Development and Evaluation of IPM Systems Components for Insect Pests and Pathogens of Cucurbit Crops in the Southeastern U.S.

Objective 1. Effects of colored plastic mulch on cucurbit production and insect communities.

Our research showed similar results as others with black and silvery reflective mulches having a beneficial impact on squash yield compared to bare ground beds in one of the years. The reasons for these differences are not completely known, but are likely related to increased soil temperature and moisture retention resulting in greater plant growth and productivity. However, greater densities of squash bugs were found on plastic mulch plots compared to bare ground beds. Squash bug adults like to seek shelter in the planting hole of plastic mulch and consequently may deposit more eggs on those plants. Our study offers new insights for cucurbit growers to use when considering whether they should implement plasticulture in their growing systems.

In our study, we observed significantly more A. tristis adults and egg masses on summer squash grown with white, black, or reflective mulch compared to bare ground, yet no differences in adult or egg mass numbers among the three mulch color treatments (Fig.1, 2).  The observed similarity in population dynamics for adults and egg masses was expected, as the two life stages are highly correlated throughout much of the growing season (Harmon et al. 2003). More specifically, once adults reach peak density, there is a subsequent peak in egg mass density (Fargo et al. 1988; Palumbo et al. 1991b).  Our findings also support a preference by A. tristis adults for squash grown in plastic mulched systems, which appears to be unaffected by mulch color.  Preference for plastic mulch systems may be a result of A. tristis adults’ propensity to congregate near the crown and lower, older leaves of their host plant (Palumbo et al. 1991a).  During the initial weeks of the summer squash vegetative growth phase, the plants are small and offer inadequate refuge for the colonizing adults.  Holes in the plastic mulch where squash is planted provide shelter for adults to feed on the young plant’s main stem and mate relatively protected from natural enemies. One of the most prominent natural enemies of A. tristis adults, parasitoid fly Trichopoda pennipes Fabr. (Diptera: Tachnidae), actively searches for the bugs in the canopy of host plants (Beard 1940).  However, it is unknown whether T. pennipes will forage for host bugs underneath the plastic mulch.  Squash bug copulation can last > 20 h for a single adult pair, leaving the bugs vulnerable for extended periods of time (Sears et al. 2020).  The protective cover of plastic mulch may then allow A. tristis to optimize its fitness and fecundity early in the season when shelter near host plants is sparse.  Future research is needed to completely discern if squash bug adults experience decreased rates of predation and parasitism when using plastic mulch as refuge.

Nymphal densities were highly variable and did not follow the pattern of the adult and egg mass numbers.  Mulch treatment was only a significant predictor of nymph counts in 2020 (Table 1; Fig. 2c).  Previous research suggests that while all A. tristis life stages exhibit an aggregated distribution, nymphs show the greatest degree of aggregation (Palumbo et al. 1991c).  For this reason, nymph distribution may be patchier and require additional sampling to accurately determine nymphal abundance (Palumbo et al. 1991c).  Therefore, a potentially patchy distribution of nymphs in conjunction with identical numbers of plants surveyed for all life stages may explain inconsistent results among nymphs between and within treatment groups.

Even with significant differences in A. tristis adult and egg mass numbers between mulched and bare ground plots, differences among mulch treatments observed in marketable fruit yield varied between years, as plants in plastic mulch produced more marketable fruit than bare ground plants only in 2019 (Table 1, Fig. 1d, 2d).  As mentioned earlier, the use of plastic mulch in vegetable systems provides numerous horticultural advantages (Jabran 2019).  In cucurbit crops, mulch increases yield and plant biomass, lowers soil moisture evaporation, and improves irrigation efficiency (Conway et al. 1989, Kirnak and Demirtas 2006, Torres-Olivar et al. 2018).  The benefits of plastic mulch may have had a compensatory effect in 2019, increasing yield to even exceed reductions resultant from increased squash bug pressure. Contrarily, even with reduced A. tristis pressure, bare ground zucchini produced similar marketable fruit yield to mulched plants in 2020, further suggesting a compensatory/beneficial effect of plastic mulch on yield. Disentangling relationships among insect pressure, plastic mulch, and yield has the added difficulty of research lacking a well-established relationship between squash bug pressure and yield. To date, research-based threshold values for adults, egg masses, and nymphs have not been firmly established.  While we did find some differences in insect pressure, mulch, and marketable yield between treatments, our study did not include numbers of fruit discarded due to A. tristis damage.  It is possible that mulched plants with large densities of squash bugs incurred greater fruit damage than those grown in bare ground with lower bug densities.  In order to correctly assess yield loss from squash bug damage, additional fruit quality predictor variables like pollination success, presence/absence of microbial pathogens, and counts of other cucurbit insect pests are required.  Future experimentation attempting to connect squash bug and its influence on damaged fruit numbers should include the aforementioned predictors to accurately elucidate how A. tristis specifically influences overall marketable yield in summer squash. 

Despite many differences in methodology and experimental design, our study and Cartwright et al. (1990) identified similar patterns in squash bug presence and yield across different mulch treatments.  Both studies observed more adults and egg masses on squash in plastic mulch versus bare soil plots, and inconsistent nymph presence among treatments between years. Although we demonstrated differences in yield between mulch treatments, methodological ambiguity in Cartwright et al. (1990) makes comparing yield between the two studies difficult. For example, they only recorded total yield in the second year of their study with no known differentiation between marketable and damaged fruit. Additionally, they offered no explanation of how or for what duration they conducted their yield collections and varied their squash planting dates and subsequent A. tristis sampling time frames dramatically between years. Overall, our study clarifies and expands the results described in Cartwright et al. (1990) by using uniform planting dates, increasing mulch treatment and sequential sampling replication, and including two years of marketable yield data.

When considering the implementation of plasticulture in summer squash production, growers should be mindful of the entire cucurbit pest complex, particularly which species are routinely economically destructive in their region.  Despite the possible horticultural attributes provided by plastic mulch, our research conducted in southwest Virginia suggests plastic mulch can negatively impact squash bug management, especially in regions with high A. tristis pressure.  Growers in such areas looking to minimize A. tristis presence may benefit from forgoing the use of plastic mulch, and instead, utilize a system that uses herbicide applications, cover crops, or other weed suppression tactics.  On the other hand, growers in locations that experience significant pressure from cucumber beetles, aphids, or whiteflies can help protect their cucurbits from pest colonization with the repellency effects offered by reflective plastic mulch (Summers et al. 1995, Caldwell and Clark 1999, Vincent et al. 2003, Frank and Liburd 2005).  Availability of essential farm equipment (e.g., plastic mulch layer) and added costs of specific plastic mulch colors (e.g., reflective, red, blue, etc.) will also inevitably impact whether a certain plasticulture system is implemented. In most cases, cucurbit growers will face a mixed assemblage of insect pests throughout the growing season, requiring them to conduct cost-benefit analyses of all possible cultural, biological, and chemical control strategies. Using the newly acquired knowledge put forth by this study, growers can more confidently decide if plasticulture fits within their squash bug management plans.  

Publication produced:
Boyle, S. M., A. M. Alford, K. C. McIntyre, D. C. Weber, and T. P. Kuhar. 2022. Effect of plastic mulch colors on Anasa tristis (Hemiptera: Coreidae) population dynamics in summer squash (Cucurbita pepo). J. Econ. Entomol. 115(3): 808–813. https://doi.org/10.1093/jee/toac036

Objective 2. Harmonize chemical and biological control in cucurbit systems by integrating plasticulture with living mulches between beds

We predicted that living mulches and pesticides would have interactive effects on natural enemies, with negative pesticide effects being attenuated in plots with untreated living mulches serving as refugia between crop rows. Surprisingly, we found that living mulches instead magnified non-target effects by reducing natural enemy densities on pesticide-treated crops adjacent to living mulches, relative to bare plots. Pesticides had no effect on spotted and striped cucumber beetles, while living mulches directly reduced them by 25%. Conversely, pesticides reduced squash bug pressure by 50%, while living mulches had no effect. Although crops were grown in plastic mulch to protect them from competition, living mulches reduced zucchini yields by 54% at sites where living mulches were less-managed. Alternatively, living mulches had more neutral effects on yields at sites where mulches were mowed monthly, suggesting that living mulches require management to minimize competition with crops. These results suggest that the grass-dominated living mulches we tested did little to harmonize chemical and biological control. While non-crop plant diversity has clear benefits for natural pest suppression in many systems, these benefits cannot be generalized across all plant and insect taxa. Future efforts to fine-tune management of non-crop habitat within fields will be strengthened by consideration of traits of key pests and their specific responses to both pesticide applications and plant diversity.

Results

Natural enemies: In contrast to our prediction, living mulches did not mitigate pesticide-mediated reductions in natural enemies in the zucchini cash crop across our sites. We observed a significant interaction between living mulches and pesticides, but instead of increasing natural enemies where pesticides were applied, living mulches decreased them by 20% relative to untreated plots (significant pesticide * living mulch interaction, Fig. 1, Table 1a). Living mulches had no effect in the absence of pesticides, and neither living mulch nor pesticides had significant main effects on natural enemies generally (Table 1a).

Key zucchini pests: Pesticide treatments had no impact on cucumber beetles, while living mulches reduced them by 25% (Fig. 2A, Table 1b). Pesticides reduced squash bugs by 50%, while living mulches subtly increased them (Fig 2B., Table 1c).

Zucchini yield: A significant three-way interaction between site, pesticide, and living mulch treatments revealed that site-specific management and environmental variables led to complex and varied effects of pesticides and living mulches (Fig. 4, Table 2). For example, living mulches led to strong yield losses at all sites but UGA, where they were mowed to keep living mulches less than 0.3 m tall to limit competition (Fig. 4). Sites with high squash bug pressure (Coastal and Inland Virginia, Appendix Fig.1) had decreased yield in the absence of pesticide applications (Fig. 4), while pesticides provided relatively limited yield benefits at sites where cucumber beetles dominated the herbivore community (Georgia and North Carolina).

Discussion

Pesticides are widely used to protect crops from arthropod pests, but can come at a cost of reducing natural enemy biodiversity (Stark et al. 2007; Mills et al., 2016). Integrating structurally and ecologically complex plants alongside crops might buffer natural enemies from collateral damage by providing refuge and augmenting food resources (Lee et al., 2001; Pozzebon et al., 2014; Gurr et al., 2017). Recent work in large-scale agronomic production reveals that intercropping can impose stronger control on pests and stronger benefits for natural enemies that outweigh effects of pesticide treatments (Alarcon-Segura 2022). By integrating living mulches with pesticide applications in zucchini crops, we expected to enhance natural enemies and partially mitigate harmful non-target effects. Instead, we discovered that living mulches backfired to magnify negative effects of pesticides on natural enemies, reducing them by 20% relative to pesticide-treated plots without living mulches (Fig. 1). 

Chemical and biological control are notoriously difficult to implement concurrently (Roubos et al., 2014a; Schmidt-Jeffris et al., 2021). One factor that might have contributed to the failure of our living mulch to augment natural enemies was the composition and structure of the living mulch itself.  At most sites, teff grass dominated the living mulch community (Fig S3); grasses provide few nectar/pollen resources (Ramsden et al. 2015; Toivonen et al., 2018) and may have formed a barrier to movement for many natural enemies, counteracting the expected benefits of rapid recolonization after pesticide treatments (Roubos et al., 2014a; Gontijo, 2019).  In some scenarios, natural enemy numbers increase in living mulches but do not move nor provide any biological control services in the neighboring cash crops (Lopez and Liburd 2021). Therefore, our intercrops may have served as a sink, rather than a source of natural enemies in adjacent zucchini plants (Schellhorn et al. 2014).

A final potential explanation for why our non-crop habitat may have failed to ameliorate non-target effects of pesticides on natural enemies could be that dense living mulch vegetation could have slowed the residual deterioration of pesticides by blocking light and/or airflow between crop rows (Takahashi et al., 2017; Leach et al., 2017). Typically, organic pesticides like pyrethrins are short-lived, degrading within 24-48 hours (Barry et al., 2005), and recolonization by natural enemies increases greatly after pesticide residuals decline to relatively benign levels (Roubos et al., 2014b). However, if drying and photodegradation of pyrethrins were delayed by living mulches, this would extend residual activity and the window of pesticide vulnerability for natural enemies. Because predators are typically more susceptible pesticides than herbivores (Starks et al. 2007. Douglas et al., 2015; Tooker et al., 2020), our living mulches may have inadvertently exacerbated non-target effects on predators without improving control of pests.

Notably, the organic pesticide we applied had no impact on cucumber beetle pests across sites. Cucumber beetles are particularly difficult to control with insecticides due to the timing of their life cycle, as larvae are protected from pesticide exposure belowground, and highly mobile adults can emerge from soil in multiple waves (MacIntyre Allen et al., 2001; Andow et al., 2016). In our study, living mulch reduced cucumber beetle numbers by 25% compared with bare-soil plots, which may indicate that intercrops served as a barrier for dispersal and recolonization and/or impaired beetles’ ability to locate their host plants (Bach 1980, Holmes and Barrett, 1997). Indeed, chrysomelid pests are commonly found to decline in intercropped systems regardless of natural enemy pressure, although declines vary depending on species (Andow 1991; Hinds and Hooks 2013; Kahl et al., 2019). Resource-mediated herbivore declines caused by intercrops might also be attributed to reduction in suitable microclimate (Hinds and Hooks 2013), or decreased host suitability from companion plant competition (Iverson et al., 2014; Kahl et al., 2019).

In contrast to patterns observed with cucumber beetles, pesticides reduced squash bug numbers by 50% compared to unsprayed plots, while living mulches increased them slightly (Fig. 2b). Organic pesticides are sometimes inadequate in controlling squash bugs (Seaman et al., 2015), but our pyrethrin applications kept them below economic injury levels across all pesticide-treated plots (Doughty et al., 2016). In contrast to our prediction, squash bugs increased slightly in the presence of living mulches compared to bare plots, potentially because the mulches formed a structural barrier to movement of natural enemies between crop rows, or provided sufficient alternative prey resources that prevented their wide dispersal (Redlich et al., 2018; Fair and Braman, 2017). Additionally, the living mulch did not provide many floral resources that would typically attract parasitoids, a key natural enemy of squash bugs (Wilson and Kuhar, 2017; Cornelius et al., 2019). Both of the Virginia sites had high yield costs in the absence of pesticides, where squash bug pressure was relatively high, whereas pesticide applications in Georgia and North Carolina had relatively limited impacts on yield, where cucumber beetles were dominant (Fig. S2). Contrasting responses to living mulches and pesticide treatments between the two key pests of zucchini highlight the need to carefully consider unique life history traits (e.g. dispersal capacity, diet breadth) and differential patterns of susceptibility among dominant herbivores when making management decisions (Cornelius et al., 2019; Kahl et al., 2019). Recommendations for managing pests and non-crop biodiversity should not be generalized across wide geographic ranges, even in similar crop systems, because the dominant pest(s) can vary by location and region (Karp et al., 2018; Hooks et al., 1998). 

At all sites except in Georgia, living mulches treatments led to yield losses of up to 72%. Living mulches were relatively unmanaged at North Carolina and Virginia sites and exceeded the canopy of the zucchini crops, causing competition for light, water, and nutrients (Gurr et al., 2017). Even where living mulches were actively managed with mowing at UGA, it is important to note that they did not increase yield, and even reduced it by 35% in pesticide-managed plots. These patterns highlight the need for active management of non-crop diversity in agroecosystems (Himanen et al., 2016), as well as a need to consider added labor costs in the deployment of living mulches (Huss et al., 2022). Economic benefits of biodiversity-based tools are highly variable across systems (Griffiths et al., 2008; Letourneau et al., 2011, Gurr et al. 2017). While ecological intensification can lead to increased and stable yields in the long-term, short-term financial incentives may be needed to facilitate wide-scale adoption and mitigate initial losses for farmers as they troubleshoot management (Rosa-Schleich et al., 2019). Economic interests of farmers are important to consider, as it is difficult to incentivize ecologically-based practices solely for the benefit of biodiversity without financial safety nets to offset potential costs (Bybee-Finley and Ryan, 2018).

Ultimately, our results emphasize the need to acknowledge and clarify where biodiversity might function to strengthen crop protection and where it might fail (Holmes and Blubaugh, in review, Tscharntke et al. 2017). While a rich body of evidence reveals pest control benefits of living mulches and intercrops in cucurbit agroecosystems (Hinds and Hooks, 2013; Hooks et al., 1998; Hooks and Wright, 2008), and in agroecosystems generally (Iverson et al., 2014; Albrecht et al., 2020, Alarcon-Segura 2022), integrating our living mulches in pesticide-treated crops failed to augment natural enemy services and incurred significant yield losses. Increased plant diversity in agroecosystems does not always lead to increased biological control (Gontijo et al., 2018; Karp et al., 2018; Lopez and Liburd, 2021), and our contrasting results with cucumber beetles and squash bugs in zucchini clarify how variability in non-crop habitat quality and unique pest characteristics contribute to context dependency in benefits of non-crop habitats in different environments (Griffiths et al., 2008).

Reducing the frequency and increasing the selectivity of pesticides (Bergeron and Schmidt-Jeffris 2020; Bilbo et al. 2022) and improving intercrop selection and management for our system may improve harmony between chemical and biological control, but our results reinforce that ‘silver bullet’ solutions are elusive in integrated pest management (Tooker et al., 2017; Koricheva and Hayes, 2018). Indeed, pests commonly diverge in their responses to biodiversity (Begum et al., 2006; Balzan et al., 2016, Alarcon-Segura 2022), and adopting practices effective against one pest may likely increase crop susceptibility to another (Simon et al., 2011; Blubaugh et al. 2021).  Ecological interactions can be highly nuanced in biodiverse agroecosystems, and growers may benefit from making management decisions that reflect the unique life history traits of dominant pests on their farms. Future research must examine whether trait-informed management decisions improve the predictability of success for ecological pest management tools.

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Results were presented in 2022 at the International IPM symposium. We also published one review article (Huss, Holmes, and Blubaugh 2022) on the risks and benefits of living mulch intercrops in the Journal of Economic Entomology.

Publication:
Huss, C.P. , K.D. Holmes, and C.K. Blubaugh. 2022. Benefits and risks of intercropping for crop resilience and pest management. Journal of Economic Entomology. toac045, https://doi.org/10.1093/jee/toac045

Hudson, TB, AM Alford, TR Bilbo, SC Boyle, TP Kuhar, L Lopez, AK Stawara, KC McIntyre, JF Walgenbach, C Walls, C Blubaugh. 2023. Can living mulches harmonize chemical and biological control? Submitted to Agriculture, Ecosystems and Environment. Accepted with revisions

Symposium Presentation:
Blubaugh, C., A. Stawara, T. Kuhar, J. Walgenbach, T. Bilbo, H. Doughty, L. Lopez, C. Walls, S. Boyle, A. Alford. 2022. Can living mulches buffer natural enemies from non-target effects of pesticides? IPM 2022: 10TH INTERNATIONAL IPM SYMPOSIUM, February 28–March 3, 2022, Denver, Colorado.

Objective 3. Assess the impact of augmentative releases of the egg parasitoid Hadronotus pennsylvanicus (formerly Gryon pennsylvanicum) on squash bug populations in squash.

In both years, we found greater parasitism rates of A. tristis eggs collected at Virginia release sites compared to no-release sites. While all eggs collected at release and no-release sites before parasitoid deployment displayed low levels of H. pennsylvanicus parasitism, eggs from release sites were significantly more parasitized than eggs from no-release sites within two weeks post-deployment.

Table: Ratio of parasitized: unparasitized A. tristis egg masses collected at each release and no-release site in 2020. Combined ratios (bold values) were compared between release and no release treatments (Fishers Exact Test of Independence). Asterisks indicate significantly larger parasitized: unparasitized egg mass ratio (*P < 0.01, *** P < 0.0001) per sample date.

Our two-year study demonstrates that the releases of lab-reared H. pennsylvanicus can increase A. tristis egg parasitism rates and subsequently decrease successful nymph hatch rates in early summer squash plantings.

Objective 3b (Impacts of insecticides on squash bug and its parasitoid Hadronotus pennsylvanicus).

Pyrethroids or neonicotinoids have been the insecticides of choice for chemical control of squash bug for the past three decades (Doughty et al. 2016, Wyenandt et al. 2016). Although these insecticides are efficacious at controlling squash bug nymphs (Eiben et al. 2004, Kuhar et al. 2005, Abney and Davila 2011, Kuhar and Doughty 2016), most are also toxic to non-target arthropods and are generally not compatible with IPM or pollinator protection plans (Smith and Stratton 1986, Fairbrother et al. 2014, Pisa et al. 2014). In this study, acetamiprid, lambda-cyhalothrin, sulfoxaflor, and flupyradifurone all provided effective control of squash bug nymphs in lab bioassays. These insecticides have also resulted in significant reductions in squash bug densities in field experiments (Kuhar and Doughty 2016).

Acetamiprid, sulfoxaflor, and flupyradifurone are considered less toxic to pollinators than pyrethroids, while also providing control of aphids, whiteflies, and other small piercing-sucking pests. Repeated applications of pyrethroids, on the other hand, often lead to outbreaks of aphids by suppressing natural enemy abundance and promoting insecticide resistance (Slosser et al. 1989, Chapman et al. 2009).

With regards to insecticide impact on the parasitoid H. pennsylvanicus, lambda-cyhalothrin and sulfoxaflor caused the highest mortality of adult wasps compared to flupyradifurone, pyrifluquinazon, and flonicamid. High rates of H. pennsylvanicus mortality in the lambda-cyhalothrin treatment was not surprising, as lethal and non-lethal effects have been observed with other scelionid egg parasitoid species (Salerno et al. 2002, Bayram et al. 2010). It was surprising to observe high mortality rates for adult parasitoids exposed to sulfloxaflor since it is absorbed systemically by plant tissues and acts on sucking pests that imbibe the compound when feeding on treated plant fluids (Sparks et al. 2013). The H. pennsylvanicus adults were exposed to all treatments via contact by walking on treated filter papers. There is recent evidence that supports this result, as detrimental effects of contact exposure to sulfoxaflor were found with three distinct Trichogramma parasitoid species (Jiang et al. 2019). Future research should add field-related complexity to laboratory assays, such as different insecticide application methods for whole C. pepo plants (e.g., foliar, chemigation, seed treatments), to elucidate more realistic insecticide exposure effects on H. pennsylvanicus adults.

Based on our bioassays, flupyradifurone was the only reduced risk insecticide found to be both highly toxic to A. tristis nymphs and nontoxic to H. pennsylvanicus adults. Our laboratory bioassays align with results from field trials displaying significant reductions in A. tristis abundance on summer squash plants sprayed with flupyradifurone (Kuhar and Doughty 2018). Although no previous research has studied the effects of flupyradifurone exposure on scelionid parasitoids, Tabebordbar et al. 2020 showed that parasitoid Trichogramma evanescens has reduced lethal and sub-lethal effects from contact exposure to field rates of the flupyradifurone compared to pyrethroid deltamethrin exposure. Additional studies are needed to identify if there are sub-lethal behavioral and life history effects of insecticide exposure on H. pennsylvanicus. In particular, research focused on determining potential effects on H. pennsylvanicus ability to successfully search for and locate A. tristis egg masses, as well as on parasitoid fecundity and longevity, would provide valuable information concerning the overall risk broad and narrow spectrum insecticides pose to H. pennsylvanicus.