Timing Fungicides And PGRs To Control Greenhouse Pests

Thrips under attack from an insect-killing fungus.

The management of pests and diseases during the production of greenhouse crops is not an isolated event. As such, interactions may occur that can have a significant impact on the success of any pest management strategy.

For example, under conditions of high relative humidity (greater than 75 percent), naturally-occurring populations of beneficial or insect-killing fungi may cause substantial mortality of insect and mite pests. Although beneficial fungi typically require high relative humidity, these fungi can be effective when the microclimate relative humidity around the spore (e.g., leaf surface) is high. This can occur even when the relative humidity within a greenhouse is less than 50 percent.

In addition, the effectiveness of beneficial fungi can vary depending on light intensity, plant size and plant architecture. Fungicides used to manage plant diseases may create issues associated with the compatibility of these chemicals and beneficial fungi, such as Beauveria bassiana, Metarhizium anisopliae and Isaria fumosoroseus, by impacting their effectiveness and persistence.

Furthermore, fungicides used to regulate or manage plant pathogenic fungi may or may not have any direct or indirect effects on beneficial fungi based on timing of application. However, not only do certain fungicides inhibit the effectiveness of beneficial fungi, but they may also affect insect or mite pests.

In fact, some fungicides have been shown to increase fecundity of insect pests, and fungicides could be potentially harmful to certain natural enemies, causing inadvertent outbreaks of pest populations.

Finally, plant growth regulators (PGRs) that are applied to regulate plant development, may increase populations of certain arthropod pests. So in this article, we will discuss both the direct and indirect impact of fungicides and PGRs, as well as possible interactions that may occur when greenhouse producers are attempting to manage pests.


Chemical fungicides may negatively impact beneficial fungi. This depends on a number of factors, including the particular fungicide (contact versus systemic) and rate used, as well as the formulation of the product.

Certain broad-spectrum fungicides might also decrease susceptibility of insect and mite pests to beneficial fungi. For example, certain scales are less susceptible to beneficial fungi after exposure to copper sprays, which may impact pest populations.

Although older fungicides such as benomyl and sulfur have been shown to have direct effects on plant-feeding mite pests, there is also the issue associated with indirect effects. There are some indirect effects that may involve increasing pest populations by inhibiting the activity of beneficial fungi through preventing infection of insect or mite pests. For example, fungicides have proven to inhibit infection by beneficial fungi of the green peach aphid (Myzus persicae), resulting in increases in aphid populations.

Still, this may not necessarily be due to the active ingredient. Inert ingredients, such as surfactants, adjuvants, carriers or solvents in the formulation may be responsible. In fact, certain surfactants and solvents have been shown to exhibit efficacy by themselves. Moreover, some fungicides may inadvertently delay plant senescence, prolonging the time that insects remain on plants and continue feeding, thus causing damage.

So what accounts for some of these interactions? Well, fungicides can affect conidial germination, or they may inhibit mycelial growth of beneficial fungi. Any inhibition of germination, growth and “killing power” of beneficial fungi may slow or prevent infection of insect and mite pests. Furthermore, fungicides may have variable effects on germination and growth of different beneficial fungi, which influences the infection process by inhibiting germination on the insect cuticle and decreasing the ability of beneficial fungi to kill insect pests.

The germination and production of conidia associated with the primary infection of beneficial fungi may be inhibited by a wide range of systemic and non-systemic fungicides. As such, conidia production affiliated with infected insect hosts may be reduced when exposed to fungicides. However, results may not be consistent, and again, application timing could influence compatibility with beneficial fungi.

Laboratory studies have reported that certain fungicides are capable of inhibiting growth, conidia germination and infection of insect hosts. Still, the results are quite variable. So it is important to note that most labels associated with the commercially available beneficial fungi products, including BotaniGard (BioWorks), No-Fly (Natural Industries) and PFR-97 (Certis USA) clearly state that fungicides can be detrimental to these products.

Companies typically provide specific information and directions for using these products in a program with fungicides. For example, both metalaxyl (Subdue) and mancozeb (Dithane) have been shown to inhibit development and mycelial growth of Beauveria bassiana, and the fungicides thiophanate-methyl (Cleary’s 3336), mancozeb (Dithane) and chlorothalonil (Daconil) inhibited mycelial growth and sporulation of B. bassiana under laboratory conditions.

Although certain fungicides may inhibit germination of beneficial fungi under laboratory conditions, they may have minimal to no affect on the ability to kill insect and mite pests under greenhouse conditions. In addition, many of these responses to fungicides are dose-dependent. For specific information on compatibility, always consult with a company representative of the product.

One of the primary means of utilizing chemical fungicides and beneficial fungi in a pest management program is to delay the timing of fungicide applications. In fact, reports indicate that delaying fungicide applications may enhance the efficacy of B. bassiana, though the mechanisms responsible for this are still unknown. Studies have shown that individual applications of the fungicides azoxystrobin (Heritage), fosetyl-aluminum (Aliette), iprodione (OHP 26 GT-O), myclobutanil (Eagle or Systhane) and fenhexamid (Decree) did not inhibit infection of B. bassiana. In addition, application of fungicides, after applying beneficial fungi, did not negatively impact efficacy nor inhibit infection or growth within the insect.

Interactions between pesticides (in this case, fungicides) and beneficial fungi may also be synergistic (enhance) or antagonistic (inhibit), which could affect insecticidal activity of beneficial fungi. For example, applications of B. bassiana two to four days before applying the fungicides metalaxyl (Subdue) or mancozeb (Dithane) appeared to synergize the activity of B. bassiana, whereas fungicide applications made before applying B. bassiana resulted in antagonism (reduced mortality). This delay appeared to allow the beneficial fungus to initiate the infection process (adhesion, germination, differentiation and penetration) without any disruption following the fungicide applications.

Fungicides may also affect arthropod pests by impacting prey quality and/or influencing plant quality. In fact, some sterol-inhibiting fungicides have demonstrated PGR activity, which may result in an increase in plant susceptibility to arthropod pests. Furthermore, effects of fungicides on beneficial fungi may be due to subtle influences on plant metabolism, resulting in an increase in susceptibility to particular phloem-feeding insects such as aphids.

In addition, certain fungicides may indirectly impact natural enemies negatively by reducing the acceptability of prey to either parasitoids and/or predators, which could alter the ability of natural enemies to regulate populations of insect or mite pests. This would allow pests to escape, attack and continue to cause damage.

Plant Growth Regulators

The interaction of PGRs with arthropod pests has not been studied as extensively compared to fungicides. However, PGRs, similar to fungicides, may change plant physiology by modifying levels of essential plant nutrients. This makes the plants more nutritious (a better food source) to insect or mite pests, particularly those with piercing-sucking mouthparts such as aphids, whiteflies and the twospotted spider mite.

This phenomenon has been demonstrated with chrysanthemum and leafminers in which the levels of amino acids were elevated following applications of PGRs, thus increasing susceptibility to feeding and subsequent plant damage. Furthermore, it has been proposed based on cultivar sensitivity that PGRs may modify or increase the attractiveness of certain cultivars, enhancing susceptibility to insect and mite pests.

Conversely, plants receiving applications of certain PGRs may actually exhibit a negative effect on insect pests via changes in the physical structure of plants. This occurs  by increasing cuticle thickness, which may make it difficult for phloem-feeding insects like aphids to insert their mouthparts into plant tissues when probing for food.


This article has presented information on the potential interactions that may occur between fungicides, beneficial fungi, plant growth regulators and arthropod pest complexes. The use of these compounds in commercial plant production is important in regulating plant pathogens, pest populations and plant growth with the end result being production of a quality and salable crop.

As such, the goal is to provide applicable information to greenhouse producers on possible inadvertent factors that may be responsible for the failure to regulate arthropod pest populations. One of the key factors to consider in order to ensure compatibility of fungicides with beneficial fungi is timing of application. Once this is understood, you can integrate both chemical fungicides and beneficial fungi into pest management programs.

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