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Bees, Beetles, and Bacteria: the Cucurbit Bacterial Wilt Dilemma

Erika Saalau Rojas, Laura Jesse, and Mark L. Gleason

Department of Plant Pathology and Microbiology
Iowa State University
mgleason@iastate.edu

THE CASE: PART B

Henry searches the internet to obtain more information about cucurbit bacterial wilt. He knows that he will not be able to control cucumber beetles effectively if he eliminates all insecticide applications, but he is willing to consider insecticide reduction strategies as part of an Integrated Pest Management (IPM) plan. Unfortunately, there is not a standard IPM plan against bacterial wilt or any other plant disease. An IPM approach is situation-specific and relies on combining management strategies to effectively control a pest or plant disease while minimizing human health and environmental risks. Henry will have to choose which strategies could be applicable to his farm and resources.

With this goal in mind, Henry skims several extension publications. He is surprised to find out that cucumber beetle populations fluctuate throughout the season. He had never paid that much attention to their activity patterns. According to the publications, the adult beetles survive the winter buried in the soil. The beetles emerge from the warming soil and start flying in the spring at about the same time the muskmelon crop is planted. These beetles carry the wilt bacterium, Erwinia tracheiphila, in their guts. When beetles feed on cucurbit plants and deposit frass (fecal pellets), the bacterium can invade and infect the young cucurbit plants. Later in the spring, a new generation of adult beetles emerges, but, to Henry’s surprise, they do not initially carry the cucurbit bacterial wilt pathogen. In order to acquire and spread the bacterium, cucumber beetles that hatch during the growing season must first feed on infected plants.

It’s clear to Henry that in order to control bacterial wilt he has to minimize the contact between the cucumber beetles that first emerge in the spring and his young melon plants by creating some sort of barrier. This barrier can be chemical (i.e. insecticides), physical, or temporal.
A temporal barrier can be as simple as delaying planting time enough to avoid exposure of plants to high springtime beetle populations. Hank likes this option because it sounds affordable, but at the same time wonders how will he know when it’s safe to plant? Also, will this strategy delay harvest? After all, most of his profits come from obtaining premium muskmelon prices early in the season.

Henry explores the possibility of using row covers. Row covers are made of a special breathable polypropylene fabric. This fabric allows light and air to go through but protects plants from insects as well as harsh weather conditions like frost, hail, and high winds. Row covers provide a physical barrier that can keep out cucumber beetles and bacterial wilt very effectively. As an added bonus, row covers may protect plants from diseases caused by fungi, and can save Henry several fungicide sprays. However, row covers have drawbacks. They are costly to buy and laborious to put on and take off. Another concern is when to remove row covers. If row covers are left in place through the entire muskmelon season, Henry can eliminate most insecticide applications and protect his crop. On the other hand, cucurbit crops are 100% bee-pollinated and if the plants are covered by row covers how will the bees pollinate them? Unless Henry solves the problem of when to remove the row covers, pollination could be delayed (resulting in a late harvest) or yield could be reduced.

A third strategy that Henry learns about is called perimeter trap cropping (PTC). The principle behind perimeter trap crops is to keep the cucumber beetles out of one crop (called the main crop) by providing a more beetle-attractive alternative crop as a barrier (or perimeter) around the main crop. Often, the trap crop border consists of two rows of the trap crop planted on the outer edge of the main crop field. The trap crop keeps cucumber beetles from entering the main crop because they enter the field from its borders and tend to remain on these highly attractive plants. The beetles that congregate in the trap crop can be killed by spraying the border rows with insecticide. Henry reads that in winter squash, using PTCs can eliminate up to 90% of insecticide applications, because the main crop needs fewer insecticide sprays than a non-PTC main crop. However, Henry is concerned that the bee keepers might object to insecticides around the edge. Henry is not sure how honey bees forage and if they would be likely to start foraging at the edge first. Some references list several varieties of winter squashes that could be used as a perimeter trap crop for muskmelon, but Henry is not familiar with these crops and does not know if his customers will be interested in buying these squash varieties. The trap crop must also be planted 2 weeks before the main crop, which means Henry will have to work around the trap crop when he decides to plant the muskmelon. Is it worth trying this strategy?

Henry’s mind is whirling with possibilities. All of these strategies sound promising, but how can he overcome the practical limitations associated with each strategy? Should he come up with his own plan or combine strategies?

In order to decide which alternatives are economically feasible, Henry pencils out a chart of additional costs per acre associated with each of the strategies he just read about, including the cost of scouting per acre:

 

*Scouting costs should be adjusted to number of acres and interval of scouting (i.e. weekly, biweekly, or triweekly).

Henry’s current management plan against cucumber beetles:

 

*Henry’s management plan consists of one neonicotinoid application at transplant and at least 8 to 10 carbamate or pyrethroid applications per season.

If you were advising Henry, what would you recommend that he do?

QUESTIONS

  1. Based on your knowledge of cucurbit bacterial wilt, in which part(s) of the growing season do you think that risk of wilt infection is highest? Why?
  2. Discuss solutions to the potential problem of delaying harvest (and possibly receiving lower prices per fruit as a result) while using row covers or choosing to delay planting.
  3. What are some practical challenges that need to be overcome when implementing a perimeter trap crop?
  4. In order for a perimeter trap cropping system to be successful against bacterial wilt; which characteristics are desirable in the perimeter plants? Hypothesize what could go wrong if Henry chooses an inappropriate cucurbit variety for his perimeter.
  5. Use all the information available and your own experiences to construct an IPM-based management plan against cucurbit bacterial wilt that could solve Henry’s situation.
  6. When choosing among different IPM strategies to combat pests and diseases, what kind of considerations do you think farmers will keep in mind when making a decision?

Answers to these questions are available to instructors in the password-protected section of this site.

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