Emmalea Ernest, Scientist – Vegetable & Fruit Crops; emmalea@udel.edu and David Owens, Extension Entomologist, owensd@udel.edu
Pole lima beans are a very profitable crop for market gardeners and produce growers across Delaware and also have an enthusiastic following among home gardeners. Unfortunately growing them has been somewhat frustrating in recent years because of their sensitivity to heat stress and attractiveness to stink bugs.
Heat Stress
Large seeded lima beans like Dr. Martin, King of the Garden, Big Mama and other strains maintained by gardeners are more heat sensitive than small seed baby limas. High night temperatures have a greater impact than high daytime temperatures. When night temperatures are high, the anthers in developing flower buds do not develop properly. When the flower opens the damaged anthers do not release pollen (Fig. 1) and/or the pollen that is released is not viable. Without viable pollen, seeds do not set, and the flowers or small pods will drop from the plant.
Figure 1. These anthers are from lima bean plants grown under controlled night temperature conditions. Anthers from plants grown in high night temperature conditions do not open when flowers open and pollen is not released. In plants grown under cool night temperature conditions, anthers open and release pollen.
The critical threshold temperature for pole lima varieties had not been established but it is probably in the 68-70°F range. To date, this year (2022) has had the lowest June and July night temperatures since 2014 (Table 1). At Georgetown, there were 15 nights above 68°F and 8 nights above 70°F from June 1 to July 20 compared to an average of 21 nights above 68 °F and 14 nights above 70 °F from 2005-2022. This means that 2022 has had some of the best conditions for early season pod set of any year since 2014. However, this week’s hot weather will put an end to the favorable conditions. Often the majority of pole bean yield is from pods set in August and it is nighttime temperatures in that month that determine September and October harvests.
Table 1. Number of days with daily lows above 68 °F and 70°F from June 1 to July 20 at the Carvel Research and Education Center, Georgetown, Delaware from 2005 to 2022.
| Number of Daily Lows Above Threshold Jun 1-Jul 20 | ||
| Year | >68 °F | >70 °F |
| 2022 | 15 | 8 |
| 2021 | 25 | 16 |
| 2020 | 25 | 18 |
| 2019 | 26 | 17 |
| 2018 | 23 | 17 |
| 2017 | 28 | 21 |
| 2016 | 18 | 13 |
| 2015 | 25 | 18 |
| 2014 | 13 | 8 |
| 2013 | 29 | 23 |
| 2012 | 21 | 15 |
| 2011 | 19 | 12 |
| 2010 | 28 | 20 |
| 2009 | 4 | 2 |
| 2008 | 17 | 9 |
| 2007 | 15 | 12 |
| 2006 | 22 | 14 |
| 2005 | 25 | 17 |
| Average | 21 | 14 |
There are variety differences in heat tolerance. The most heat tolerant large-seeded lima available to growers currently is Christmas Lima. I (Emmalea) am breeding and testing some pole lima varieties that are more heat tolerant as a part of the UD lima breeding program. These lines have heat tolerance derived from large seeded lima varieties from Africa, the southern U.S.
Stink Bugs
Although temperatures have been favorable for pod set in the last six weeks, we are also seeing unusually high numbers of stink bugs this year and they are a major threat to developing pole lima pods. Three of our common stink bug species are pests of pole lima bean: green stink bug, brown stink bug, and brown marmorated stink bug (Fig. 2-7). Stink bugs can reduce lima bean yields significantly.
Figure 2. Stink bug eggs on a pole lima pod.
Figure 3. Recently hatched stink bug nymphs on a pole lima pod.
Figure 4. Green stink bug nymph
Figure 5. Brown marmorated stink bug nymph
Figure 6. Adult green stink bug.
Figure 7. Adult brown stink bug.
These insects feed by piercing developing pods with their needle-like stylets, sucking sap out of the pods or young seeds. This causes misshapen seeds in more developed pods and dropped pods when seeds inside young pods are killed. Prior to pod set, stink bugs should not cause any yield loss, but if they are present in a planting prior to the podding stages, they will be there for when the plants start to produce small pods. Hot weather will also cause poor pod set, but if conditions are cool and pods and flowers are dropping from plants, stinkbugs are frequently the cause of the problem. Also, larger pods that wilt and drop from the plant are usually the result of stink bug feeding, not heat stress. Scouting for stinkbugs in a mass of thick pole be foliage can be difficult. Instead, look for the damage they cause to confirm they are a problem (Fig. 8-9).
Figure 8. Stink bug feeding damage on baby lima pods (left) and undamaged pods at a similar developmental stage. Stink bug feeding can kill developing seed and cause fluffy white growth inside of pods at the feeding site.
Figure 9. Pole lima bean pod with stink bug feeding damage indicated by arrows.
Insecticide sprays are used for control. Commercial applicators may consult the Mid-Atlantic Commercial Vegetable Recommendations for insecticide options (http://extension.udel.edu/ag/vegetable-fruit-resources/commercial-vegetable-production-recommendations/). Home gardeners and small scale growers can look for products containing bifenthrin which are labeled for use on lima beans. On most home garden type products, the active ingredient is listed on the bottom left or bottom right of the label panel. Follow the product directions for application and days until harvest. Such products are available in some garden centers. If treating for stink bugs, scout the plants carefully for spider mites. Insecticides targeting stink bugs reduce predator activities allowing mites to quickly increase in population growth.
Finally, reduce your impact on pollinators. Spaying at dusk is the best way to reduce pollinator exposure.