Ants at the honeydew. Scales and parasitoids on Myrtillocactus geometrizans

Four organisms connected by food

In the studied system, the cactus supplied food to the soft scale Toumeyella martinezae, a member of Coccidae. The insect pierced plant tissues and extracted resources. Its honeydew fed ants of the species Liometopum apiculatum. Workers groomed the scales, moved them to favourable settlement sites and defended them against natural enemies. Another participant was the wasp Mexidalgus toumeyellus. It is a parasitoid, an organism whose development occurs at the expense of a host and ultimately kills it. Its presence connected the fate of scale colonies with a potential reduction in feeding damage to the plant [Callejas-Chavero et al. 2023].

The benefit of protection has to be assigned to the appropriate participant. For a scale insect, enemies being deterred by ants means an improved opportunity to survive. For the ants, maintaining the colony sustains access to honeydew. The cactus bears the cost of feeding. A parasitoid using the scale can reduce that cost if its activity actually limits herbivore abundance. This produces indirect plant protection. The strength of the relationship depends on encounter rates and successful attacks. A list of organisms present on a stem alone does not reveal the eventual outcome for cactus growth and reproduction [Callejas-Chavero et al. 2023].

An experiment in the dry scrubland of Hidalgo

Alicia Callejas-Chavero and colleagues worked near Zequetejé, in the municipality of Huichapan, Hidalgo. The plants grew in dry scrubland at an elevation of 2100 m. 20 cacti at least 2.5 m tall were selected for comparisons of plant performance. Ten carried Toumeyella martinezae, while ten were free of these scales. Four branches were marked on each plant. The researchers measured branch growth and the area covered by fungi, and counted buds, flowers and fruits. This allowed several aspects of plant response to be compared in relation to herbivore presence [Callejas-Chavero et al. 2023].

Ant access was manipulated at branch level. Ants were removed from two branches of each scale-bearing plant, and an unscented petroleum jelly barrier prevented their return. Workers could move freely on the other two branches. A separate component examining parasitoid dynamics involved 12 randomly selected shrubs. From June to November 2016, the researchers assessed scale abundance, developmental stage and parasitoid presence, again comparing branches accessible and inaccessible to ants. This manipulation supported an assessment of ant effects, whereas the comparison of naturally infested and uninfested cacti remained a field observation [Callejas-Chavero et al. 2023].

Costs expressed in growth and flowers

Branches of scale-free plants grew by an average of 4 cm, compared with 2 cm in infested plants. Among infested plants, greater insect abundance was associated with less branch growth. The researchers also found more buds and flowers on scale-free plants. These results concern two routes of resource use by the cactus. Growth extends the stems, while bud development initiates further reproductive opportunities. Feeding was therefore associated with reduced investment both in vegetative structures and in the structures preceding fruit formation [Callejas-Chavero et al. 2023].

The number of mature fruits, however, did not differ statistically between the groups. Fruit size likewise showed no demonstrated difference. This separation matters to the biological interpretation. Successive reproductive stages have their own limitations, and an observed reduction in flower number need not produce a proportional change in final fruit number within a particular sample. In this study, conclusions concerning growth, buds and flowers had stronger support. Assessing the production of offspring also required examining seeds and the seedlings that developed from them [Callejas-Chavero et al. 2023].

The response of the next generation

The authors followed the germination of 2000 seeds, with 1000 each from scale-free and infested plants. Observations continued for 44 weeks. Germination did not differ significantly between the two origins in this sample. Seedlings were then measured 30 days after emergence, including 94 from scale-free mothers and 82 from infested mothers. Offspring of the first group were larger. This result links the condition of the maternal plant with early offspring growth despite a similar germination outcome under the procedure used [Callejas-Chavero et al. 2023].

Young seedling size represents a distinct stage of assessment. It connects the earlier provisioning of the seed with growth after germination. The study demonstrated a size difference but did not separate the mechanisms linking feeding on the maternal plant to that outcome. The subsequent fate of offspring in the field also remains unresolved. A measurement after one month describes early development under the study conditions. Following further seasons would be needed to determine whether the difference persists, affects survival or diminishes as water and other resources change [Callejas-Chavero et al. 2023].

Honeydew connects insects and fungi

Honeydew created another connection among participants in the system. Excess remaining on stems favoured the development of dark fungal growth. Fungi covered an average of 15% of the assessed area on scale-bearing branches, compared with approximately 0.1% on scale-free branches. Greater insect abundance was associated with a larger affected area. The observation therefore connects feeding with an additional change in conditions on the green stem. The plant loses resources extracted by scales while its surface becomes covered by microorganisms growing on the secretion [Callejas-Chavero et al. 2023].

The authors linked extensive fungal coverage with impaired functioning of photosynthetic stems and declining plant condition. In the experiment itself, however, they measured affected surface area and growth responses without directly measuring gas exchange. Reduced photosynthesis therefore remains an interpretation of the mechanism. Ants collecting honeydew simultaneously help maintain the colonies producing it. The final amount of secretion on a stem depends on both colony size and its removal. The relationship can consequently change during the season as scale numbers increase and worker activity changes [Callejas-Chavero et al. 2023].

What changed when ants were excluded

The parasitoid assessment focused on scales in their second and third developmental stages, the stages in which host use by the wasp was observed. During the first two months, the proportion parasitised was similar in both treatments, at approximately 15%. The trajectories then diverged. With ant access, the mean proportion fell to approximately 10%, whereas on exclusion branches it rose to approximately 25%. The change therefore concerned parasitoid effectiveness within the colony, and detecting it required observations extending across part of the herbivore’s life cycle [Callejas-Chavero et al. 2023].

The authors proposed that continuous patrolling makes it harder for wasps to land and lay eggs. The ant-exclusion experiment supports a role for ants in reducing parasitism, while the detailed sequence of encounters requires direct behavioural observation. Protection did not eliminate parasitoids completely. Some scales were still used when ants were present. The system therefore maintained both processes simultaneously, care by workers and attacks by wasps. Their relative effectiveness influenced how much of the colony could continue feeding on the cactus [Callejas-Chavero et al. 2023].

Host density changes attack success

The number of parasitised scales increased with the availability of suitable developmental stages, but the relationship was curved. With ants present, the increase in attacked individuals levelled off at intermediate colony densities. After workers were excluded, the parasitoid response was stronger, but it also weakened at very high host abundance. Separating the number of attacked insects from their proportion in the colony helps explain these results. A large colony can supply more hosts while still leaving a substantial proportion of individuals beyond the reach of successful attack [Callejas-Chavero et al. 2023].

Several explanations were possible for the decline at the highest densities. Excess honeydew could impair the condition of scales or interfere with egg laying. The authors also considered effects of other natural enemies on developing parasitoids. The study did not distinguish among these mechanisms. It did show clearly that wasp effectiveness depends on host density and the presence of the hosts’ attendants. Applying a single fixed rate of protection to every colony size would therefore overlook changes documented during the field observations [Callejas-Chavero et al. 2023].

The scope of indirect protection

In this system, the parasitoid removed part of the herbivore population, while ants reduced the incidence of parasitism. The authors interpreted the wasp’s activity as a potential indirect defence of the cactus. The signals guiding females to colonies remained to be established. No volatile compounds were identified that would demonstrate active attraction of the parasitoid by the plant. The documented starting points are therefore the effect of ants on parasitism and the association between scales and cactus condition. The chemical mechanism of host location and the full balance of plant benefits require further experiments [Callejas-Chavero et al. 2023].

The source of sugar changes the connection between ants and herbivores. On M. geometrizans, honeydew comes from a scale insect drawing resources from the cactus, so ant attendance favours persistence of the pest. In the cholla study on Cylindropuntia echinocarpa, the starting resource was the plant’s extrafloral nectar. Braun and Lortie added sugar food to alternating cacti in two Mojave field experiments, involving 60 plants in 2023 and 80 different plants in 2024. This treatment supplied sugar independently of insect secretions [Callejas-Chavero et al. 2023] [Braun and Lortie 2025].

Supplementation increased observed ant numbers in some species, including Solenopsis xyloni, while Crematogaster depilis showed no clear increase in abundance. The presence of C. depilis was nevertheless associated with fewer herbivores and more seeds per fruit. The food supplement itself did not significantly increase seed number. Separating these outcomes follows two stages of the relationship, the ant community’s response to a resource and the association between attendant identity and plant reproduction [Braun and Lortie 2025].

The scale-insect experiment restricted ant access and measured the change in parasitism. The cholla experiment manipulated a sugar resource, while the relationship between a particular ant species and seed number came from observed worker presence. Both systems involve sugar flowing through a food web. The direction of its effects depends on whether an ant protects a honeydew producer at the cactus’s expense or accompanies a reduction in other herbivores. Measurements of parasitism, herbivore abundance and seeds describe successive, distinct parts of that balance [Callejas-Chavero et al. 2023] [Braun and Lortie 2025].