Striped Cucumber Beetle and Bacterial Wilt

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

Striped cucumber beetles (Acalymma vittatum) (SCB) are the most important insect pests of muskmelon and cucumbers in our area. They overwinter as adults and emerge when temperatures reach 54–62°F at which time they begin searching for cucurbit hosts. Volatiles produced by the plant attracts SCB to cucurbits initially, then male SCBs produce an aggregation pheromone attracting more beetles. The beetles tend to mass on small plants where they eat, mate and defecate (Fig. 1).

Figure 1. Early season feeding of SCB on cucumber

This type of frenzied activity where there are many beetles feeding on a few leaves or a small plant leads to increased chances of bacterial wilt development. The bacterium that causes bacterial wilt in cucurbits, Erwinia tracheiphila, is in the cucumber beetle’s feces. As the beetles defecate on the leaves where they are feeding the bacteria can be moved into open (feeding) wounds with water that is in the form of precipitation or dew. The more beetles that are feeding and opening wounds on susceptible crops like cucumbers and cantaloupe the greater the chance of bacterial wilt infection. The bacteria multiply and block plant xylem, restricting water flow to the rest of the plant; plants wilt and eventually die (Fig. 2). The wilting usually starts with just one heavily chewed upon leaf wilting and then this wilting progresses to the stem of the leaf and then to major vines of the plant. This process of vines and the entire plant wilting down can take 2-6 weeks after initial infection, but because the non-infected parts of the plant continue to grow growers might think when they see a plant wilt down that infection took place just within the last few days (Fig. 3).

Figure 2. Cantaloupe plant killed by bacterial wilt infection

One additional problem with SCB and why control sprays may not work as well as they should under some conditions is that the beetles are consistently hiding at the base of the plant (in the plastic hole) where they are feeding on the stem (Fig. 4). Sprayers are set up usually to cover a lot of leaf canopy and often do not do a very good job of putting chemical down in the plant hole. This stem feeding can be severe enough to cause some wilting. It is hard enough to control cucumber beetles with a good cover spray, but when only small amounts of spray are reaching them down in the plastic hole they will not be controlled.

Melon cultivars have different susceptibilities to bacterial wilt infection. Watermelon is almost immune to infection while squash and pumpkin are moderately susceptible. Cantaloupe and cucumbers as well as some of the specialty melon types are much more susceptible. Among the most susceptible cultivars are, Honeydew 252 and HD150 which are honeydew melons; Da Vinci which is a Tuscan type melon and Miracle and Sheba which are a netted yellow-green melons. Among the most tolerant cantaloupe cultivars are Aphrodite, Athena, Accolade and Astound which are all eastern cantaloupes and just happen to all start with A. The management methods that are recommended for bacterial wilt control for standard cantaloupe varieties (using seed treatments and insecticides when beetles reach 1 per plant or using kaolin clay or row covers before beetles appear) work well. For the specialty melons more attention is needed to carefully follow management recommendations.

Figure 3. Only the leaves at the base of the plant (arrows) were initially infected with E. tracheiphila but the whole plant eventually will die.

Figure 4. Striped cucumber beetle feeding damage at base of small plants

Pollination in Seedless Watermelons and Honey Bee Placement, Bumble Bees as Pollinators

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

A female watermelon flower will need around 500-1000 pollen grains to be fertilized effectively. This will require a minimum of 8 visits by a honey bee for seeded watermelons. In seedless watermelon more visits will be required. The pollen produced by seedless watermelons is not viable. To fertilize seedless watermelon, pollen must be transferred from viable male flowers in standard or special pollinizer seeded types to triploid seedless female flowers. Because bees foraging in seedless watermelon plantings carry a mix of viable and non-viable pollen, more pollination visits (16 to 24) by honey bees are needed to set fruit.

First planted watermelons are now flowering in Delaware and Maryland. Honey bees should be placed when the first female flowers appear to achieve good crown sets without defects (i.e. prominent lobes or hollow heart). Placement should be made before 10% of plants are in bloom.

The crown set in watermelon is fruit that set on one of the first 8 nodes of the plant. This is often the most profitable, especially early in the season. Poor crown sets in watermelon can occur when there is poor weather during early flowering. Honey bee flights are reduced significantly in rain and when winds are 15 mph or greater. Cloudy weather also reduces bee activity. Honey Bees also do not fly much below 55°F, so on cold mornings, as we often have in June, bee activity will not pick up until later in the morning. Unfortunately, female watermelon flowers open early in the morning, are most receptive before 10 am, and then close in the afternoon.

In addition, in early mornings and during poor weather, bees usually visit plants closest to the hives. As the temperature rises or the weather improves, the bees will forage further from the hive. This means that in bad weather watermelons closest to the hives will have the best set and furthest from the hives will have reduced set.

Another problem that causes crown set reduction is the loss of pollenizer plants due to unfavorable weather conditions during or after planting. This means that pollen will be limiting. Research has shown that were pollen is limiting, fruit numbers will be reduced with distance from a pollen source. In fields with limited pollen, expect reduced fruit set or reduced fruit size in areas where pollenizers are missing.

Watermelon growers can manage crops for improved pollination and fruit set with honey bees by:

  • Increasing the number of honey bee hives for early watermelon crops. A minimum of one strong hive per acre is recommended in general and 2 hives per acre can be justified for early planted fields.
  • Placing hives in several locations in a field rather than just on one edge. While bees will fly over a mile, the best pollination activity is closest to the hives. Hives placed within the field will provide more bee visits to the crop compared to edge placements. Place hives in groups of 4-8 in good locations throughout the field to have even distribution of bees.
  • Having ample sources of pollen by planting pollenizers at a minimum ratio of one pollenizer per every 3 seedless plants. Use the most effective pollenizers as shown by local trials. In-row pollenizers should have limited competitiveness with the seedless melons.

Bumble Bees
Compared to a honey bee, bumble bees are about 10 times more efficient as a pollinator due to their size, the speed at which they transfer pollen, the efficiency with which they gather pollen within various crops, and their increased endurance to fly in adverse weather for longer periods of time. The bumble bee also has the ability to buzz pollinate the flower for pollen, a pollination technique not seen in honey bees. Buzz pollination occurs by bumble bees vibrating the flower by pumping their wings at a certain frequency, to dislodge pollen. Bumble bee foraging activity starts earlier and ends later in the day than managed honey bees and they forage in lower temperatures. Because of these characteristics, fewer bees are needed to achieve the same crop pollination and commercial colonies only have about 200 bees each (800 per quad).

When assessing bumble bee activity, flag out 10 areas in your field and observe each area on three different days during bloom. These observations should last one minute under sunny, windless conditions, between 9 a.m. and noon. Approach each plot with care so as not to disturb the foraging bees. Stand about three feet from the crop to avoid blocking the flight path of the bees. Count and record the number of bumble bees at each flag, then calculate the average for your observations. You should an average one bumble bee per ten flags (0.1 bees per flag) to have adequate pollination.

Bumble bee colonies should be shaded and can be placed along shaded field edges. However, if there are other wild flowers nearby, they will also work in those areas, reducing their field effectiveness. Therefore, when placing bumble bees in watermelons or other flowering vegetable or fruit fields needing pollination, it is recommended that bumble bee quads be placed in the field middles under a shade canopy to have more foraging in the target field. Bumble bees should be placed far from honey bee hives to avoid honey bee pollen theft from bumble bee nests.

 

 

 

 

 

Vegetable Crop Insect Scouting

David Owens, Extension Entomologist; owensd@udel.edu

Cucurbits
Cucumber beetles continue to move into fields. It is important to base a treatment decision from 5 to 10 locations per field. I visited a couple of fields this week where cucumber beetle activity was heavy, but only on a couple of rows and a few plants within those rows. For watermelon, we use an action threshold of 2 beetles per plant for young transplants. Other states use a threshold of 5 beetles per plant once vines start running. Unlike watermelon, cucurbits such as summer squash, cucumber, and cantaloupes are susceptible to bacterial wilt. These crops are also more attractive to cucumber beetle than watermelon, thus you may need to treat them more often or see more aggressive cucumber beetle populations. Pumpkins are not very susceptible once they have two true leaves (not counting the cotyledons), and so a seed treatment should provide sufficient control to avoid bacterial wilt. We have been relying heavily on the Group 4A chemistry early, products like imidacloprid, acetamiprid, and thiamethoxam, followed by Group 3 pyrethroids late. There are a couple of other chemistries that may provide good cucumber beetle control: Lannate, Sevin XLR plus, and two diamides, Exirel and Harvanta. Our group is in the process of collecting cucumber beetles from across the area to test susceptibility to the commonly used insecticides. If you have a treatable population, please let me know before or within 1 day of treating and I will gladly take some cucumber beetles off of your hands. Thanks!

Spider mites can be found in some fields, most likely coming in with the transplants. At this stage, you will see the light yellow stippling on the leaves. This stippling is not as apparent on older melons once the leaves thicken up until there are very high populations underneath the leaf. Agrimek or other Abamectin containing Group 6 products can provide excellent control and are systemic miticides. They are hard on bees, so if you are using them during the season, the best window for using them is prior to bee arrival and to apply in the evening when pollinators are not as active.

Sweet Corn
Sweet corn pheromone and blacklight traps are checked twice weekly on Mondays and Thursdays. By Tuesday and Friday morning, data is uploaded to our website: https://agdev.anr.udel.edu/trap/trap.php. For reference, action thresholds based off of blacklight and pheromone trap can be found here: http://extension.udel.edu/ag/insect-management/insect-trapping-program/action-thresholds-for-silk-stage-sweet-corn/. Silking sweet corn is highly attractive to moths, a trap that is nearby but not adjacent to sweet corn may not be entirely representative of the population in your block. Thursday trap counts are as follows:

Trap Location BLT – CEW Pheromone CEW
3 nights total catch
Dover 0
Harrington 1 0
Milford 0 4
Rising Sun 0 1
Wyoming 0 1
Bridgeville 0 0
Concord 2 1
Georgetown 0 0
Greenwood 2
Laurel 2 10
Seaford 2 2
Harbeson 0
Trap Pond 0 0

Guess the Pest! Week 6 Answer: Soil Compaction

David Owens, Extension Entomologist, owensd@udel.edu and Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Congratulations to Will Carlisle for correctly answering soil compaction. Will will receive a sweep net and be entered along with all correct guessers for the end of season raffle. Unfortunately, this is one case where a sweep net is not going to do much to alleviate the problem, unless you put a shovel or soil corer on the end of the handle.

This from Gordon Johnson:

Peas do not perform well in soils that are worked when they are too wet. Compaction will lead to poor emergence and reduced growth. Wet soil conditions, compaction, and poor drainage are also associated with higher rates of infection of root rots in peas such as Aphanomyces root rot, or common root rot. Soil compaction limits root development and root function and will reduce yield potential in vegetable crops such as peas.

There are two processes at play when soils are compacted by equipment. The first is destruction of soil structure. In most Delaware soils, our surface soil structure is granular or crumb in nature and consists of small aggregates. It takes considerable time and good cropping practices to build up soil structure. When compacted by equipment, structure is destroyed, making soils denser. Excessive tillage also destroys soil structure.

A second compaction process is the compression of soil particles, pushing them closer together. This happens with equipment traffic across fields. The heavier the loads carried by equipment passing over soils, the more the compaction. With large equipment and heavy axle loads, significant soil compaction is expected; the heavier the weight on an axle, the more the compaction. Other equipment factors affecting compaction include tire size, tire pressure and operating speeds. Wider tires or dual tires will distribute weight over larger areas, reducing deep compaction but increasing the amount of area with shallow compaction. Higher tire pressures will result in more deep soil compaction and slower speeds will also result in more compaction.

In wet soil, there is less resistance to soil particle movement and soil is more “plastic”. This means that potential for compaction is greater in wet soils than dry soils. It is important to wait until soil conditions are favorable for tillage. Waiting a day or two for soils to dry will improve yield potential by reducing compaction.

Subsoiling in the fall is a short-term solution to deep compaction. The use of forage radish cover crops has shown great potential to reduce shallow and deep compaction. Research in Delaware has shown that peas can be no-tilled after a winter-killed radish cover crop successfully with equivalent or better yields than conventionally tilled peas.

Flea Beetle Feeding and Tomato Early Blight

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

I visited a few tomato fields this week and found 2 to 4-week-old tomato plants with some early blight (Alternaria solani) and in some cases bad early blight lesions. This is very early in the season to be seeing this level of early blight. Many of the plants had a few flea beetle adults on the plant (Fig. 1) and in the areas where the early blight was found also had moderate to high flea beetle feeding (Fig. 2). In some cases I could not find any flea beetles after the rains we have had and in other cases I could find a few of them. Normally the amount of flea beetle feeding I saw would not have been of much concern, but flea beetles can cause increased infections of Alternaria leaf blight in tomatoes and potatoes and possibly other early blight susceptible crops. I found that there was a strong relationship between the amount of flea beetle feeding and the amount of early blight on tomato plants in different fields of a few farms. If you have moderate flea beetle feeding damage to your Solanaceae plants and you see any early blight starting you’ll need to control both the beetle and the disease. Pyrethroids should work well in controlling flea beetles. There is not much organically that will control flea beetles once they are causing economic damage (there are some things that can be done though, to reduce flea beetle problems before flea beetles cause damage, more at: https://attra.ncat.org/attra-pub/download.php?id=135). Using kaolin clay (Surround) before beetles begin to feed on plants is one organic possibility as is using spinosad on beetles after they start to feed.

Flea beetle adults are generally small and range in size from 0.05 to 0.15 inch. They overwinter as adults on weed hosts surrounding the field, on residues of a previous tomato crop, or in the soil if the previous crop was a flea beetle host. Some flea beetles (Systena blanda – the pale striped flea beetle being one) can feed on amaranths or pigweeds (Fig. 3) and will readily move from them over to your crops. Other flea beetles are more host specific (the eggplant, potato and tobacco flea beetles feed on Solanaceous plants while others prefer broccoli, cabbage and other cole crops). However all adult flea beetles have similar damage patterns, they chew small round holes in leaves, which make them look as if they have been damaged by fine buckshot, called “shot-holing”. The white larvae feed on underground parts of the plant, but this damage is usually not economically significant. There is normally a second generation during the summer and at times even a third depending on species. Normally foliar damage to larger plants is not considered to be economically important but feeding damage to small plants or seedlings can reduce stand or vigor of the plant. The other exception about flea beetles not being economic pests is when Alternaria is associated with their feeding on smaller tomato plants.

Figure 1. Underside of tomato leaflet with two flea beetles (Epitrix sp)

Figure 2. Tomato leaf with old flea beetle feeding and early blight

Figure 3. Pale striped flea beetle feeding on amaranthus weed

Allium Leafminer Moving South in Maryland

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

The new pest of onion, leek and garlic, the Allium leafminer, is moving south in Maryland. It was first observed in Maryland in Cecil Co. in 2017, but now the fly’s tell-tale marks (Figs. 1 and 2) have been found in a Baltimore City chives planting. This new pest was first found in Lancaster County, Pennsylvania in December 2015. Unfortunately, it is my guess that the pest is now probably in many northern/central areas of Maryland. New transplants or seedings of onions or leeks should be watched closely for the tell-tale signs of the fly’s damage which are several very small white dots in a row along the leaf of an allium plant (Figs. 1 and 2).

Figures 1 and 2. Tell-tale marks on allium leaf made by Allium leafminer females

Penn State has a great deal of good information about the new pest which can be found at: Penn State Allium Leafminer Pest Alert page. Growers should look for these tell-tale signs on any newly planted allium species, but especially on leeks. You can cover any Allium planting with row cover to keep the flies off or if needed treat with insecticides as found in the 2019 Mid-Atlantic Commercial Vegetable Recommendations guide.

Yellowing in Peas

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Pea harvest is nearing and we are seeing yellowing and poor growth in many pea fields due to wet conditions. Peas do not perform well in soils that are worked when they are too wet or when they receive heavy rainfall after planting. Compaction and crusting over will lead to poor emergence and reduced growth. This is evident in many Delmarva pea fields in 2019.

Recently, heavy rains have caused some pea fields or parts of fields to turn yellow, particularly were there was compacted soil or poor drainage. Peas are effective at fixing nitrogen; however, we normally apply 40-80 lbs/a of fertilizer nitrogen (N) prior to planting thus reducing N fixation contributions from Rhizobium nodules on the roots. With the frequent rainfall, some fields have remained saturated and denitrification has occurred, reducing available N from the initial fertilizer application. In addition, root function and Rhizobium nodulation is further impaired in saturated soils, thus limiting any potential N fixation contributions.

In pea fields that have had a past history of root rot, we have the potential to see problems in 2019. According to the Crop Profile for Peas in Delaware: “Aphanomyces root rot, or common root rot, is one of the most destructive diseases of peas. It occurs in most pea producing regions of the U.S., including the Mid-Atlantic. In the Northeast, average annual yield loss to this disease is about 10%, though losses in individual fields may be up to 100%. Wet soil conditions and poor drainage are associated with higher rates of infection. The disease is most damaging in years when a cool, wet spring is followed by an early, warm summer with low rainfall.”

Good pea growth and development.

Yellowing in peas in wet soils

Vegetable Crop Insect Scouting

David Owens, Extension Entomologist; owensd@udel.edu

Potatoes
Colorado potato beetle adults are active and laying eggs in potato fields. Most of the insects observed this week were confined to the edge of the field, in part because beetles do not fly when temperatures are below 80 degrees. Sample a field from 10 locations. At each location, count beetle adults and larvae per stem from 1 stem on each of 5 adjacent plants. Thresholds are 50 adults, 75 large larvae, and 200 small larvae per 50 stems. There are many non-neonicotinoid options available that can be used in a rotation strategy to ease selection pressure off of neonicotinoids. They can be found here: https://cdn.extension.udel.edu/wp-content/uploads/2012/03/23152414/Potato.pdf.

Sweet Corn
Scout for cutworm injury. Thresholds are similar to field corn. Trapping data can be found here for European corn borer and corn earworm; trap data is updated by Tuesday and Friday mornings: https://agdev.anr.udel.edu/trap/trap.php.

Watermelons
Continue scouting for cucumber beetles, especially on transplant trays that are hardening off on wagons. Recent cool, wet weather has slowed beetles down, but the weather is warming back up. This year, the Extension entomology team is looking to collect several hundred beetles from individual farm sites throughout Delaware and Maryland to conduct insecticide bioassays. If you have a cucumber beetle infestation, feel free to contact me at owensd@udel.edu. We’d love to take some beetles off your hands!

Seed Maggot Problems in Some Fields

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

This spring has been pretty good for pests of early planted seeds and bulbs—the seed corn maggot Delia platura (SCM) and other seed maggots such as Cabbage maggot Delia radicum (it prefers to feed on cole crops) and Onion maggot Delia antiqua (it feeds on crops in the onion family). Seed corn maggot has the earliest peak flights in the spring and has a host range of over 30 crops, which include alliums and brassicas. This maggot also is often the cause of poor stands of peas. All species overwinter in the soil as a maggot inside a brown case. In March and April small, grayish-brown flies emerge (Fig. 1), usually early March for SCM and mid-April for cabbage and onion maggot. A good indicator for the first cabbage maggot peak flight and onion maggot beginning flights is the blooming of yellow rocket or wintercress. Adult flies are most active from 10 a.m.–2 p.m. and are inactive at night, in strong winds and when temperatures are below 50 °F or above 80 °F.

Female cabbage maggot flies seek out and lay eggs on the lower portions of stems of young host seedlings or in nearby cracks in the soil. Within a few days the eggs hatch and the tiny maggots burrow down to the roots and begin feeding. SCM eggs are oviposited in soils with decaying plant material or manure. The adults are also attracted to the organic media around the roots of transplants and germinating seeds. Maggots will move into small stems and move up the plant causing a swelling of the stem just above ground level, while also causing root collapse and decay. If these stems are split you will usually find the white cylindrical larvae (Figs. 2 and 3).

Figure 1. Adult seedcorn maggot fly

Soil temperatures three inches deep in the planting hole that are at or above 70o F reduce SCM egg laying and larval survival. If soil temperatures are above 70o F at planting but fall below this level for several days in a row SCM adults will begin to oviposit eggs at the base of transplants. When wilted transplants are inspected in the field, maggots are often not found (they have already pupated), but their tell-tale damage can be seen as a hollowed-out stem or root held together by a few strands of plant material.
Seed maggots cause damage by burrowing into seeds or cotyledons and hollowing them out. Although it can take 5 maggots per snap bean seed to cause significant damage, once the seed has been opened up by the maggots the seed becomes much more susceptible to invasion from
soil borne pathogens. The maggots also can burrow into the bulb or stem of transplants such as watermelon or cantaloupe as well as cole crops or garlic and onions.


Figures 2 and 3. Seedcorn maggots in stems of transplants

Management: The use of treated seed or in-row banding of an insecticide gives some control of SCM, however, once seed corn maggot damage is noticed, it is too late to apply control procedures. Thus, economic thresholds are not useful and all management options are preventative. Replacing the dead seedlings or transplants is the only solution after SCMs kill a plant. Wait at least 5 days if maggots are a quarter inch long; if they are smaller than that, wait 10 days to make sure they have pupated and cannot damage the new seeds or plants. Overcast wet conditions and fields with moist, heavy-textured soil usually have the worst problems. Avoid planting a crop following root crops or cole crops such as cabbage and cauliflower or after fall tomatoes. Later-season plantings may avoid the early season infestation of SCM. For crops like onions or garlic row covers can be used as soon as transplants are put in the field. Plants can remain covered until the ground warms. Diazinon as a broadcast application before planting can be used with some vegetables, as well as post planting options using applications of Malathion or pyrethroids (be sure to check the label for each crop and see the 2019 Commercial Vegetable Production Recommendations guide).

2019 Seedling Diseases of Watermelon and Melon on Transplants

Kate Everts, Vegetable Pathologist, University of Maryland; keverts@umd.edu

There has been a report of a bacterial seedling disease on muskmelon ready for transplant this week. The sample in question turned out to be angular leaf spot (ALS) caused by Pseudomonas syringae. Recent research from University of Florida demonstrated that populations of P. syringae that cause ALS are very diverse. It isn’t uncommon to see this disease every few years in Maryland and Delaware. Our experience has been that if infected plants are transplanted to the field and the weather turns hot and dry, damage due to ALS may be minimal. However, prolonged cool and wet conditions will result in losses.

If conditions favor disease development, apply the labeled rates of fixed copper plus mancozeb. (Some coppers are OMRI-approved and may be able to be used in organic systems to help suppress ALS.) Continue applications at weekly intervals. In addition, to reduce ALS spread, avoid overhead irrigation when symptoms are present and avoid working in field while foliage is wet.

Nathan Kleczewski and I wrote an article a few years ago about several greenhouse diseases that occur during transplant production. The link to that information and many images of these seedling diseases can be found at: http://extension.udel.edu/weeklycropupdate/?p =6727