Management of Mexican Bean Beetle, Epilachna varivestis Mulsant, in Snap Beans Using Cultural Control Strategies

Management of Mexican Bean Beetle, Epilachna varivestis Mulsant, in Snap Beans Using Cultural Control Strategies

Summary

Snap beans are an important fresh-market crop in Virginia with more than 5,500 acres grown and one of the largest packing facilities in the U.S. located in the state. Mexican bean beetle (MBB), Epilachna varivestis Mulsant, is one of the most serious pests of that crop, particularly in higher elevations in the state. Although this insect can be managed with foliar sprays of insecticides, more sustainable approaches are needed to alleviate the reliance on chemical control.  Small plot field experiments were conducted in 2013 and 2014 to better understand the ecology of this pest, and how it can be managed.

Objectives/Performance Targets

1. 1. (2013 and 2014) Evaluate differences in Mexican bean beetle developmental success and/or host resistance amongsix Phaseolus garden bean varieties.

Treatments:

Phaseolus vulgaris:
1. ‘Caprice’ (2013 and 2014) 
2. ‘Dragon’s Tongue’ (2013 and 2014) 
3. ‘Rocdor’ (2013 and 2014)

Phaseolus lunatus:
4. ‘Fordhook’ (2013 and 2014)
5. ‘Henderson’ (2014 only)
6. ‘King of the Garden’ (2014 only) 

2. 2. (2013 and 2014) Determine if MBB will exhibit host preference among various bean crops, using mark-release-recapture.

Treatments:

Phaseolus vulgaris:
1. ‘Caprice’ (2013 and 2014)
2. ‘Dragon’s Tongue’ (2013 and 2014)
3. ‘Rocdor’ (2013 and 2014)

Phaseolus lunatus:
4. ‘Fordhook’ (2013 and 2014);

Glycine max:
5. ‘Hutcheson’ (2013 and 2014).

3. 3. (2013 and 2014) Evaluate the effects of systemic neonicotinoid insecticides on arthropod communities in snap beans, including key pests, non-pests herbivores and beneficial arthropods.

Treatments:

1. 1. Untreated ‘Caprice’ snap bean seed (2013 and 2014).

1. 1. Thiamethoxam and Fungicide treated ‘Caprice’ snap bean seed (2013 and 2014).

1. 1. Fungicide only ‘Caprice’ snap bean seed (2014 only).

4. 4. (2013) Determine if delayed planting may reduce Mexican bean beetle damage in snap beans.

Treatments:

1. 1. ‘Caprice’ snap beans planted on May 3, 2013.

1. 1. ‘Caprice’ snap beans planted on July 16, 2013.

5. 5. (2014) Evaluate the utility of metalized plastic mulch to manage Mexican bean beetle populations and Increase Snap bean yields.

Treatments:

1. 1. ‘Caprice’ snap beans planted on bare soil

1. 1. ‘Caprice’ snap beans planted on black, polyethylene mulch

1. 1. ‘Caprice’ snap beans planted on white, polyethylene mulch (white top, black backing)

1. 1. ‘Caprice’ snap beans planted on metalized, polyethylene mulch (aluminum top, black backing)

Accomplishments/Milestones

At least two seasons of data have been collected on all objectives of this project except Objective 4 and 5. 

Objective 1:   

This variety trial has yielded interesting trends over the past two years.  Though we have not seen any significant (p<.05) differences in MBB’s developmental success among snap bean varieties, trends suggest that wax beans are more susceptible to MBB damage than common green beans.  Also, certain lima bean varieties exhibited a level of resistance to the beetles.  In 2013 and 2014, we witnessed beetles laying as many or more eggs in ‘Fordhook’, ‘King of the Garden’ and ‘Henderson’ lima beans as seen in all snap bean varieties.  We then found significantly (p<.05) lower numbers of each larval instar and pupae in ‘Fordhook’ and “King of the Garden’ than the other varieties (this trend occurred for ‘Fordhook in both 2013 and 2014).  

We have started conducting greenhouse trials of this experiment to isolate factors that may limit the developmental success of MBB the two lima bean varieties.  Some factors we are looking into are leaf texture, canopy light penetration, predator attractiveness, nutrient content and phago-stimulants. 

Objective 2:

            The mark-release-recapture experiment has been very successful in terms of recapture success and result significance.  In 2013 and 2014, we observed a clear host-selection preference of adult MBB toward the purple wax snap bean variety ‘Dragon’s Tongue’.  In both years, of all the beetles that moved from one variety to another, significantly more (p<.05) invaded ‘Dragon’s Tongue’ than any other variety.  Also, nearly 80% of beetles originally released into ‘Dragon’s Tongue’ remain in that variety.  For all other varieties, less than 50% of beetles remained in the variety of in initial release.

            We have already started to look at what may be causing differences in host selection.  Though it is unknown what drives host selection for adult MBB, one study suggests that fructose and sucrose concentration in host plants can elicit host selection by larvae.  We have started running sugar content assays using GCMS to determine if sugars concentrations correlate to adult preference.  These assays are currently in progress. 

Objective 3:

            This objective had a “set back”: in the first season, 2013.  We planned to test the effects of thiamethoxam seed treatments against untreated seeds.  However, after the first season of sampling, we realized that our treated seeds were also treated with a fungicide.  Because of this, we were unable to see the effects of the insecticide alone (i.e. any trends observed could have been related to the fungicide alone, or the combination of the fungicide and insecticide).  To correct this, we added a fungicide only treatment in 2014 (seeds treated with just thiamethoxam were not available).  Despite all this, we did not find any stand-out differences in arthropod communities among our treatments in either season.  Though some sample dates yielded significant differences among certain predator and pest groups, no consistent trend was observed that would suggest a major ecological impact on the arthropod community from seed treatments. 

            The most notable difference we witnessed was in regard to stand and yield in 2013.  Seed treatment plots produced significantly more pods (both per replicate and per plant) and exhibited greater numbers of plants surviving to maturity.  Again, we do not know if this is the result of the fungicide, insecticide or both.  Also, neither yield nor stand were significantly different in 2014. The 2013 season was very cool and wet, causing plants to remain in the ground for nearly three weeks before emergence.  The pre-emergent stage makes young plants highly vulnerable to both pathogens and pests, including seedcorn maggot and wireworms. 

Objective 4:  

            Though, in theory, late planting may work to control MBB on certain years, we were unable to successfully test it.  Because our farm produces snap beans each year starting in late April, our population of MBB grows to huge levels.  The late planted beans get wiped out by this pest.  With our resources and time, it is impossible to keep these treatments (early and late planting) independent.

From further research in the literature and discussions with growers, late planting can be effective on very hot years, or if the planting occurs so late that beetles leave the premises in search of hosts.  Lab studies show that MBB survival is reduced when temperatures progressively increase above 76, which was our basis for this experiment.  However, we have since found out that MBB actually develop faster and eat more when temperatures increase in this way.  So it is possible that the benefits would be canceled out.

Objective 5:

            The goal of Objective 4 was to develop a management strategy for MBB, which utilized this insect’s known intolerance to high heat.  Though this first attempt was unsuccessful, we believed the natural limiting factors of MBB could still be used for management techniques.  After further research, we discovered that exposure to direct light also reduces MBB’s fitness, both in conjunction high heat and independently.  We hypothesized that certain plastic mulches, which reflect light back toward the plants (metalized and white surface color), would limit MBB’s ability to damage crops. 

            Our results from 2014 were very promising on two fronts.  First, we witnessed significant (p<.05) reductions in MBB adults and eggs laid on beans planted on metalized plastic mulch than black plastic and bare soil.  Second, beans planted on metalized mulch produced significantly (p<.05) greater yields than all other mulch treatments and bare soil.  This experiment will be continued in 2015.   

Impacts and Contributions/Outcomes

Our studies are going to benefit growers in the southern region (and other regions) for many reasons.  Most importantly, we a probably the only applied entomologist from a research university studying Mexican bean beetle.  MBB is an extremely important pest throughout much of the eastern United States, and yet it is largely ignored by most research entomologists simply because it is easily controlled by conventional insecticides.  Our research is the starting point for implementing useful management strategies to control this pest without the excessive use of insecticides.  Our results thus far can be described as the “tool kit” for developing and evaluating functioning management strategies, which we are already beginning to test.

            In Objective 1, we determined that MBB are less likely to survive from egg to pupa on ‘Fordhook’ or ‘King of the Garden’ lima beans than any snap bean variety.  Growers may implement either of these varieties as a “dead end” trap crops for MBB.  Planting a small section of these varieties near overwintering sites, while the main bean crop is planted later, and farther away, should attract the majority of emerging spring beetles to deposit F1 generation eggs.  The result should be a much smaller F1 generation, due to these lima beans resistance to MBB development.  Not only will this limit the population, but the commodity beans will be ignored in the early season.

            Our findings from Objective 1 and 2 suggest that Dragon’s Tongue (purple wax snap) beans are highly preferred by, and susceptible to, MBB compared to Rocdor (yellow wax snap) and Caprice (green snap) beans, respectively.  Previous studies have also shown that wax beans varieties are, overall, more susceptible to MBB damage than common green varieties.  This information is important for growers to know, especially if they grow in a region that experiences economically important levels of MBB.  Growers can use this knowledge to implement cultural strategies such as a trap crop, or simply refraining from planting highly susceptible varieties.  

            Though Objective 3 produced less lucid conclusions, our results are still important to growers and to future research. We determined that seed treatments may increase plant survival and yield; however, it may not always have this result.  Both weather and the time of planting may alter the effects of treated seeds.  On cool, wet years, where seeds/seedlings remain in the ground for long periods of time, a seed treatment may be necessary to avoid mortality or reduced plant vigor from pathogens and/or pests.  However, some of these problems may be mitigated by simply waiting a little longer to sow the seeds, when weather is warmer and seeds will germinate and emerge faster.  This objective will continue to undergo testing in the 2015 season to clear up uncertainties. 

            Objective 5 offers a growing strategy for that manages MBB while enhancing snap bean yields.  It is essentially ready for on-farm implementation.  We plan conduct an enhanced version of this study in the summer of 2015.  We will plant an early and late crop of snap beans (first planting in May, second in July) for this experiment.  Our hope is to determine if this strategy yields similar results throughout the entire growing season.  This is important information, because many growers either plant two crops of beans per season, or plant on varied dates throughout the season.    

Collaborators: