High population levels lead Glycaspis brimblecombei (Hemiptera: Aphalaridae) to unrecorded feeding and oviposition behaviors on Eucalyptus urograndis plants

The red gum lerp psyllid, Glycaspis brimblecombei Moore, 1964 (Hemiptera: Aphalaridae), an insect pest originating in Australia and which feeds only on Eucalyptus L’Hér. (Myrtales: Myrtaceae) plants, has spread to several countries. The populations of this insect commonly reach high populations on Eucalyptus plants since its entry into Brazil, and also indicated an unrecorded behavioral. The objectives of this study were to describe a peculiar adaptation in the feeding habit of G. brimblecombei and to register the new habit. The oviposition and feeding by G. brimblecombei , commonly, on the leaves of Eucalyptus , started to occur, also, on lignified twigs. This suggests a not yet recorded adaptation of this insect to reduce insect × plant intraspecific

The objectives of this study were to describe an unprecedented adaptation in the feeding habit of G. brimblecombei and to register the new habit.

Experimental site
The feeding habit of G. brimblecombei was examined on two-year-old clone hybrid plants of Eucalyptus urograndis (E. urophylla × E. grandis)  Two stands, each with a group of plants "a" or "b", were examined per municipality. The average area per stand was 40 ha. The distance between stands in the same municipality was 1.0 and 2.0 Km in Bom Despacho and Luiz Antônio, respectively. The damage was observed from July to September 2018 and 2019, months of the greatest occurrence of this pest in Brazil, due to the low intensity of rain and relative humidity (Masson et al., 2009;Oliveira et al., 2012;Silva et al., 2013). No control method was applied in the plantations since 30 days before and during the evaluation period. All photographs were taken using an HP Photosmart 945 -5 megapixel digital camera, 8 × zoom (Palo Alto, California, United States of America).

Population density
The population density of G. brimblecombei was evaluated by sampling 100 leaves per plant (20 plants per municipality, 10 plants per stand, randomly selected within them) in the middle third of the crown (Oliveira et al., 2012;Boavida et al., 2016), with leaf collection, manually, using a ladder. One collection was carried out per month per year of evaluation.

Population density
The population density of G. brimblecombei was high in both municipalities, with more than 100 nymphs per Eucalyptus leaf ( Figure 1A).

Unrecorded feeding and oviposition habits
Unrecorded feeding behavior and oviposition of G. brimblecombei were seen during the observation period of this insect in the field. This insect oviposited and nymphs  (Moore, 1970;Brennan and Gill, 1999;Brennan et al., 2001).
The area planted with Eucalyptus in Brazil was 7.5 million hectares in 2021, with an average productivity of 39 m 3 per ha (Pavan et al., 2021;André et al., 2021). The entry of G. brimblecombei in Brazil, through the state of São Paulo in 2003 (Santana and Burckhardt, 2007), started to demand the adoption of control strategies, burdening the cost of this culture (Boavida et al., 2016;Dias et al., 2017). The cost of applying systemic insecticide to control of G. brimblecombei ranged from R$40.00 to R$150.00 per round in the state of Santa Catarina, Brazil in 2011 (Garcia et al., 2011). Nymphs and adults of this insect suck the sap causing discoloration, drying and leaf fall, and top dieback of Eucalyptus plants (Pereira et al., 2013). In addition, excrement (honeydew) from the initial feed of G. brimblecombei, which is rich in sugars, facilitates the development of sooty mildew by reducing the rate of photosynthesis and the development of the plant (Huerta et al., 2010;Spodek et al., 2015). Successive damage with high infestations of G. brimblecombei can cause the fall of 20 to 30% of the leaves and up to a 40% mortality rate of Eucalyptus plants (Laudonia et al., 2014;Ferreira-Filho et al., 2015).
Glycaspis brimblecombei lays eggs on the surface of the leaves, where their nymphs insert the stylets into the phloem directly through the sheath to feed (Perris-Felipo et al., 2011;Ribeiro et al., 2015), avoiding reaching the oil glands (Brennan and Weinbaum, 2001a). Younger nymphs introduce the stylet between mesophilic cells and those in the fifth stage use an intracellular route (Pereira et al., 2013) with greater survival, of adults on mature Eucalyptus leaves (Brennan and Weinbaum, 2001a, b). Glycaspis brimblecombei reproduces sexually, laying six to 45 eggs per leaf, preferably on those younger (Firmino-Winckler et al., 2009). The nymphs of G. brimblecombei use honeydew and other chemical components produced by Malpighian tubules to build were observed feeding on lignified twigs of plants of both E. camaldulensis and E. urograndis clones ( Figures 1B-1F). The presence of adults on these twigs indicates that G. brimblecombei completed its cycle on this plant part ( Figure 1G). Leaf fall was more pronounced from plants with a high population density of G. brimblecombei (i.e. insects on leaves and lignified twigs), with drying of the entire twig (twig dieback) and a greater reduction on the emission of new leaves by the plants (Figure 1H), than in those with damage, by this pest, which was restricted to the leaves.

Discussion
Reports that adults and nymphs of G. brimblecombei feed only on leaf tissue are common (Perris-Felipo et al., 2011;Pereira et al., 2013;Ribeiro et al., 2015). The population of G. brimblecombei, in plantations of E. camaldulensis and E. urograndis, severely damaged by this pest (i.e. nymphs on leaves and lignified twigs) had a drastic reduction, after an intense leaf fall at the end of the dry season, probably due to food depletion and that herbivores feeding only on a plant genus are more sensitive to changes in the food availability (Steinbauer et al., 2016). Feeding by hemipterans on lignified twigs requires more energy to produce a greater amount of allelochemical detoxifying enzymes. The feeding on phloem suggests a high potential for these enzymes to bind to the Ca ++ of this plant vascular tissue to make sugars as food available to hemipterans (Sharma et al., 2014). In addition, the association of Aphalaridae species with endosymbiont bacteria induces changes in plant tissues. These bacteria and enzymes, like the pectinases in these insect salivary glands, allow them to mobilize primary metabolites quickly (Sharma and Raman, 2017).
The high density of G. brimblecombei, from the beginning to the end of the dry season, may have been due to the abundance of food, the low population of its parasitoid, Psyllaephagus bliteus Riek, 1962 (Hymenoptera: Encyrtidae) and the non-application of insecticides. This parasitoid was found, for the first time in Brazil, in 2003 in Piracicaba, state of São Paulo, shortly after the detection of G. brimblecombei (Berti Filho et al., 2003). However, its population has remained low in the field in Brazil (Ferreira-Filho et al., 2008;Silva et al., 2013;Ferreira-Filho et al., 2017).
The adaptation of feeding site in the same host by G. brimblecombei is apparently due to the reduction of its favorite food, as the damage by this insect was greater on the abaxial surface of E. camaldulensis and on the adaxial surface of the E. urophylla × E. camaldulensis and E. urophylla × E. grandis hybrids. This was attributed to the ability of this insect to tolerate physical and chemical defenses due to the long coexistence with E. camaldulensis in Australia and to the greater flow of nutrients and its low desiccation on the abaxial surface (Firmino-Winckler et al., 2009;Tuller et al., 2017).
The uncommon adaptation in the feeding habit of G. brimblecombei in the present study was similar to that reported for the syzygium leaf psyllid, Trioza eugeniae Froggatt, 1901 (Hemiptera: Triozidae), which moved from the abaxial part of the plants, preferred for feeding, to the adaxial in conditions of greater intraspecific competition with Sygygium paniculatum Gaertn. (Myrtales: Myrtaceae) (Luft and Paine, 1997). Factors such as water stress (common in the Brazilian midwest region during winter) and/or genetic improvement of plants (production of Eucalyptus hybrids susceptible to the pest with branch morphology that allows them to feed), in addition to competition, may have contributed to the adaptation in feeding behavior by G. brimblecombei.
The adaptation in the feeding site, by nymphs of G. brimblecombei, from the leaves to lignified twigs, seems to be more drastic than those of other hemipteran, mainly due to the difference in hardness and rigidity between the tissues of leaves and twigs of the Eucalyptus. An adaptation of feeding location has also been reported for the soybean pests, Dichelops melacanthus (Dallas, 1851), Euschistus heros (F., 1798), Nezara viridula (L., 1758) (Hemiptera: Pentatomidae), and Neomegalotomus parvus (Westwood, 1842) (Hemiptera: Alydidae) that prefer seeds and fruits, but in the absence of this resource they feed on vegetative tissues (Panizzi, 2000). Empoasca fabae (Harris, 1841) (Hemiptera: Cicadellidae) feeds on twigs of the alfalfa plants, Medicago sativa L. (Fabales: Fabaceae), but migrates to the petiole and, later, to the leaves of this plant, the least preferred place for feeding, in highly infested genotypes or with high trichome density (Shockley et al., 2002).
The high susceptibility of E. camaldulensis and E. urograndis plants and favorable climatic conditions for the development of G. brimblecombei may facilitate the population increase of this pest. This insect, in high populations, also uses alternative resources and reproduces and develops on the lignified twigs of the plants of these species.