Biostimulants enhance the action of fungi against Diaphorina citri

The association of entomopathogenic fungi with biostimulants based on silicon dioxide and chitosan increased the mortality of Diaphorina citri in some of the bioassays carried out in a greenhouse. However, the performance depended on environmental conditions. A temperature of 34 degrees Celsius and relative humidity close to 35 percent limited the action of the biological agents. At 26 degrees Celsius and relative humidity around 67 percent, the fungal treatments reduced the survival of the psyllid.

This study evaluated commercial formulations of Beauveria bassiana, Metarhizium anisopliae, and Cordyceps fumosorosea, also presented in the work as a synonym for Isaria fumosorosea. The researchers applied the fungi alone or in mixtures with biostimulants containing silicon dioxide and D-glucosamine derived from chitosan. The objective involved evaluating the compatibility of the tank mixture and its effects on the mortality of adult Diaphorina citri, the main vector of Huanglongbing, HLB (doi 10.3390/insects17090947).

Six bioassays

The researchers conducted six bioassays in greenhouses at the State University of Londrina, in Paraná. The host plants used were Murraya paniculata. The insects came from a colony free of Candidatus Liberibacter asiaticus. The adults were between seven and ten days old after emergence.

The first bioassay was conducted in a greenhouse without climate control, at a temperature of 34 degrees Celsius, with a variation of two degrees, and relative humidity close to 35 percent. Under these conditions, Beauveria bassiana, applied alone or with biostimulants, did not increase the mortality of Diaphorina citri compared to the untreated control. The researchers attribute the result to the unfavorable environment for the development of entomopathogenic fungi.

The second assay was conducted at a temperature of 26 degrees Celsius, with a variation of two degrees, and relative humidity close to 67 percent. In this environment, Beauveria bassiana and the combination of the fungus with biostimulants caused higher mortality than that recorded in the control. Subsequent bioassays maintained these environmental conditions.

Experiments three to five

In experiments three through five, the researchers also evaluated *Metarhizium anisopliae* and *Cordyceps fumosorosea*, in addition to *Beauveria bassiana*. In the sixth bioassay, the experimental design was reduced to six treatments, including only *Beauveria bassiana* and *Cordyceps fumosorosea*—alone or combined with biostimulants—in addition to controls with water and with biostimulants alone; *Metarhizium anisopliae* was not part of this last trial. The protocol included two sprayings. The first occurred 24 hours after the release of the insects. The second occurred 192 hours after release. Each application used ten milliliters of spray solution per cage, with five milliliters directed from top to bottom and five milliliters from bottom to top.

The results indicated no antagonism between the biostimulants and the commercial fungi. In some assays, the combination increased psyllid mortality compared to the fungus applied alone. In the fourth bioassay, the combination of Beauveria bassiana with the biostimulants showed higher mortality than the isolated application of the fungus in the evaluation performed after 192 hours. In the fifth assay, the same combination also outperformed Beauveria bassiana applied without the biostimulants.

Sixth bioassay

In the sixth bioassay, Cordyceps fumosorosea associated with biostimulants reduced insect survival in evaluations performed after 24, 192, and 480 hours. The combination of Beauveria bassiana with biostimulants showed a significant effect in the 192 and 480-hour evaluations. At the end of the experiment, the treatments consisting of fungi and biostimulants outperformed both the untreated control and the isolated application of biostimulants.

The inclusion of an organosilicone adjuvant, evaluated in the fourth trial, did not increase insect mortality. The study also showed an advantage of the synthetic insecticide used as a reference. The combination of lambda-cyhalothrin and chlorantraniliprole showed faster initial mortality and, in several trials, higher mortality than that recorded with the biological agents.

According to the researchers, this difference in the time of action represents a significant limitation in the management of Diaphorina citri. Entomopathogenic fungi require a longer period for infection and death of the host. For an insect vector, the speed of control becomes important due to the risk of transmission of the agent associated with HLB.

Influence of microclimate

The data also reinforce the influence of microclimate on biological control. The study does not allow for defining exact limits of protection offered by biostimulants in situations of heat and low humidity. The researchers themselves highlight an experimental limitation: the first two bioassays took place in different greenhouses and at different times. Thus, the effects of temperature and humidity were also associated with spatial and temporal variations.

Research suggests that tank mixing of commercial entomopathogenic fungi with silicon dioxide and chitosan is a viable alternative for integrated management programs for Diaphorina citri. However, the outcome depends on environmental conditions favorable to fungal activity. Researchers recommend further studies in climate chambers with different combinations of temperature and relative humidity, as well as field trials to evaluate the persistence, operational efficiency, and viability of this strategy in commercial orchards.

The work was carried out by Éder M. Carrilho, Paulo SG Cremonez, Luciano M. de Oliveira, Fernando T. Hata, Maurício U. Ventura and Humberto G. Androcioli.