitinoid insecticides provide some protection against potato leafhopper, but it does not last the entire season. Throughout the summer, growers should check snap beans and potato fields for leafhoppers (and tarnished plant bugs) when nearby alfalfa fields are cut.

Weather fronts may have also brought in high populations of another migratory pest, aster leafhopper. Large populations are now reported from lettuce fields in Macomb County, and growers are encouraged to check their celery and carrot fields, as recent weather conditions were perfect for aster leafhopper migration. Aster leafhoppers transmit the disease aster yellows to vulnerable crops, such as lettuce, celery, and carrots. The May 24 issue of the Vegetable CAT Alert discussed how to determine the treatment threshold, which is dependent on the inherent susceptibility of the crop, the number of leafhoppers and the proportion of leafhoppers that carry aster yellows. The last factor is unknown at this time because few aster leafhoppers have been found up until now. Diagnostic Services is ready to test leafhoppers for infectivity rate (see the May 24 issue for details), and growers and scouts are encouraged to send leafhoppers for testing. A conservative estimate of 2.5% infectivity can be used until the true number is determined. This translates into a treatment threshold of 10 leafhoppers per 100 sweeps for lettuce, 14 per 100 sweeps for celery and 20 per 100 sweeps for susceptible varieties of carrots. Insecticide treatment is warranted if numbers meet or exceed this. See Bulletin E-312, 2006 Insect, Disease and Nematode Control for Commercial Vegetables, for insecticides registered to control insect pests on various vegetable crops. This bulletin can be viewed at http://veginfo.msu.edu

This article was initially published in The Vegetable Growers News April 2006 issue P23-24.

Records as early as 1927 show Michigan as the leader in United States pickling cucumber production. In 2005, Michigan pickle production was valued at over $35 million. Michigan’s dominance in the pickle industry is the result of many factors, including use of high quality seed, good agronomic practices, and mechanical harvesting. Once over mechanical harvesting is without a doubt the single most important factor that improved pickling cucumber production in Michigan. This technology has reduced the labor inputs needed for multiple hand picks and, most importantly, significantly increased planting densities. Until 1948, the USDA recommendation for pickling cucumber spacing was 6 to 7 feet between rows and 3 to 4 feet between plants in the row, for a planting density of 1,500 to 2,500 plants per acre. In contrast a 2003 Michigan survey found spacings as close as 12 inch rows with 4 inches between plants within the row (about 130,000 plants/acre) were being used successfully.

The need for high planting densities
Before mechanical harvesting, pickling cucumber was grown at low densities and yield was maximized by multiple hand picks. In machine -harvested cucumber production systems: however, all plants are harvested destructively when they average 1 to 1.5 fruit per plant. Under such a system, yield is maximized by increasing plants per acre rather than number of fruits per plant. High densities are achieved by reducing between and in row plant spacing. In 2003 and 2004, we conducted an experiment with a total of 12 densities using combinations of 4 row spacing s (12, 18, 24 and 30 inches) and 3 plant spacing s inside the row (4, 5, and 6 inches). Spacings were selected based on current practices in Michigan and to achieve specific plant populations. We found that number of fruits per plant quickly becomes a limiting factor for yield increase at low planting densities. Even though increased planting density reduces the total number of fruits per plant, the total number of fruit per acre increases. Maximum yield was found at a density of about 130,000 plants/acre. Beyond this density, competition among cucumber plants becomes a limiting factor and can result in significant yield reduction.

Maximize economic value not yield
If seed were free, then planting densities with the highest yield would also produce the highest economic (dollar) value. (see Figure) However, profitability of pickling cucumber is not just a function of total fruit weight but is also dependent on seed cost and selling price. Therefore, seed cost should be included in the analyses of studies designed to identify optimum pickling cucumber densities. In our economic analysis we used an average cost of $1.65 per thousand seeds and a selling price of $3.25 per bushel for grade 2 and 3 (marketable grades for machine-harvested cucumbers). Economic value was obtained by multiplying yield by selling price and subtracting seed cost. We found optimum planting density required to maximize economic value was significantly lower than that required to maximize yield. With a 5% margin of error, optimum economic value is obtained with densities between 72,000 and 120,000 plants/acre. Optimum density for highest economic value varies depending on seed cost and cucumber selling price. The higher the seed cost the lower the optimum density. Also the lower the selling price the lower the optimum density. For pickle production to be a viable business, growers must use planting density as a tool to maximize economic value instead of to maximize yield.

Other factors that should be taken into account when selecting pickling cucumber planting density include: cultivars and growing conditions. We used the cultivar Vlaspik with indeterminate growth habit. Other cultivars, especially those with determinate growth habit, would respond differently. Growing conditions (especially fertilization), pest management, irrigation, and harvester efficiency also impact optimum planting density. Finally, experienced growers will maximize economic value at a specific planting density through adequate timing of harvest.

Between May 28 and 29 the weather favored a clear and sunny sky with maximum temperatures exceeded 90°F in many parts of the state. These conditions in early season are conducive to heat injury with plastic mulches because the crop canopy is not yet large enough to cover an important portion of the mulch. Black plastic mulches are widely use in vegetable production to control weeds and to warm the soil in early season. However, under hot and sunny days they can cause significant injury to the crop. The injury is usually related to heat and is either direct or indirect or both.

Direct crop injury
Black mulches allow little light wave lengths to pass through. They first absorb the light that is then converted into heat. The heat is finally transferred to the soil. For greatest efficiency it is usually recommended to have close contact between plastic and soil. Under a clear and sunny day, the black mulch surface temperature can exceed 130°F. This results in injury or desiccation of most plant parts (root, stem, leaves and fruits) in direct contact with the mulch.

Indirect crop injury
Crop injury can still occur even if the crop is not in direct contact with hot plastic mulch. During a sunny day, hot air builds up between the plastic and the soil. If the plastic is loose, its flapping (even under a light wind) creates movement of hot air and funnels it through crop holes. This causes stem girdling and transplants desiccation, a problem commonly observed with young pepper transplants.

Hardened transplants will tolerate heat injury better than young and tender ones. Crop holes on the plastic should be round, cut evenly and large enough so that there is no direct contact between the plant and the plastic. The transplant should be centered in the middle of the hole.

Infra-red transmitting (IRT) mulches maintain a cooler surface temperature than black mulches under hot and sunny days. But they are also more expensive than black mulches.

After the injury: Minimize the effects. Depending on the importance of the injury growers might consider replanting. If injury is extensive and occurs when the transplant is very young, replanting might be a good option. When replanting is not possible, it is better to minimize the effects of other stresses (drought, diseases, nutrients) on the plant. For peppers, the plants become very susceptible to wind damage and should be staked and tie as early as possible.

Soil and climatic conditions have varied greatly so far this growing season. Rainfall and soil temperatures have been variable. Over the last few days with high temperatures and plenty of moisture, crops have grown tremendously. Soil microorganisms have also become much more active in converting organic nitrogen into plant available ammonium and nitrate N. Where cover crops, compost or even crop residues have been incorporated into the soil, considerable nitrogen may become available. Nitrogen provided by one of these sources or even soil organic matter is nitrogen that does not need to be provided by purchased nitrogen.

Soil sampling is the only way to know for sure how much plant available nitrogen is becoming available in the soil. Collect 15 to 20 soil cores to a depth of 12 inches from the area of interest not to exceed 20 acres. Dry the sample as quickly as possible before sending the sample to the soil test lab. Also, keep the samples cool as possible. If the soil will be delivered directly to the soil test lab, then drying is not necessary. At a value of 40 to 50 cents or more for each pound of nitrogen, finding out that the soil already contains 30, 40, 50 or more pounds per acre can have a significant financial impact.

EPA has approved a Section 18 Emergency label for use of Spartan 4F herbicide on strawberry in Michigan in 2006. The Section 18 label is similar to those issued in the past few years. Spartan is labeled primarily for control of common groundsel, field pansy, mayweed chamomile, redroot pigweed, white campion and yellow woodsorrel. Spartan also will aid in suppression of yellow nutsedge and other broadleaf weeds.

Spartan may be used at renovation or after dormancy has set in late in the season. The actual labeled dates are June 25 to July 25, and October 15 to December 15. Do not apply to green strawberry leaves, which can cause crop injury.

Regional reports
1 -- Southwest
Ron Goldy

Weather
Temperatures through the period were generally above normal with highs ranging from 68 °F to 93 °F. This was our first above 90 °F for the season. Lows ranged from 47 °F to 70 °F. There was 0 to 0.75 inches of rain across the area depending on where the thunderstorms tracked. Some areas are in need of rain. Soil temperatures are in the mid- to upper 60s.

Asparagus harvest continues but at a much faster pace due to higher temperatures. Harvesting is nearing completion in some younger fields.

Fields are being prepared for pumpkin planting next week or the week after.

Watermelon and cantaloupe transplanting is underway.

2 -- Grand Rapids Area
Amy Irish-Brown

Weather
Air temperatures have been much above normal – quite a change from early last week when we had frost. We’ve moved back ahead of normal on degree day accumulations and are running about a week ahead of normal averages for degree accumulations of all bases. Rains across the region have been variable. There was some hail in spots Tuesday afternoon.

Radish planting continues.

Lettuces on muck soils are in various stages of growth.

Sweet corn continues to emerge. There is some slight damage from frosts last week, but overall early sweet corn is looking good. Additional planting continues.

Vine crop seeding and transplanting continues.

Asparagus harvest continues. Picking was slowed a bit last week by the cooler than normal weather, but has made a comeback this week with warmer temperatures and some rain.

Tomato transplanting continues both on plastic and without.

3 -- Oceana County
Norm Myers

Weather
Oceana County experienced widespread rain last evening and overnight. I had over an inch in my gauge this morning. Parts of Mason County should be pretty wet, since most areas there received over an inch of rain over the weekend as well as last night’s rain. The rain was welcome in Oceana County since it was getting dry enough to begin irrigation in a lot of places.

In carrots, we did have a little damping off show up during the cool snap, and even one field replanted because of it. Aster leafhopper numbers are increasing but were still below threshold before last night’s rain.

Planting of winter squash is underway, and last night’s rain should help that crop get off to a good start. Planting of pumpkins and zucchini should begin next week.

In snap beans, early crops have been planted.

5 -- Macomb, St. Clair and Lapeer counties
Hannah Stevens

Weather
We won’t soon forget the record-high temperatures last weekend continuing through Tuesday with a seasonal high of 95 ° F on Monday (May 29) and soil temperatures on bare ground (mineral soil) reaching 89 ° F. Humidity was high as well, so “sweltering” would be a good descriptor. Rainfall totaling 1.2 inches in central Macomb slowed fieldwork once again, and some plastic that should have been laid is just going down. Thunderstorms yesterday evening dumped 0.75 inches in a short period of time in Macomb and soil moisture is adequate. Growers north of Macomb County have not seen measurable rain this week, and irrigation pipes are being laid on some crops.

In lettuce, a ster leafhoppers were in abundance rather suddenly on Tuesday. Other susceptible crops should be scouted at once.

Unfortunately some early seeded squash was lost to frost last week. No striped cucumber beetles at this time.

Garlic scapes are beginning to emerge in the crop

Tomatoes, peppers and melons continue to be set on plastic as weather allows.

Pumpkin seeding continues, and rye cover crops are beginning to pollinate.

6 -- Monroe County
Paul Marks

Weather
We received about three quarters of an inch of rain this last week in several showers on Thursday and Friday (May 25-26). Most vegetable growers are just now back in fields attempting to finish transplanting warm season crops. Accumulated degree days now total 497 using a base of 50 which is almost exactly at the 30-year normal for this date. Daily high temperatures as well as nighttime temperatures in the low 70s have brought on several new pests for this season.

Potatoes planted early are fully budded with some flowers showing. Late planted fields are still emerging. Leafhoppers have appeared this week and are above threshold in more than half of the early fields I have checked in the last several days.

Cabbage is developing small heads in early fields. Harvest is still two weeks away. Root maggot damage is very evident in many fields and serious in those which were untreated. Check out this photo at the ends of rows where insecticide was not applied to see how serious the damage might have been without controls. (Photo 1)

Sweet corn has a number of problems developing. Armyworms (Photo 2) have appeared in several fields and seem to be more of a problem in fields planted in killed rye cover crops. Corn borer traps averaged more than 140 adults per trap this week indicating the first generation in our area has great potential.

Cucurbits have been invaded this week with stripped cucumber beetles (Photo 3). Damage appeared almost overnight in most fields. Those treated with preplant insecticides seem to be free of the pest for now.

The upper air ridging feature that brought hot and humid weather to Michigan during the Memorial Day weekend has moved off to the east of the region. This will signal the beginning of a new upper air weather pattern that will lead to cooler, more seasonable temperatures during most of the week ahead. Wednesday morning (May 31), a cool front stretching from northern Lower Michigan southwestward to the southern Great Plains was making its way very slowly to the south and east. Large quantities of tropical-origin moisture remain in place ahead of the front, which will set the stage for another round of showers and thunderstorms Wednesday afternoon through early Thursday. Best chances for rainfall will be over southern and eastern sections of the state Wednesday. With very slow movement expected, some thunderstorms will lead to some heavy rainfall totals and possibly to some flooding, especially across central and southern Lower Michigan Wednesday with some 2.0-inch totals possible.

Drier weather is expected statewide by late Thursday into early Friday. An upper air troughing feature is expected to form over the region by late Friday, which will bring relatively cold air aloft into the region and an almost daily chance for scattered showers or thundershowers Friday, Saturday, and Sunday, especially during the late afternoon and early evening hours. Temperatures during the next several days will slowly fall back to near normal levels. Highs Wednesday will be in the 70s to low 80s over most locations, falling back to a range from near 70 in the north to the upper 70s south Friday through Sunday. Low temperatures will remain in the upper 50s and 60s overnight Wednesday, falling back to the upper 40s north to mid-50s south this upcoming weekend. The next chance for significant rainfall will come late Monday or Tuesday as an area of low pressure and frontal system approaches from the west.

Medium‑range forecast guidance for at least the next one to two weeks is suggesting the formation of an upper air ridging pattern across the Great Plains region with more of a west-northwesterly component across the Great Lakes.

The NOAA 6-10 day and 8-14 day outlooks (covering June 5-9 and 7-13) both call for temperatures to range from above normal in extreme western sections of the state (closer to the axis of the upper air ridge) to near normal elsewhere. Precipitation totals during the 6-10 day time frame are forecast to remain at near normal levels, and range from near normal across Upper Michigan to below normal over the Lower Peninsula during the 8-14 day period.