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

Vegetable Insect Update – May 25, 2018

David Owens, Extension Entomologist, owensd@udel.edu

Striped cucumber beetles have been roaring into watermelon fields now that the sky has cleared. Our threshold for watermelon, which is not susceptible to a bacterial wilt, is 2 per plant. A lot of folks are going out with or planning to go out with a foliar neonic. Be careful with the product you use. Some only permit soil applications (imidacloprid – Admire Pro and other labeled formulations) while other active ingredients can be applied by soil or foliar depending on the specific trade name you use. The difference is in the formulation and how the product interacts with leaf tissue (ex: thiamethoxam – Platinum is for soil, Actara is for foliar application). Also check your application restrictions. Some products can only be used at the full rate once in the field while others can be applied several times in-season.

Another question that comes up is residual control. Efficacy trials are hard to get good data on with this pest in a field setting due to the unreliable nature of cucumber beetle pressure. Some data suggests that products should be effective for up to 5 or more weeks, other data and personal anecdotes say no more than about 2 weeks. I would be very interested to hear your input. Please also note that Vydate, effective on nematodes, does not have cucumber beetle on the label. While it does affect cucumber beetles, it is not consistent.

The question has come up regarding application timing and bee safety. A foliar application is going to be a bit more toxic because of the additional exposure route. If you are within 2 weeks of flowering, either a chemigation treatment or switching products to acetamiprid (ex. Assail) would be a little more bee-friendly. A great table indicating insecticide toxicity to bees can be found here: http://msue.anr.msu.edu/uploads/resources/pdfs/Minimizing_Pesticide_Risk_to_Bees_in_Fruit_Crops_(E3245).pdf. Other products that you can use with less risk to bees BEFORE flowering include Sevin and various pyrethroids. Pyrethroids and Sevin are very toxic to bees, but the residues are short lived, and effective for no more than about a week under ‘good’ conditions. Apply no later than several days before hives are placed. Unfortunately, that limited activity may not give us adequate cucumber beetle protection, necessitating follow up applications in shorter intervals – increasing risks to bees.

Live mating cucumber beetles and a dead individual on a leaf.

Mild Winter Induces Three Pest Problems This Year

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

There have been problems in our vegetable fields with three pests: striped cucumber beetles, leafhoppers and twospotted spider mites. I think most of the problems we are having with these three comes from our mild winter, as each has had an outbreak population after a mild winter sometime in the last 12 years.

So far this has been a particularly bad year for striped cucumber beetles in squash, cucumber and watermelon, and, when they are up, pumpkins. Some fields have been hit particularly hard with beetles causing 5-12% plant loss due just to their feeding. The cool wet May we had slowed the emergence of the beetles from their overwintering sites. When you combine the delayed emergence of the beetles with the slow planting schedule of squash and cucumber because of the wet weather you end up with a massive movement of beetles into some fields as soon as there are any cucurbit plants available. The beetles tend to mass on small plants where they eat, mate and defecate (Fig. 1). This type of frenzied activity where there are many beetles feeding on a few leaves or on a small plant leads to increased chances of bacterial wilt infection. The bacterium that causes bacterial wilt in cucurbits, Erwinia tracheiphila, is in the cucumber beetles’ feces. As the beetles defecate on the leaves where they are feeding the bacteria move into the open (feeding) wounds with the help of water that is in the form of precipitation or dew. The more beetles that are feeding and opening wounds on susceptible crops like cucumber, cantaloupe and squash the greater the chance of bacterial wilt infection. In a few small cucumber fields I saw as much as 30% of the plants with bacterial wilt. One additional problem with these pests and why control sprays have not worked as well as they should have 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. Sprayers are set up usually to cover the 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.


Figure 1. A frenzy of striped cucumber beetle

The second outbreak pest has been potato leaf hopper. When looking on the underside of leaf-scorched leaves (Fig, 2), I found many potato leafhopper Empoasca fabae nymphs (but no adults) (Fig. 3). Potato leafhoppers prefer warm, dry conditions and are commonplace in southern states where they overwinter; leafhoppers do not overwinter in our area, but the more mild the winter the better able they can overwinter close to us. PLH are generally first seen in late April or early May, but are arriving on average 7-10 days earlier in our area than just 20-30 years ago. Females lay 2-4 eggs per day in the leaf stems or veins of plants. In 7 to 10 days nymphs emerge. Nymphs undergo five instars and reach maturity in about 2 weeks. The newly emerged nymph is nearly colorless with red spots that fade. Nymphs then become yellow, finally changing to pale green in the third and later instars. There are 3-4 generations each summer. Leafhoppers are capable of very rapid population increases so scouting is important to control the pest to avoid damage to crops. Alfalfa and a few other forage legumes are the primary hosts for the potato leafhopper and once the first cutting of the forage is done, PLH will move into other susceptible crops such as potato, raspberries and hops.

Damage: The most obvious symptom of potato leafhopper feeding is hopper burn. Hopper burn is the yellowing of the leaf margin (Fig. 2). This damage is followed by leaf curling and necrosis. Hopper burn occurs because potato leafhoppers feed by sucking the juices out of leaf veins and blocking the veins with a toxin in their saliva. Once hopper-burn is seen the plant has been damaged.

Monitoring and Management: Because potato leafhoppers can have very rapid population surges, it is important to scout and control them before major damage can occur. While there is no agreed upon threshold for leafhoppers in several of our crops such as eggplant, raspberry or hops, most recommendations have a threshold at 2-3 PLH per leaf. Fields should be scouted weekly by checking the undersides of 5-10 leaves per 10–20 plants. If the average number of leafhoppers per leaf is at or above the threshold, then a control is needed. Because hops are a newer crop in our area, states may differ in what they allow to be used, so be sure to check the label to see what your state will allow to be used on hops for PLH control. In general, neonicotinoids, pyrethroids, or spinosyns could be used. Organic growers could use spinosad or pyrethrins that are OMRI approved for potato leafhopper management. If PLH are more of a consistent problem for you one suggestion is to plant red clover in drive rows (do not mow) as potato leafhoppers prefer to feed on the red clover than most of our vegetables.

 




Figure 2. Leaf scorching ‘hopper-burn’ on hops, eggplant and raspberry


Figure 3. Immature leaf hoppers on underside of hop leaf

The third pest problem is twospotted spider mites (TSSM) Tetranychus urticae. Even in May, after all that rain, there were reports of mite infestations on a few crops which included hops — yes not a good year so far for hops. Spider mites overwinter as adults in the soil or leaf litter, although they may remain somewhat active in high tunnels through the winter. The light yellowish eggs are pearl-like in appearance and are attached to the undersides of leaves or stems (Fig. 4). The most difficult thing to accomplish for good TSSM control is getting adequate spray coverage. Many of the spray applications do a good job of covering the top of the leaves, but do a poor job of reaching the underside of the leaves. Good coverage is essential. One grower used a leaf blower-like back pack sprayer and applied two sprays of 1% (by volume) horticultural oil 7-10 days apart. He got excellent spray coverage on the underside of his leaves and consequently excellent control of the mites that were present. By using two applications about one week apart it was possible to control not only the adults and nymphs, but the eggs as well. Oil is a good management tactic to use at this time of year as plants are small. An added benefit of the oil is that it is rather inexpensive. Using oils now also will greatly reduce any development of mite resistance to other chemicals over the course of the season. If miticides are needed Kanemite, Acramite and Portal are all excellent miticides.


Figure 4. Twospotted spider mite eggs on underside of leaf

High Populations of Striped Cucumber Beetle Early This Year

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

If you think striped cucumber beetles seemed to be in greater than normal numbers this year you are not alone. This has been a particularly bad year for striped cucumber beetles in squash, cucumber, watermelon and, lately, pumpkins. The beetles have been ravaging cucurbit fields in southern and central Maryland as well as the eastern shore. Some fields have been hit particularly hard with beetles causing 10-15% plant loss due just to their feeding. The cool wet spring we had slowed the emergence of the beetles from their overwintering sites. When you combine the delayed emergence of the beetles with the slow planting schedule of squash and cucumber because of the cooler weather you wind-up getting a massive movement of beetles into some fields as soon as there are any cucurbit plants available.

The beetles tend to mass on small plants where they eat, mate and defecate (Fig. 1). 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 problems (Fig. 2). 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 feeding and opening wounds on susceptible crops like cucumbers and squash the greater the chance of bacterial wilt infection. In a few small cucumber fields I saw as much as 45% of the plants with bacterial wilt.

One additional problem with these pests and why control sprays have not worked as well as they should have 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. 3). Sprayers are usually set up 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. On many of the farms that were hit hard with early beetle populations, beetle numbers seem to be much lower the last week or so.

cucbeetle1

Figure 1. Feeding frenzy of striped cucumber beetles-eating, mating, defecating

cucbeetle2a

cucbeetle2b

Figure 2. Small and large cucurbit plants infected with bacterial wilt. At first a few leaves or one vine will wilt and eventually the whole plant

cucbeetle3

Figure 3. Feeding at the base of a young cucumber plant by striped cucumber beetles.

Striped Cucumber Beetle Populations Still Very High

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu and Karen Rane, Extension Specialist Entomology, University of Maryland rane@umd.edu

We have seen very high populations of striped cucumber beetles on squash, pumpkins, cantaloupe, watermelon and other cucurbits over the last few weeks. These populations at times have reached over 20-30 beetles per plant. If a spray was missed or plants were not thoroughly covered with an insecticide application the beetles would soon consume that unprotected area very rapidly (Fig. 1). This area is often times the base of the plant. This is especially true if the cucurbits are sprayed with an air-blast sprayer. While air-blast sprayers do a good job of covering leaves with material, they often do not do a great job of covering the base of a plant and heavy feeding can occur (Fig. 2). The feeding can lead to plants being girdled by beetles or can lead to bacterial wilt infection—even though the leaves of the plant show almost no feeding. This feeding by the beetles also opens the base of the stem to infection from soil organisms and greater rates of Fusarium and bacterial soft rots are possible. When beetle populations are this high the base of the plant, even more so than the foliage, needs to be protected from heavy feeding.

Figure 1. Two squash leaves fed upon heavily by striped cucumber beetles because of the lack of good spray coverage.

Figure 2. Base of pumpkin plants damaged by striped cucumber beetles due to poor spray coverage

Watch for Striped Cucumber Beetle and Squash Bugs at Base of Cucurbit Plants

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

I talk about this every year it seems, but I still see cucumber beetle and squash bug problems at the base of growers’ cucurbit plants. So far this has been a ‘good’ year for striped cucumber beetle and squash bug populations in just about every cucurbit field. Some fields have been hit particularly hard with beetles causing 5-10% plant loss due just to their feeding. The biggest problem with these pests, and why control sprays have not worked well, is that they are consistently hiding at the base of the plant where they are feeding on the stem. Most of the time we look for the foliage damage to tell us how well our spray program is working. Sprayers are set up usually to cover a lot of leaf canopy and do not do a very good job of putting chemical along the base of the stem. This stem feeding can be severe enough on small plants that either pest alone could cause some wilting, but with both feeding on this relatively small area of the stem they are causing considerable damage (Fig. 1). Even on larger plants the feeding can still cause significant damage (Fig. 2). It is hard enough to kill squash bug adults with a good cover spray, but when only small amounts of spray are reaching them on the lower stem they will not be controlled. Often it is possible to walk by plants and even inspect them and still see no beetles or squash bugs, as they will stay down at the base of the plant and only move when the base is exposed. In a couple fields about 10% of the plants were wilting (Fig. 3) due to squash bug and cucumber beetle feeding. These pictures are from a squash field but the same problem is occurring in watermelon and cantaloupe fields with both striped cucumber beetles and squash bugs feeding at the base of a plant. Growers need to check to see if this type of feeding is occurring in their fields and if so insecticide applications (pyrethroids such as Asana, Warrior, etc.) must be directed at the base of the plant.

Figure 1. Striped cucumber beetle feeding damage at base of a small squash plant

 

Figure 2. Larger cucurbit plant with feeding at its base by cucumber beetle

Figure 3. Wilted squash plant due to squash bug and cucumber beetle feeding at its base