Study area

This research was carried out from June to November, 2012, in a 3 ha sesame smallholding, in the Ocolome ejido, El Fuerte Municipality (26° 27’ 0.57’’ N, and 108° 35’ 20.1’’ W, at an altitude of 102 m). In Ocolome, 50 ha are sowed, mainly with corn, watermelon (Citrullus vulgaris Schrad), cucumber (Cucumis sativus L.), summer squash (Cucurbita pepo L.), bean (Phaseolus vulgaris L.), and sesame. The maximum temperature is 43 °C, and can fall to 4 °C. This is a flat terrain with elevations; the region has a dry and warm weather, and it can rain up to 616 mm per year.

Larvae harvesting

Larvae from the three stages were harvested monthly from June to October, 2012. Thirty samples were taken from the plot (30x30x30 cm sections), in a (systematic) respondent-driven sampling, across 30 transect separated 10 m from each other. Each sample was meticulously examined, according to the manual separation method, in order to count the larvae, before fixing 50 % of them in Pampel fluid, for 72 hours; then, 70 % were preserved in ethyl alcohol. In the laboratory of the Colección Entomológica (CEVF) of the Universidad Autónoma de Sinaloa, they were taxonomically identified, using the dichotomous keys proposed by ^{Boving (1942)}, ^{Ritcher (1966)}, and ^{Morón (1986)}, as well consulting the data base. The other 50 % of the third instar larvae were taken to the laboratory alive, in order to form similarity groups, based on raster and epipharynx characteristics. The larvae were put on plastic terraria, with sterilized organic soil, and they were fed carrot (Daucus carota L.) bits, according to the methodology proposed by ^{Aragón and Morón (2004)}, to continue the breeding process and to verify their identity.

Adults harvesting

In order to have a reference sample which would facilitate the larvae identification, beetles were harvested at night, from June to October, 2012, using a funnel trap, with a 20-watt fluorescent black light, connected to a harvester container (19 L.). The trap with a 45 m harvesting effectiveness (^{Aragón et al., 2010}) was placed in the center of the smallholding, and it was operated twice a week during the beetle’s main activity hours (19:00 to 23:00 hours). The captured adults were kept in 70 % ethylic alcohol (^{Morón and Terrón, 1988}), and their containers were labeled with the harvest data (date, place, capture hour, and harvester). In the laboratory of Colección Entomológica del Valle del Fuerte (CEVF-UAS), individuals were processed using conventional techniques. Representative series were nailed to insect pins for their conservation; their taxonomic identification was carried out using the keys proposed by ^{Morón (1986)}, ^{Deloya and Ratcliffe (1988)}, ^{Morón et al., (1996)}, the collections of the Instituto de Ecología A.C. de Xalapa (IXEA), and the Instituto de Ciencias de la Benemérita Universidad Autónoma de Puebla (BUAP). The specimens identified were deposited in the entomological collections of Inecol, Xalapa (IEXA), and the Universidad Autónoma de Sinaloa (CEVF-UAS).

Third instar larvae

Of the 265 larvae harvested, five subfamilies, six tribes, seven genera, and fifteen species of the Melolonthidae, Cetoniidae, and Hyborsoridae families were represented (Table 1).

Table 1 Melolonthidae, Cetoniidae and Hyborosidae species associated, as larvae, with sesame (Sesamum indicum L.), crops, in Ocolome, El Fuerte, Sinaloa, México 

The identification of white grub complex larvae in sesame crops was partial. Larvae breeding only proved that there is a relation between larvae and adults in some species, based on raster and epipharynx (Figure 1 and 2); and, in other cases such as Euphoria leucographa and Hybosorus illegueri the descriptions that would identify a relation between larvae and their corresponding adults do not exist; meanwhile, the taxonomy of genera with higher diversification such as Phyllophaga, Cycocephala, and Paranomala, whose species are frequently associated with agricultural crops was not settled. In this research, the following larvae were identified: 8 Euphoria leucographa, 7 Ligyrus sallei, 65 Hybososrus illegeri, 83 Phyllophaga, 26 Cyclocephala, 50 Paranomala, 19 Euphoria, 15 Cotinis, and 2 Gymnetini.

Figure 1 Larval epipharynx of the species collected in Ocolome, El Fuerte, Sinaloa, Mexico. A) Phyllophaga sp. 1; B) Phyllophaga sp. 2; C) Phyllophaga sp. 3; D) Phyllophaga sp. 4; E) Cyclocephala sp.; F) Paranomala sp. 1; G) Paranomala sp. 2; H) Euphoria sp. 1; I) Euphoria sp. 2; J) Cotinis sp. 1; K) Cotinis sp. 2; L) Gymnetini sp. 1. 

Figure 2 Raster of the species collected in Ocolome, El Fuerte, Sinaloa, Mexico. A) Phyllophaga sp. 1); B) Phyllophaga sp. 2; C) Phyllophaga sp. 3; D) Phyllophaga sp. 4; E) Cyclocephala sp.; F) Paranomala sp. 1; G) Paranomala sp. 2; H) Euphoria sp. 1; I) Euphoria sp. 2; J) Cotinis sp. 1; K) Cotinis sp. 2; L) Gymnetini sp. 1. 

Phyllophaga genus larvae prevailed (31.2 %), followed by Hybosorus (24.5 %), Paranomala (18.8 %), Cyclocephala (9.8 %), Euphoria (7.1 %), Cotinis (5.6 %), and, Ligyrus (2.6 %). In Mexico, the white grub complex is diverse: there are species with rhizophagous, saprophagous, and detritovore larvae (^{Morón, 2001}). Phyllophaga prevailed over Cyclocephala and Paranomala species, probably because they have strictly rhizophagous larvae. These larvae can cause important agricultural losses, and their prevalence is due to their adaptability, and ecological aggressiveness, as well as the easiness with which their populations increases. This situation is caused by monoculture and indiscriminate application of agrochemicals, which favour species selection (^{Morón, 1986}). The Phyllophaga genus is important because the lineages provide aggressive rhizophagous species, which damage the crops, and because it is possible that some groups, apparently with exclusive conditions and crops, have specialized and adapted to certain forest and xerophilous conditions, and they could hardly survive without them (^{Morón, 2010}).

The seasonal distribution or phenology of the white grub complex showed the highest density in August and the lowest in October, with 125 and 17 Phyllophaga spp. and H. illigeri larvae, respectively. H. illigeri’s, Euphoria leucographa’s, and L. sallei’s first instar larvae were harvested in July, while second and third instar larvae were harvested from August to October. This proved that the species has an annual life cycle (Table 2). We bred insects, in order to find out the larva-adult ratio for third instar larvae of the most abundant species, but we only obtained four adults out of five Ligyrus sallei larvae, three adults out of five Euphoria leucographa larvae, and 17 adults out of 32 Hybosorus illigueri larvae.

Table 2 Seasonal distribution of Melolonthidae, Cetoniidae and Hyborosidae larvae, per genus, associated, as, with sesame (Sesamum indicum L.) crops, in Ocolome, El Fuerte, Sinaloa, México 

Adults recorded in sesame cultivation plots, in El Fuerte, Sinaloa, México.

The 769 specimens harvested represented five subfamilies, eight tribes, 13 genera, and 17 species of the Melolonthidae family (Table 3).

Table 3 Melolonthidae, Cetoniidae, and Hyborosidae species harvested with a fluorescent black light trap in sesame (Sesamum indicum L.) crops, in Ocolome, El Fuerte, Sinaloa, México. 

The Phyllophaga genus contributed the highest number of species (17.6 %), as well as the highest number of specimens (21.4 %). The following species were recorded: Phyllophaga opaca, Ph. lenis, Ph. cristagalli, Oxigrylius ruginasus, Paranomala flavilla, Euphoria leucographa, and Hybosorus illigeri; they represent 89.72% of the insect sample (total number: 690 individuals).

An outstanding finding was that none of adults attracted by the light belonged to any of the six species of larvae found in the cultivation soil. This suggests that they had developed in neighbouring plots, in other crops, or in wild plants (^{Morón et al., 1997}). The richness of rhizophagous species in the light trap could be the result of the mixture of the species established in sesame crops with those that originate in the wild plants that surround sown plots. The coincidence of Lygyrus sallei, Euphoria leucographa, and Hybosorus illigeri larvae and adults was confirmed, but their relation was inaccurate in other cases. One such case was the recording of the larvae of four Phyllophaga species in the light trap and only adults from three species. Likewise, larvae belonging to one Cyclocephala species were harvested, but the captured adults belonged to two species. Larvae from two Paranomala species were extracted from the soil, and only one species was found in the light trap.

Grouping species based on their feeding habits (^{Morón, 2013}) allowed us to calculate that 41.17% of the adults captured had phyllophagous-saprophagous-rhizophagous habits and their larvae develop in the soil; that 29.41% was represented by saprophagous-rhizophagous-wood-boring species; and that 17.6 and 11.7% are saprophagous-nectarivore and phyllophagous-wood-boring species. The presence of species with phyllophagous adults and rhizophagous larvae of the Phyllophaga, Paranomala, and Cyclocephala genera in sesame crops was interpreted as the result of zone’s relatively recent colonization: these species have a wide ecological valence, and have started a transition period, in which a typical natural vegetation community was replaced to engage in rainfed agricultural production (^{Lugo et al., 2016}). This would create a favourable atmosphere in which Phyllophaga, Paranomala, and Cyclocephala larvae found in sesame and recorded in several Mexican states as harmful for crops, would eventually represent a phytosanitary problem in northern Sinaloa, as a result of corn and sesame monoculture.

In Mexico, edaphic species belong to 68 genera; Phyllophaga alone encompasses 369 species, out of which 25 are recorded as rhizophagous agricultural pests that cause damage to a wide variety of plants important for agriculture and forests. Damage is classified as minor, moderate, or severe, depending on the crop, environmental conditions, and its development state (^{Morón, 2010}). Twenty-two Dynastinae species were collected from cultivated plots. In this group, the species belonging to the Cyclocephala genus stand out, as a result of their diversity, abundance, and wide distribution (^{Morón et al., 1997}). There are doubts about the feeding habits of the larvae of several species, mainly Cyclocephala lunulata. This is the species most frequently found in corn crops, where it has rhizophagous habits; however, in sugar cane plantations, it has saprophagous habits (^{Deloya, 1998}). Based on this difference, we can infer that the Ligyrus sallei larvae studied here could mainly feed from agricultural waste and manure, instead of roots. In other regions of the country, farmers blame them for damages to tilled plots with high organic matter content (^{Morón, 2010}). Twenty-one Rutelinae species mainly Paranomala were recorded in agricultural plots; however, the larvae’s habits have not yet been specified, and identifying the adults and non-mature insects of several species is difficult (^{Morón et al., 1997}).

According to ^{Ramírez-Salinas and Castro-Ramírez (2000)}, emphasis should be put on the species of the Phyllophaga genus, because they are high-impact agricultural pests. However, attention must be paid to the species of the Paranomala, Euethola, and Dyscinetus genera, because their larvae show detritivore-rhizophagous habits in soils with scarce organic matter, particularly when they are subject to the pressure of inter- or intraspecific competition. Phyllophaga ravida Blanchard, Ph. detex Bates, Ph. polyphylla Bates, Ph. fulviventris Moser, Ph. setifera Burmeister, Ph. lenis Horn, Ph. rubella Bates, Ph. misteca Bates, Ph. vetula Horn, Ph. pruinosa Blanchard, and Ph. obsoleta Blanchard are recorded as agriculturally important species in the states of Jalisco and Veracruz. ^{Morón (2010)} indicated that, in Mexico, the Phyllophaga genus is made up by eight subgenera, 41 species groups, and 386 species, out of which 50 make up the species complex with rhizophagous larvae known as white grub, which is a major pest on several agricultural crops. Phyllophaga lenis stands out among this complex as the species that could have a short-term impact on sesame roots in this region, since it cause recurrent damage in other crops in Mexico.

The larvae of the Phyllophaga genus represent 51 % of the species that damage the roots of crops and forest systems in the state of Puebla. In Puebla’s Sierra Norte, the white grub complex is recognized as a major problem for corn production, because Ph. vetula and Ph. ravida caused 48 % losses (^{Aragón et al., 2010}). ^{Aragón et al. (2009)} reported that Cyclocephala lunulata, Diplotaxis angularis LeConte, Macrodactylus ocreatus Bates, Parachrysina parapatrica Deloya and Morón, Paranomala sp., Ph. cuicateca Morón and Aragón, Ph. ilhuicaminai Morón, Ph. misteca, Ph. obsoleta, and Ph. ravida are associated with the cultivation of amaranth (Amaranthus hipocondryacus L.), but the damage were caused by Phyllophaga cuicateca, Ph. ilhuicaminai, and Ph. ravida, which feed on roots, causing them to become flaccid, wilted, and knocked over.

The damages caused by white grub to sugar cane yield in the Tepic region during the 1993-1994 sugar harvest amounted to 20-70 %, and 70-100 % root system destruction was recorded; the approximate loss reached 14 920 t of cane (^{Morón et al., 1998}). The C. sinaloae and Phyllophaga sp. recorded in Valle del Carrizo, northern Sinaloa, could potentially damage corn crops, as most of the Phyllphaga genus in Mexico (^{Lugo et al., 2012}). The presence of four Phyllophaga species in the area under study indicate that, in order to identify economically important species, further research is required. Therefore, since the regional production of sesame and corn is high, it is also a favourable environment for the development of Phyllophaga, Cyclocephala, and Paranomala.

Although the damage caused by Phyllophaga, Cyclocephala, and Paranomala larvae to seasame were not quantified, the damage they caused to the roots was evident. Large stains were also observed in withered plants and they had negative repercussions in yield.

Euphoria, Cyclocephala, Ligyrus, Cotinis, and Hybosorus larvae are strictly saprophagous. They appear regularly in humidified soils or in soil fertilized with organic matter (mainly manure). The scarcity of organic matter and the abundance of roots in agricultural smallholdings promote the prevalence of rizhophagous larvae; in soil with an abundance of organic matter and high root density, rhizophagous and saprohagous larvae may predominate. However, determining this habit is difficult, because plants with root damage do not frequently show obvious weaking symptoms (^{Morón, 2010}). In amaranth production zones in Puebla and Chiapas, Euphoria basalis Gory and Percheron, Euphoria sp., and Strigoderma sulcipennis Burmeister had an outstanding role in the decomposition of organic matter in smallholdings fertilized with compost (^{Ramírez-Salinas et al., 2001}). When they feed, Dynastinae larvae participate in the recycling of nutrients and in the fragmentation of great volumes of dead leaves; they transform them into organic matter thus increasing soil fertility (^{Morón, 1985}). Scarabaeiform larvae or rhizophagous, saprophagous, or detritivore white grubs are beneficial to tilled and virgin soil: they turn over the soil (favouring water and air flow), and they incorporate their faeces with high assimilated nitrogen and microorganism content into the soil; plants make use of both of them (^{Tapia-Rojas et al., 2013}).

A fundamental aspect is the identification of white grub complex species in each region. Therefore, confirming and accurately identifying which Phyllophaga sp. species is harmful is necessary; additionally, a diagnosis must be carried out to find out the infestation degree. This information will allow the proposal of actions for the development of comprehensive management strategies and the establishment of their environmental importance. Overall, this would contribute to increasing the information about the rainfed agricultural zone at Sinaloa.