Squash bugs can be a serious insect pest for organic summer squash growers. The purpose of this research was to evaluate two methods to control squash bug populations. The first experiment involved planting cover crops adjacent to summer squash. Diversifed plantings have been shown to attract natural enemies, which sometimes help to keep insect populations low, or break up the farm landscape and make it more difficult for insect pests to find their way to the crop. Natural enemies were attracted to the plots, but did not significantly reduce squash bug populations. This may have been due to other food sources in the plots, such as pollen, nectar, and aphids. Also, summer squash yields were negatively affected by the cover crop treatments. The second experiment evaluated the efficacy of organic insecticides on squash bug adults and nymphs. Results of this study showed pyrethrin-based sprays are best for controlling squash bugs.
Squash bugs are the most notorious of the summer squash insect pests, inflicting serious damage. They cause feeding damage and transmit Serratia marcescens, which causes cucurbit yellow vine disease (CYVD). This disease has varying effects on yield, with crop losses from 5-100% (Bruton et al., 2003). CYVD has been identified in several states, including Georgia (Besler, 2014; Bruton et al., 1995). Plants infected with CYVD turn yellow, wilt and eventually die. Squash bugs adults and nymphs obtain the bacterium when probing the plant for feeding. The bugs harbor the bacterium while overwintering and transmit it to cucurbits when they begin feeding the next year (Pair et al., 2004).
Organic farmers utilize several methods to manage this insect pest including row covers, crop rotation, and organic insecticides, but substantial yield loss due to squash bugs is common. One method that may help reduce squash bugs involves conservation biological control. This method focuses on enhancing natural enemy populations to decrease natural pest populations. Squash bug adults secrete a foul odor when handled, so predation of adults is rare, however, nymphs and eggs have several natural enemies. Lady beetles, big eyed bugs, damsel bugs, web building and hunting spiders have been shown to be top squash bug predators (Schmidt et al., 2014). There are several other squash bug predators including: ground beetles, rove beetles, green lacewings, and spined-soldier bugs (Beard, 1940; Decker et al., 2008; Snyder, 2014; Snyder et al., 1999).
One way to increase natural enemy populations is to diversify the farm landscape by planting floral resources or cover crops near cash crops. There has been a great deal of research investigating how diversified plantings affect insects; clear benefits are debatable. In a survey of 219 peer-reviewed studies, 51% of the studies found lower insect pest populations in polyculture plantings compared to monoculture, while 49% of the studies found higher insect pest populations, no change in insect pests, or mixed results (Andow, 1991).
Non-organic growers do not have similar problems with squash bugs because there are several conventional insecticides available for squash bug control, but are not permitted for organic production. Studies using insecticides on squash bugs have focused primarily on conventional insecticides, however, there are several insecticides approved by the Organic Materials Review Institute (OMRI) that are labeled for use on squash. Some of these may provide adequate control of squash bug adults and nymphs. OMRI-approved insecticides include Monterey neem oil, Monterey horticultural oil, Monterey garden insect spray, PyGanic, Azera, and Safer Soap. There has been little research done involving squash bug and most OMRI approved insecticides, but pyrethrin has been shown to be effective at killing squash bug nymphs (Watkins, 1946).
The goal of this project was to evaluate practices to reduce squash bug populations in organic summer squash plantings. Project objectives were to:
- Evaluate cover crops as a strategy to decrease squash bug populations in summer squash
- Evaluate cover crops as method of attracting natural enemies
- Evaluate cover crops as method to discourage squash bug populations
- Evaluate the effect of cover crops on summer squash yield
- Evaluate OMRI-approved insecticides for squash bug adult and nymph control
Three field trials were planted at UGArden demonstration farm in Athens, Georgia. Trial 1 was planted in early summer 2014 (April-July), Trial 2 in late summer 2014 (July-September), and Trial 3 during early summer 2015 (April-July). The three cover crops selected for this study were buckwheat, cowpeas, and sunn hemp, based on previous studies supporting their ability to attract natural enemies that prey on squash bugs.
Each treatment and control had four replicates for a total of 16 plots. Plots were arranged in a complete randomized block design with 4 blocks. For Trial 1, each replicated plot was 4.3 x 6.7 m with 0.91 m space between treatments and 1.5 m between replicated plots. Cover crops were planted using a hand-held broadcast seeder in the entire plot. Three strips were tilled in each plot and then on 2 June 2014 three-week old squash transplants (C. pepo ‘Multi Pik’) were planted. In total, there were 30 squash plants per treatment for a total of 480 plants in the trial.
Changes were made to the experimental design for Trial 2 due to observed competition between squash and cover crops. Each replicated plot was 4.6 x 5.8 m with 0.91 m space between treatments and 1.5 m between replicated plots. Cover crops were sown in two 5.3 m2 blocks adjacent to squash instead of the spacing done in Trial 1. Squash were planted in two rows in between the cover crop blocks, with 0.91 m between the two rows of squash and 0.91 m between cover crop and squash rows. To increase germination and stand, cowpeas and sunn hemp were planted with a seeder 14 days before planting squash since these cover crops took longer to establish than buckwheat. On 11 Aug., 10 squash/row were planted for a total of 20 per plot and 320 plants in the trial. One week after squash planting, a seeder was utilized to sow buckwheat.
The design of Trial 3 was similar to Trial 2, with a few additional modifications. Each replicated plot was 4.6 x 6.7 m with 0.91 m space between treatments and 1.8 m between replicated plots. Cowpeas and sunn hemp were planted with a Yang seeder on 28 days before squash planting. On 5 May, squash transplants were planted. Buckwheat was seeded the day after squash planting using the Earthway seeder. Sunn hemp was cut back twice to reduce shading effects on squash.
Natural enemies were collected in traps (yellow pan traps, sticky traps, pitfall traps, sweep nets). Traps were placed within squash and cover crop rows to determine if natural enemies were moving from cover crop to squash. Squash bugs were monitored through visual inspection of all squash plants, counting adults and egg masses. For trial 1, traps were placed in all plots on squash planting day and collected for five weeks. For Trial 2, insect traps were placed in plots 2 weeks after squash planting and collected for 4 weeks. For Trial 3, traps were placed on in plots 2 weeks after squash planting and collected for 5 weeks. Due to low squash bug populations in Trials 1 and 2, for Trial 3, 182 squash bugs were brought in from neighboring farms and placed in the plots.
Squash were harvested on a Monday-Wednesday-Friday schedule. For Trial 1, harvest began 19 days after squash planting and lasted 3 weeks. For Trial 2, harvest began on 21 days after squash planting and lasted 4 weeks. For Trial 3, harvest began on 22 days after squash planting and lasted 5 weeks. Marketable fruit was free from blemishes and within the diameter of 3.8 – 6.4 cm (Boyhan et al., 2004).
Suspected CYVD infected plants were initially confirmed with PCR as outlined in previous work (Besler, 2014). Later infected squash were diagnosed based on visual symptoms (yellowing, collapse, honey colored phloem). Plots were monitored for disease until the end of harvest.
Count data was analyzed using Poisson regression or negative binomials. Yield results were analyzed using one-way analysis of variance and Tukey HSD. To analyze comparisons between counts in squash row and cover row, difference was taken between the rows and log transformed. Linear regression was then run on the log-transformed data. Disease incidence was analyzed with binomial regression. In all analyses, significance was determined if P < 0.05.
Six commonly used organic insecticides advertised to kill squash bugs were selected for this study. Water was used as a control treatment. Highest labeled rates were applied for all treatments.
Squash bugs were collected from field plots or from farms near Athens, Georgia. The trials were completed between June and July 2015. Squash bugs were treated on the same day of collection to reduce likelihood of other causes of death. Nymphs in stage one, two, and three were classified as ‘young’ or ‘old’ if in stage four or five. Five nymphs of the same age group were put into a petri dish and sprayed once with an insecticide, which coated each of the bugs. Death rates were recorded after 1 and 24 hours. Squash bugs were determined to be dead when not able to stand within 30 seconds of being placed on their back, as outlined in previous work (Watkins, 1946). Each trial consisted of the same age group sprayed with each of the 6 treatments plus the control with a total of 35 bugs in each trial replication. Six replications were completed with young nymphs, 7 for old nymphs, and 8 for adults, for a total of 735 nymphs and adults.
Death rates were analyzed using binomial regression and Tukey HSD. Significance was noted when P < 0.05.
Squash bug adult populations
Overall, squash bug populations were low in Trials 1 and 2, with increased populations in Trial 3, in part, due to supplementation from outside farms. Total squash bug counts were 36, 17, and 123, respectively, for each of the trials. Squash bug numbers did not differ between cover crop plots compared to the control for Trial 1 (p=0.670, p=1.00, p=0.121). For Trial 2, squash bugs were significantly less abundant in buckwheat plots compared to control (p=0.0371). In Trial 3, even with an increase in the general population, squash bug adults did not differ among plots (p=0.769, p=0.633, p=0.077).
Eggs and nymphs
As with the adult numbers, egg and nymph counts were highest in Trial 3. Only 14 egg masses were counted in Trial 1, and 19 in Trial 2, while Trial 3 had 430. Nymphal data was only analyzed for Trial 3 because 6 nymphs were counted in Trial 1, 0 in Trial 2, and 88 in Trial 3. Egg masses were counted, as opposed to individual eggs. For Trial 1, there was no difference among egg counts for the plots (p=1.00, p=0.0971, p=0.0971). In Trial 2, all cover crop plots had significantly lower counts of egg masses and nymphs than control (p<0.01). Egg masses for Trial 3 were found to have no significant differences among the different plots (p=0.9865, p=0.8838, p=0.8947). Nymphs for Trial 3 were highest in sunn hemp plots compared to control (p=0.0224). Overall, squash bug results were mixed, with adults, eggs, and nymphal data all suggesting different trends. However, overall, we did not see squash bugs consistently lower in treatment plots, suggesting that cover crops and natural enemies did not help to decrease squash bugs.
Only those squash bug natural enemies with adequate numbers were counted. Among those counted were: spiders, ground beetles, big-eyed bugs, lady beetles, fire ants, and rove beetles. Overall, total counts within cover crop and squash rows of the predators present for Trial 1 showed that cowpea and sunn hemp plots had significantly higher levels than control plots (p=0.042, p=0.02). Total counts for Trial 2 showed cowpea plots with significantly higher counts than control (p<0.01). For Trial 3, all cover crops plots were shown to have significantly higher counts of natural enemies compared to control (p<0.01).
Counts were compared within the cover crop, or blank space for control, and within the squash rows to determine where insects were spending most of their time. For Trials 1 and 2 there were no significant differences between squash and cover crop rows (p>0.05). For Trial 3, cowpea and sunn hemp plots had significant differences between squash and cover crop rows (p=0.0140, p=0.0302).
For Trials 1 and 3, big-eyed bugs were significantly higher in all cover crop plots (Trial 1: p<0.01 and Trial 3: p<0.01). For Trial 2, big-eyed bug populations were higher in buckwheat and cowpea plots (p<0.01 for both). Spider and lady beetle did not show strong, consistent trends of preference during the three trials.
When big-eyed bug counts were compared within squash and cover crop row, big-eyed bugs were usually highest within the cover crop row. For Trial 1 and 2, the difference in count numbers between squash and cover row were significantly higher for all cover crops compared to the control (Trial 1: p<0.01, p<0.01, p=0.0146, Trial 2: p<0.01, p<0.01, p=0.0217). For Trial 3, buckwheat and sunn hemp plots had significantly higher counts than control (p<0.01). Spider and lady beetle counts were overall lower than big-eyed bug counts and few differences were noticed between cover and squash rows.
Overall, natural enemies were higher within cover crop plots. With higher natural enemy counts, a lower squash bug population was expected, however, this was not the case. Though impossible to know how often the natural enemies were traveling between cover crop and squash row, some natural enemies may have spent most of their time within the cover crop row, eating very few squash bug eggs or nymphs within the squash row. Similar findings were found when flowering plants were planted adjacent to cash crops and ground beetles were present within the flowering plant strip but levels of ground beetles were not higher within the cash crop (Carmona et al., 1999). Of the top squash bug predators in our study, big-eyed bugs were generally highest in cover crop rows, in particular buckwheat, for each trial. The other top predators did not show consistent trends among the three trials in terms of their preference for cover crop or squash row.
Aphids were noted in all three of the trials, and had population levels that exceeded squash bugs. Aphid populations were highest in Trial 3, with each cover crop plot having an average of at least 100.
The diet of the natural enemies should be considered in analyzing the results. The natural enemies counted in the study are generalist predators that feed on a wide variety of different insects. The cover crops could have been attracting other insects that the natural enemies were preying upon. Aphids were present within cover crops and squash rows, and the natural enemies could have taken advantage of this more abundant food source.
Many natural enemies, such as lady beetles and big-eyed bugs, rely on pollen and/or nectar as an important part of their diet. It has been shown that the presence of the buckwheat flowers can reduce predation by lacewings on aphids, probably due to lacewings feeding on pollen and nectar of buckwheat (Robinson et al., 2008).
In general, yields were higher in Trials 1 and 3 than Trial 2. Since Trial 1 involved 3 rows of squash in every treatment plot, the total yields were reduced by 1/3 to make the data comparable among trials. Also, the trials had different timelines for harvesting, based on growth of squash. For Trial 1, all cover crop plots had significantly lower squash yield than control (p<0.01). Buckwheat plots had the highest yield of the cover crop treatments (p<0.01). In Trial 2, which took place later in the summer, yields were overall lower than Trial 1 and 3. Buckwheat plots had comparable yields with control plots (p=0.216), while cowpea and sunn hemp plots were again lower than control (p=0.049, p=0.003). Trial 3 had highest yields of all the trials and all cover crop plots had significantly lower yield compared to control (p<0.01). Overall, yield was highest in control plots. The cover crops shaded out the squash crops and provided less space for growth of the squash, which dramatically reduced yield.
CYVD was only noted during Trial 3 starting on 12 June. The rate of disease was highest in buckwheat plots, with an average of 25% of plants becoming infected. Both buckwheat and sunn hemp plots had a significantly higher incidence of disease (p=0.009 and p=0.031) compared to cowpea and control. Lowest incidence of disease was in control plots, with only 6% of plants infected.
Death rates after 1 hour
After 1 hour, young nymphs had the highest death rates compared to other stages of bugs tested. Among the insecticides, PyGanic had the highest average death rate (76%) and was significantly higher than all other insecticides (p
After 1 hour, PyGanic and Azera did kill older nymphs, but the death rates were not significantly higher than control. PyGanic killed an average of 25% of old nymphs, while Azera killed an average of 8%. Only PyGanic and Azera killed adults after 1 hour, however there were no significant differences among these rates.
Death rates after 24 hours
After 24 hours, the highest death rate was seen in young nymphs. Death rates for young nymphs were significantly higher with PyGanic, Azera and Monterey Garden Insect Spray than the control (p<0.01, p<0.01, p=0.01135). PyGanic had the highest average death rate, 100%. Azera had the next highest average death rate, 80%.
The pattern of death is similar for old nymphs, with PyGanic, Azera, and Monterey Garden Insect Spray killing significantly more squash bugs than the control (p<0.01, p=0.01202, p=0.04858) (Figure 3.3). Death rates were significantly higher with PyGanic than all other insecticides (p<0.01).
After 24 hours, death rates for adults showed a similar trend, but the averages were lower compared to nymph data. PyGanic, Azera, and Monterey Garden Insect Spray had death rates significantly higher than control (p<0.01, p=0.00189, p=0.02938, Fig. 3.3). PyGanic killed significantly more adult squash bugs than all insecticides except Azera (p<0.01).
Results of this study suggest that PyGanic is the most effective insecticide against squash bugs and Azera was the next best insecticide. Monterey Garden Insect Spray did little to kill bugs after 1 h but killed significantly more squash bugs than control after 24 h. Overall, young nymphs were more susceptible to the insecticides. This is to be expected since as insects molt they are replacing their exoskeleton with a stronger one (Palumbo et al., 1993).
Azera is a combination of pyrethrin and azadirachtin. Pyretherin is the main ingredient in PyGanic, while azadirachtin is the main ingredient in Neem oil. Since Neem showed very low death rates, it is likely that azadirachtin does little to kill squash bugs and Azera is only effective because it contains pyrethrin. Although pyrethrin-based sprays appear to be the most effective at killing squash bugs, pyrethrin is a broad-spectrum insecticide that can have a negative effect on non-target species. Frequency of use and timing of pyrethrin needs to be closely monitored. Honeybees can be killed from pyrethrin-based sprays (Casida et al., 1995). Pyrethrin can also kill natural enemies such as spiders and parasitic wasps and is also very toxic to fish (Cox, 2002).
Educational & Outreach Activities
Thesis: Davies, L, D. Berle, P. Guillebeau and E. Little. 2016. Effects of Cover Crops and Organic Insecticides on Squash Bug (Anasa tristis) Populations. Graduation date: May 2016.
Abstracts: Davies, L, D. Berle, P. Guillebeau and E. Little. 2015. Effects of Cover Crops on Squash Bug (Anasa tristis) Populations. Poster presentation at American Society for Horticultural Science in New Orleans, LA. 3 August- 7 August 2015.
Presentations: Davies, L. D. Berle, P. Guillebeau and E. Little. Effects of Cover Crops on Squash Bug (Anasa tristis) populations. Oral presentation at E. Broadus Browne competition in Athens, GA. 25 March 2016.
Davies, L. D. Berle, P. Guillebeau and E. Little. Efficacy of Organic Insecticides on Squash Bug Adults and Nymphs. Poster presentation at Georgia Organics conference in Columbus GA. 26-27 February 2016.
Davies, L, D. Berle, P. Guillebeau and E. Little. Effects of Cover Crops on Squash Bug (Anasa tristis) Populations. Oral presentation at Interdisciplinary Graduate Plant and Soil Symposium in Athens, GA. 13 November 2015 and 2014.
Davies, L, D. Berle, P. Guillebeau and E. Little. Effects of Cover Crops on Squash Bug (Anasa tristis) Populations. Poster presentation at American Society for Horticultural Science in New Orleans, LA. 3-7 August 2015.
Cover crop treatments provided a few environmental benefits, however, contributed negatively to squash growth. There were more natural enemies in cover crop plots, most notably big-eyed bugs. However, the big-eyed bugs may have been preoccupied with other food sources and tended to stay within the cover crop strip. The composition and numbers of natural enemies varied greatly from trial to trial, suggesting environmental conditions and season plays a great role in the numbers of these insect populations. Yields in the cover crop treatments were lower and the prevalence of CYVD greater, which could be due to squash bugs being imported from other farms. At this time, planting cover crops alongside summer squash is not recommended, as based on this study, this practice proved more harm than good.
Results from the insecticide study demonstrate that PyGanic and Azera, both containing pyrethrin, are best for controlling squash bugs. Use of these organic insecticides can be an important part of an organic grower’s squash bug management plan, if used safely.
Areas needing additional study
Future studies should focus on adjusting spacing of cover crops and squash to maintain adequate yields. Different types of planting schemes, for example, planting the cover crops in a border around the squash plots, could help decrease competition. Also, the cover crops may provide more benefit if they are planted earlier to allow natural enemy populations to become established before planting summer squash. It may be important to establish natural enemies very early; Besler showed that timing is important in terms of CYVD is passed on to the plants. If squash bugs do not have contact with the plants during the first three weeks of growth, the squash plants do not acquire the CYVD bacterium (Besler, 2014). Also, more work needs to be done in determining how to attract natural enemies to the squash plants, and not just to the cover crop areas near the squash. Studies should focus on the movement of squash bugs and natural enemies to learn more about their preferences and how far they can travel.
While PyGanic and Azera were effective in lab conditions at killing squash bugs, this needs to be analyzing further in field settings. It would also be useful to compare timing and targeting of spray materials to catch squash bugs in the most vulnerable stage. Also, an economic analysis could be conducted after field trials to determine how often the grower needs to spray and how much it will cost.